A Volpe

F Berne

C Faisse

J Guillon

P Morales

B Moreau

Santraine

C Gil

G Colledani

M Domenes

D Volpe

A Berne

F Faisse

C Guillon

J Morales

B. Santraine P Moreau

Renewal of the interfero-polarimeter diagnostic for WEST

Keywords: tokamak, FIR diagnostic, interferometry, polarimetry

1) Introduction

On fusion plasma tokamak machines, the measurement of the electron and current density can be obtained by interferometry and polarimetry technics, using the information given by Far Infrared (FIR) laser beams crossing the plasma [START_REF] Veron | Infrared and Millimeter Waves[END_REF]. On the French Tore Supra tokamak, a ten channel poloidal Interfero-polarimeter routinely run till 2011, using the following principle of measurement [START_REF] Gil | Retro-reflected channels of the Tore Supra FIR interfero-polarimeter for long pulse plasma operation[END_REF][3]:

There are 3 dedicated detectors for each channel. After exiting the plasma, each probing FIR beam is recombined with a 100 KHz frequency shifted beam in front of the detectors to deliver synchronous amplified signals that are measurable in phase and amplitude.

On the interferometry part, the line integrated electron density (LID) is calculated for each channel using the phase information Φ of two probing beams that have different wavelengths 𝜆 (195 and 119 microns). They are superposed to cross the plasma and then they are separated toward the detectors [ref 3 p. 1228]. This double phase information enables to calculate the LID by eliminating the path length component  in equation [START_REF] Veron | Infrared and Millimeter Waves[END_REF]. Φ = 2.82 × 10 -15 𝜆 ∫ 𝑛 𝑒 (𝑙) 𝑑𝑙 + 2Π 𝛿 𝜆 [START_REF] Veron | Infrared and Millimeter Waves[END_REF] Where n e is the electron density, all parameters in MKSA unit.

On the polarimetry side, the Faraday Angle (FA)  is obtained by measuring the rotation of polarization of a 195 micron wavelength beam through the plasma. A grid separates the 2 perpendicularly polarized output beams toward 2 distinct detectors, of which one is also used for interferometry [ref 3 p. 1237]. To take into account a possible elliptization of the beams, both amplitudes and relative phases are measured to calculate  [START_REF] Brault | Evaluation of the Faraday angle by numerical methods and comparison with the Tore Supra and JET polarimeter electronics[END_REF]:

tan (𝛼) = 𝐾 abcos(ψ-ψ 0 ) a 2 (2)
Where a and b are the amplitudes of the 2 FIR beams, 𝜓 the relative phase and 𝜓 0 the initial one. K is a coefficient that is deduced from a quartz plate rotation calibration before each plasma.

Then one can calculate by the Ampere's theorem the current density with the magnetic poloidal field that can be deduced from equation (3):

𝛼 = 2.62 × 10 -13 𝜆 2 ∫ 𝑛 𝑒 (𝑙) 𝐵 // (𝑙) 𝑑𝑙 (3)
Where 𝐵 // is the magnetic component parallel to the FIR beam propagation.

In the former Tore Supra configuration, there were five plasma traversing chords going through vertical ports and five chords through a horizontal port to be reflected on the inner wall by corner cube mirrors (CC). For WEST [START_REF] Bucalossi | The WEST project: Testing ITER divertor high heat flux component technology in a steady state tokamak environment[END_REF], a lower and an upper toroidal tungsten divertor, with their supporting structures implemented in the vertical ports, have been installed and the inner panels have been displaced. Therefore the chord trajectories of the beams inside the vessel had to be modified and optimized to recover a good spatial resolution. To minimize the cost, efforts have been done not to modify the optics in the laser source and detector areas.

To increase the precision and reliability of the diagnostic, a new digital electronics with FPGA processors has been installed. The embedded algorithms benefit from several year experiences on the JET tokamak, as they were initially developed and successfully tested for it [START_REF] Gil | Upgrade of the electronics of the JET far infrared interferometer[END_REF]. An industrial PC has been configured to receive the output data from the boards, to calculate the LID and FA and to dispatch the results toward the WEST real time system and the WEST data base.

2) Mechanical modifications 2.1 Trajectories of the chords inside the vessel.

In order to reuse at most the former 10 channel diagnostic, 10 chords could be positionned to optimize the spatial resolution.The figure 1a shows the chord trajectories of the FIR beams inside the vessel with respect to a typical WEST plasma, in which the equally spaced in radius magnetic surfaces have been drown. To complete the profile characterization and because the edge of the plasma cannot be seen from the 0.8 m high horizontal port, 2 chords go through a vertical port. The incoming beams are reflected by internal mirrors (chords 1 and 2 in figure 1b) before reaching a CC. One is for giving information on the plasma edge, the other one on the density inside the X point.

Corner cube mirrors

All the FIR beams are now reflected on CC inside the vessel. Figure 2a shows the 1300 mm high specific internal panel on which 9 CC could be fixed. Cooling water flows inside a sandwich of 2 stainless steel sheets at a temperature of 70-90°C during plasmas and up to 200°C when conditioning. To save space and ensure the water tightness, the CC are TIG welded onto the panel. Figure 2b and 2c show one CC. As in the previous Tore Supra configuration [START_REF] Gil | Retro-reflected channels of the Tore Supra FIR interfero-polarimeter for long pulse plasma operation[END_REF], the bulk part is made of Cu-Cr alloy to get a sufficient hardness. To allow the welding to the panel, a 62 mm Nickel ring is first welded outside the CC by electron beam. Then the print of the 3 perpendicular faces is dug by electro-erosion to obtain an optical diameter of 49 mm. The trihedral corners are dug perpendicularly to the incoming IR beams. But one CC is oriented on the bisector of the 8 degree angle formed by 2 incoming beams that share the same mirror. The 90 degree angles between the faces are manufactured with a precision of 50 second of arcs to obtain a low divergence of the reflected beam. Finally, the faces are manually polished down to a 10 micron peak to peak roughness. It is not good enough for visible reflection when alignment and thus movable optics had to be positioned outside the windows to send back the light and enable a visible alignment toward the detectors.

Windows

In the former Interfero-polarimeter set up, the windows were made of natural crystal quartz, brazed by gold bonding on the flanges [3]. 3 of them have been reused for the WEST configurations. As the number of channels increased in the horizontal port, one had to position 3 channels per window (figure 3). To avoid spurious reflections that could reach the detector and spoil the measurement, the disks and supports are 1.5 degree oriented perpendicularly to the plane defined by the beams.

The angles between the beams that share the same window vary from 3 to 5 degrees and the maximum incidence angle is 4.5 degrees on a quartz disk. Its thickness of 6959 microns is optimized by calculation of the transmission for the expected incident angles (figure 4).

Edge channels

Although the WEST divertor support occults most of the vertical port and no direct line of sight to plasma is possible with the baffle, 2 channels could be installed to diagnose the edge plasma.

Internal flat mirrors reflect the FIR beams towards the CC (figure 5). The 220X105mm and 110X85 mm flat mirrors are screwed on the divertor leg (figure 5b). They are made of stainless steel, polished at lambda for visible alignment and they can be orientated (figure 5c). They are presently inertial but will be actively cooled for the long pulse second phase of WEST. Channel 1 is almost parallel to the divertor, 50mm away, and thus the beam crosses the X point region. Channel 2 is pointing the 0.8-0.9 normalized radius plasma region and therefore completes the profile information, which is especially useful for electron density profile reconstruction. It crosses the baffle through a 30 mm diameter hole. Only interferometry signals are presently operational but polarimetry will be added in the future.

External FIR beam transport

The trajectory changes inside the vessel induced modifications of the ex-vessel FIR beam transport. Effort has been done to keep most of the diagnostic installation. The synoptic (figure 6) of the modified diagnostic is very similar with the former one given in [3]. The DCN and H2O laser sources remain unchanged as well as the analyser FIR beam set up and the 4K cryogenic detector installation. The former five traversing channels had to be transformed in reflected channels with new calibration tables and separating optics.

Out of the overall 450 existing optics, 90 had to be repositioned because of the different chord trajectories in plasma, among which 40 new mirrors, mainly the focusing ones. Their dimensioning has been done by considering the plasma refraction and the Gaussian beam expansion:

To calculate the refraction, the REMA raytracing code was used. It is adapted in wavelength from an electron cyclotron wave propagation code [START_REF] Krivenski | Improving current generation in a tokamak by electron cyclotron waves[END_REF]. For the typical expected WEST electron density, the deviation is not more than 1.5 mm on the CC (figure 7 b) and therefore less than 3 mm on the output windows. Gaussian beam calculations [START_REF] Veron | Infrared and Millimeter Waves[END_REF] were performed to focus on the CC while fitting the incoming FIR beam in size from the lasers and towards the detectors. Figure 8a gives an example for one channel at 195 micron wavelength: 4 new focus mirrors are needed. On the CC, the 1/e diameters of all the beams is always less than 14 mm. Therefore, the 3/e value of 42 mm, in which all the beam energy is contained, is lower than the 49 mm CC diameter. Possible refraction or misalignments will thus not induce beam losses. The external optics and windows have also been dimensioned at 3/e. Therefore the signals are not lost despite the induced deviations caused by 3 mm refractions in plasma. (figure 8b)

Figure 8: Channel 1 beam transport simulations from the laser the detector, a: Gaussian beam expansion, b: Induced deviation due to a 3 mm refraction

In the torus hall in front of the ports, 3 existing crates have been modified to position the new mirrors and their supports inside (figure 9) A new tower,2.5 m high, has been also added. It connects 3 channels to the former traversing channel detectors. It has an aluminium frame composed of welded 50 mm square section tubes. The base is screwed on steel girders that are fixed on the torus hall concrete to reduce at most the oscillations when plasma.

Figure 9: Overview of the mechanical modifications for WEST

3) Modification of the signal processing 3.1 Electronics and acquisition system

The new electronics arrangement is a mixing of the old electronics and acquisition with new ones, as seen in Figure 10: The detectors, the preamplifiers, the programmable amplifiers, the calibration system and the 1MHz data acquisition have been kept unchanged. But the first generation digital interferometric signal processing electronics had to be upgraded to implement more complex real time algorithms, in particular to correct fringe jumps. Moreover, the analogue polarimetric electronics and its 12 bit data acquisition was also to be improved to avoid long term component drifts and to gain precision. In the new settlement, for each channel, a unique digital board (DGB), which has been developed by CEA [START_REF] Barbuti | Opportunist combination of electronics technologies for real time calculations in the Tore Supra tokamak[END_REF], calculates in real time the interferometric phases for the 2 wavelengths and the amplitude and phase values that are needed for polarimetry. A FPGA processor is embedded, which enables to program algorithms in VHDL language. For interferometry calculation, the inputs are the zero crossing times of the 100 KHz reference and probe sine signals. They are sent from the programmable amplifiers by optical fibres. For polarimetry, the analogue sine signals are digitalized in the Mezzanine and then are sent as inputs to the DGB. The Mezzanine also reconstructs the analogue amplitudes from the ones calculated in the DGB in order to control by industrial programmable logic controller the (1-80) amplifiers.

The DGB are connected to the WEST timing network to start the calculation and to date the measurements (figure 11). The sampling time of the output data is 1ms although the cycle time in the FPGA processor is governed by a 25 MHz clock, fast enough to both analyse the behaviour of the 100 KHz signals and to apply real time filtering . The results are sent as dated packets via an Ethernet connection to a network switch and then transmitted to a new dedicated industrial PC computer, as the upgrade of the Real Time network in the frame of the WEST project required the old Tore supra VME units to be migrated to PC architecture. Its function is to transform the incoming measurements into the LID and FA values that are defined in equation 1 and 3. Algorithms check the result consistency before sending them at the sampling time of 1ms to the WEST real time shared memory network to be used as control and protection by other systems. The time delay that is induced by the PC calculations is at most 1ms. The PC also controls via FPGA acquisition boards the half-wavelength rotating quartz used for the polarimetry calibration that is performed before each pulse. At the end of the pulse, the PC transfers the DGB outputs to the WEST data base for post pulse analysis.

Methods of calculation employed in the DGB

The algorithms implemented in the DGB for WEST have been adapted from those initially developed for the JET tokamak interfero-polarimeter, which works at similar frequencies and wavelengths.

Figure 12: Principle of the of the interferometric phase real time calculation

The interferometric phase is reconstructed with the help of slow counters K and J and fast counters Nk and Ni (figure 12) that are trigged by the fronts generated when the sine signals cross zero :

Φ = 2Π(K -J + Ni/Nk ) (4)
The K-J counting follows the 2 phase variations, which are called fringes. Non-physical fringe jumps can occur when the signal is lost or degraded. Therefore, an additional fast counter measures the probe signal time delay between 2 zero crossings to check the anomalies. When these are detected, the J counting is corrected, assuming that the phase change has been lesser than  in 500 s [START_REF] Gil | Upgrade of the electronics of the JET far infrared interferometer[END_REF]. From the experience of JET, where high frequency oscillations induced by plasma turbulence were observed on the probe signals and potentially could mislead the checking, a 5 KHz low pass filter is applied on the time delay counting before detecting the anomalies.

For polarimetry, the amplitudes and relative phase of the 2 perpendicularly polarized sine signals are calculated after digitalization (figure 13). After 80-120 KHz bandpass filtering, the 2 rectified signals are 586 Hz low pass filtered in 3 steps and 3 sub-samplings to obtain the amplitudes. Simultaneously, the straight multiplication of the 2 signals and low-pass filtering enables to obtain a value proportional to cos , where  is the relative phase. The sin information is obtained by multiplying the first signal by the second one, whose value is taken some time steps before [START_REF] Brault | Evaluation of the Faraday angle by numerical methods and comparison with the Tore Supra and JET polarimeter electronics[END_REF]. The 1.086 MHz digitalization frequency has been chosen not to be an integer ratio of 100 KHz to avoid stroboscopic effects. Moreover, the sub-sampling ratios are defined to avoid layovers.

𝑆 𝑛 = 𝑎 0 𝑋 𝑛 + 𝑎 1 𝑋 𝑛-1 + 𝑎 2 𝑋 𝑛-2 + 𝑏 1 𝑆 𝑛-1 + 𝑏 2 𝑆 𝑛-2 (5)
Where X n is the initial signal, Sn the filtered signal and a i and b i the filter coefficients.

One can see in figure 13, in which the characteristics of the 80-120 KHz band pass filter are given, that the a i and bi coefficients are small and thus justify a 32 bit precision in the computation. The results of the multiplications of the 2 signals are in 64 bits not to lose the precision and then the low-pass filters are in 32 bits and the outputs as well.

4) Experimental results

The diagnostic has routinely been running during the 2017-2018 WEST restart campaigns and helped to improve the plasma performances up to a 10 s duration and a 3 × 10 19 m-2 central LID. With the help of the last closed surface (LCS) that is given in real time by the magnetics diagnostic [START_REF] Moreau | The new magnetic diagnostics in the WEST tokamak[END_REF] -figure 16a shows an example for pulse 53259 -, the distance of each chord in the plasma can be estimated to calculate the averaged LID. Moreover, the LID integration of the 8 almost horizontal channels along the vertical line defined by the centre of the plasma enables to calculate the volume average density by dividing by the surface, assuming that the LID is null on the LCS. The peaking factor is the ratio of the average LID by the average volume density. It gives information on the peaking of the profile (figure 16 b).

Real time calculations.

The consistency of the measurements can be verified by plotting the LID and FA profiles versus the vertical position of the 8 almost horizontal chords found at radial position of 2.5 m (figure 17). 18a shows there are significant path length oscillations only at the beginning and at the end of the pulse. They are correlated to sudden inversions of the current in the poloidal coils. The exceptions are the edge channels (figure 18a blue line) because the internal mirrors are fixed on a divertor leg and therefore vary with the divertor current. Therefore the central LID range error of a few 10 18 m-2 (figure 18b) allows density control with one wavelength, except at the beginning of the pulse.

Post pulse processing

After each pulse, the LID and FA angles are recalculated. This allows correcting possible additional fringe jumps or failures not detected by the real time algorithms. During the first campaign, the plasmas were quiet enough, only L mode, not to generate fringe jumps that would be uncorrected by the DGB. The LID were only in default when the plasma was ending by a disruption, during which the FIR beams are strongly deviated from the detectors while the density strongly varies.

The 1MHz data acquisition is also used to diagnose fast variations of the electronic density. The interferometric phase is calculated with the digitalized sine signals by the same zero crossing technics as the electronics one. This allows a sampling time of 15 s. As the acquisition is not continuous but is composed of several triggered 1s data packets, the results of the calculations are mixed with the low acquisition to obtain an absolute LID. As an example, the slow and fast acquisitions are superposed during a ramp down (figure 19). A growing 6 KHz mode, followed by a crash, can be observed. The consistency of the interfero-polarimetry measurements is analysed with the code outputs. Figure 21 is an example of the obtained electron density and safety factor profiles. Some discrepancies of the reconstructed LID and FA with the experimental measurements can be observed (figure 21 c andd).

Indeed the value differences cannot be explained by the instrumental precision of the diagnostic:

For interferometry, the counting of the phase with a 25MHz clock induces a precision 1/250 of a fringe for a 100KHz signal , which can be converted in a one way through LID precision of 3.2 × 10 16 m-2 . For this pulse, the measured LID typical noise that is calculated by standard deviation is 1 × 10 17 m-2 before plasma and 2.5 × 10 17 m-2 during the plasma plateau.

For polarimetry, the 32 and 64 bit calculations in the electronic boards provide high precision outputs. The 5 point averaged FA noise is typically 0.01 degrees before plasma and 0.02 degrees on the plateau. This leads to think that these values are due to the infrared beams intrinsic noise and are due to the plasma fluctuations as well.

Another possible error could be caused by the uncertainty on the alignment of the chords inside the plasma. As it is estimated to less than 1 cm, one can see on figure c and d that a 1 cm vertical displacement does not significantly help reducing the profile differences.

Therefore, the observed discrepancies need further investigations and cross checking with other diagnostics during the following plasma campaigns to get a better comprehension of the plasma. For the new WEST tokamak configuration, the diagnostic successfully modified the positions of the chords inside the plasma by adding plane mirrors and cooled retroreflectors inside the vessel and by modifying the beam transport outside the vessel whilst keeping a great part of the old optics and most of the initial mechanical structure. 10 channels could be reinstalled to achieve a 0.1 rho spatial resolution and a X point diagnose. Solutions such as shared windows and retroreflectors by several beams proved their efficiency to solve the lack of space issues.

The data processing has been improved for higher precision and reliability. The new digital boards with their embedded interferometric and polarimetric algorithms found to be efficient and trouble free.

The new PC that is connected to the WEST real time network is now routinely working.

For 2 years the diagnostic has been routinely exploited for real time control. The post pulse analysis has started and will be continued during the next campaign. It is an efficient tool to be used for characterizing the electron density at different time scales and to contribute to the full consistency of the equilibrium codes.

Figure 1 :

 1 Figure 1: Beam chord trajectories in the plasma, a: Overview versus the magnetic surfaces, b: Normalized positions of the chords in the plasma

Figure 2 :

 2 Figure 2: CC implantation, a: specific panel with the CC, b: outer CC view, c: inside CC view

Figure 3 :

 3 Figure 3: Twin flange supporting the 2 synthetic quartz diameter 111.5 m disks

Figure 4 :

 4 Figure 4: Transmission of a 6959 micron thick Zcut crystal quartz, Refraction index: n195=2.1116, n119=2.1256, absorption: a195=.30, a119=.63

Figure 5 :

 5 Figure 5: Edge channels, a: General implantation, b: Zoom on the reflection under the baffle, c: Flat mirrors fixed on a common support

Figure 6 :

 6 Figure 6: Synoptic of the interfero-polarimeter adapted to the WEST configuration

Figure 7 :

 7 Figure 7: Calculated refraction, a: Chords in plasma, b: Beam deviation inside the plasma, c: Expected electron density profile

Figure 10 :

 10 Figure 10: Synoptic of the data processing for one channel

Figure 11 :

 11 Figure 11: Interfero-polarimetry hardware integrated in the WEST network

Figure 13 :

 13 Figure 13: Principle of the characterization of the polarimetric signals

Figure 14 :

 14 Figure 14: Real time 80-120 KHz band pass filter characteristics, a: High pass transmission, b: Low pass transmission, c: Transmission when the filtering is repeated twice.

Figure 15 :

 15 Figure 15: WEST #53259 raw real time signals, a: Line integrated density, b: Faraday angles As an example, on figure 15, one can see the 10 channel LID and the 8 channel FA that are calculated in real time in the PC. The 2 most central plasma LID (red curves almost superposed in fig15a) are routinely used for plasma gas injection control and to authorise the electromagnetic heating system power coupling. In figure 15 b, the 1ms rate FA have been averaged over 5 points

Figure 17 :Figure 18 :

 1718 Figure 17: Vertical profile of the 8 almost horizontal channels, pulse 53259 at t= 5s, a: LID profile, b: FA profile.As the LID information is mandatory to control the gas injection, it is important to have redundancy on the central LID information even when partial diagnostic failure. Channel 3 and 8 are almost central (superposed red curves on figure15 a) and thus can be equally used if one fails. Moreover, if one of the lasers or detectors is out of order, the real time software automatically switches to the LID calculated with only one wavelength. The induced errors, due to the path length variations, are analysed in figure18:

Figure 19 :

 19 Figure 19: Comparison of the fast and slow LID measurement during a density crash.

Figure 20 :

 20 Figure 20: Magnetic surfaces obtained at t=5.1 s by the NICE code, red line: LCS , dash line: real time LCS

Figure 21 :

 21 Figure 21: NICE outputs for pulse 53259 at t=5.1s, a: Electron density profile, b: Safety factor profile, c: Vertical LID profile comparison, d: Vertical FA profile comparison 5) Conclusion

For the next phase of WEST, which consists in long pulses with high heating power, only few internal mirrors will have to be modified and cooled.


Lonni Besançon

Amir Semmo

David J Biau

Bruno Frachet

El Virginie Pineau

Hadi Sariali

Rabah Taouachi

Tobias Isenberg

Pierre Dragicevic

El Hadi Sariali

Reducing Affective Responses to Surgical Images through Color Manipulation and Stylization

Keywords: Computing methodologies → Non-photorealistic rendering, Image processing, • Human-centered computing → Empirical studies in HCI, Stylization, affect, empirical study, surgery

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Many non-photorealistic and expressive rendering techniques deal with the stylization of 2D images or videos [START_REF] Kyprianidis | State of the "art": A taxonomy of artistic stylization techniques for images and video[END_REF]Rosin and Collomosse 2013]. While much of this work was initially motivated by the desire to replicate artistic techniques and was only guided by a subjective visual comparison to existing artwork, researchers have begun to empirically evaluate the effects of stylization [START_REF] Gooch | Viewing progress in non-photorealistic rendering through Heinlein's lens[END_REF][START_REF] Isenberg | Evaluating and validating non-photorealistic and illustrative rendering[END_REF][START_REF] David | Non-Photorealistic Animation & Rendering: 7 Grand Challenges[END_REF]. Some researchers argue, however, that controlled experiments are difficult in the context of expressive rendering [START_REF] Mould | Authorial subjective evaluation of non-photorealistic images[END_REF]], and that we should rather concentrate on subjective evaluation [START_REF] Mould | Authorial subjective evaluation of non-photorealistic images[END_REF]] and on the appreciation of resulting graphics [START_REF] Hall | Don't measure-Appreciate! NPR seen through the prism of art history[END_REF]. While such forms of evaluation arguably have their place in the context of the often art-inspired field of expressive rendering, the goal of creating expressive graphics is increasingly understood to incorporate more than the "support of artists (or illustrators)" or the "creation of tools for visual expression for non-artists" and to include, e. g., also "illustrations [...] to inform [...] patients" in a medical context [START_REF] Isenberg | Interactive NPAR: What type of tools should we create?[END_REF]]. In this latter case it is then essential that we understand how different stylistic filters are perceived and experienced by real people, and that we thus study them empirically, through controlled experiments [START_REF] Gooch | Viewing progress in non-photorealistic rendering through Heinlein's lens[END_REF]].

In the past, in fact, some researchers have already examined such effects of stylization. [START_REF] Mandryk | Evaluation of emotional response to non-photorealistic images[END_REF] resp. [START_REF] Mould | Emotional response and visual attention to non-photorealistic images[END_REF], for instance, studied people's emotional response to stylized images and found that emotional responses was generally muted, and that responses concentrated around neutral feelings. Others [START_REF] Dragicevic | Illustrative data graphic style elements[END_REF]] argue (motivated by the results of earlier studies [START_REF] Schumann | Assessing the effect of non-photorealistic rendered images in CAD[END_REF]) that stylization may affect people's attitude toward a data visualization and result in longer times they spend looking at the visuals. Here, however, we are less interested in potentially positive effects of stylization, but instead in how much it can diminish negative affects caused by unpleasant pictures. Such pictures are involved when surgeons inform their patients before surgical procedures-because many people find surgery pictures repellent (e.g., [START_REF] Sawchuk | Emotional responding to fearful and disgusting stimuli in specific phobics[END_REF][START_REF] Tolin | Disgust and disgust sensitivity in blood-injection-injury and spider phobia[END_REF]), effective communication can suffer. This context would seem like an ideal application case for expressive rendering [START_REF] Isenberg | Interactive NPAR: What type of tools should we create?[END_REF]]. Nonetheless, the creation of effective illustrative visualizations of a wide variety of surgical procedures is still beyond our abilities. We thus study whether it is possible to use existing stylization techniques for 2D images-applied to real surgical images-to achieve a similar effect, and diminish the negative affect that surgery pictures can elicit. Applications go beyond patient information and include student training, media communication, and public education.

Below we discuss various image filtering and stylization techniques that can be used to dampen the negative affect elicited by surgery pictures. We then report on an interview session with four surgeons, who helped us differentiate between techniques that can preserve important information, and techniques that are unusable because they obfuscate too much. We then report on an experiment where the most promising techniques were tested on ordinary subjects. We found that all techniques can reduce the repulsiveness of surgery pictures as judged by participants, although spatial-domain techniques appear to be more potent than color manipulations. We conclude by a discussion and opportunities for future work.

BACKGROUND

In this section, we review related work in non-photorealistic rendering, before reviewing work from other areas on how people perceive, experience, and deal with surgery and injury images.

Non-photorealistic Surgery Illustrations

Medical illustration has long been one of the primary motivations for non-photorealistic and expressive rendering [START_REF] Gooch | Non-Photorealistic Rendering[END_REF][START_REF] Strothotte | Non-Photorealistic Computer Graphics[END_REF] and, consequently, many researchers have developed rendering techniques for this purpose. Several surveys and tutorials have covered the field in detail (e. g., [START_REF] Sousa | Illustrative visualization for medical training[END_REF][START_REF] Viola | Illustrative visualization for medicine and science[END_REF][START_REF] Preim | Illustrative medical visualization[END_REF][START_REF] Viola | Illustrative visualization for medicine and science[END_REF]) and we refrain here from citing specific techniques. Common among them is, however, that they are inspired by traditional, usually hand-made illustration techniques, styles, and examples and that they thus focus on clarification and explanation, rather than on emotional aspects or on reducing the negative affect that some types of content could induce in people.

Another common characteristic of many illustrative techniques and also traditional illustration styles-for medical application and otherwise-is the use of abstraction and emphasis. These aspects have been discussed in the visualization and expressive rendering literature, such as in the contributions by [START_REF] Rautek | Illustrative visualization: New technology or useless tautology[END_REF] and [START_REF] Viola | Pondering the concept of abstraction in (illustrative) visualization[END_REF]. Here, abstraction is "a transformation which preserves one or more key concepts and removes detail that can be attributed to natural variation, noise, or other aspects that one intentionally wants to disregard from consideration" [START_REF] Viola | Pondering the concept of abstraction in (illustrative) visualization[END_REF]]-to allow viewers of a visualization to focus on major or important aspects. In this work, however, we explore the abstracting qualities of image filters for the removal of details such that the images are perceived as less offensive-potentially because they no longer depict surgery situations in all their details.

Non-photorealistic Techniques and Affect

In the past, researchers have studied how stylization can influence how people perceive images. [START_REF] Gooch | Evaluating space perception in NPR immersive environments[END_REF], for example, showed that a line-based rendering of a virtual scene leads participants to underestimate distances by about a third, quite similar to what happens in 'photorealistic' VR settings. Later, [START_REF] Gooch | Human facial illustrations: Creation and psychophysical evaluation[END_REF] showed that non-photorealistic illustrations and caricatures of people's portraits could be learned faster than real photographs. We cannot deduct from these results, however, that stylized images would lead people to feel differently about what is shown.

Already early work on non-photorealistic rendering, however, discussed this very effect. [START_REF] Duke | Rendering and affect[END_REF] and [START_REF] Halper | Psychology and non-photorealistic rendering: The beginning of a beautiful relationship[END_REF], for example, described how the (non-photorealistic) depiction style can affect people's assessment of danger and safety as well as strength and weakness, and can change their participation and interaction behavior (for study details see Section 2 of [START_REF] Halper | Supportive Presentation for Computer Games[END_REF] thesis). Even before this work, [START_REF] Schumann | Assessing the effect of non-photorealistic rendered images in CAD[END_REF] provided evidence for stylization to increase people's willingness to interact with visuals. More recently, [START_REF] Mcdonnell | Render me real? Investigating the effect of render style on the perception of animated virtual humans[END_REF] showed that an increased abstraction of virtual characters (according to their participants' classification of "realism") decreases appeal, friendliness, and trustworthiness up to a point; for highly abstracted depictions people again feel similar about the stylized virtual characters as they do for realistic depictions-similar to what the Uncanny Valley theory predicts. Like the perceptual studies discussed before, however, these approaches do not shed light on whether stylization changes people's negative emotions toward disturbing images.

Most relevant for our own work, out of the expressive rendering literature, is Mandryk et al.'s [2011] and Mould et al.'s [2012] work who demonstrated that stylization can affect the emotional interpretation of images. Similar to what we do in our experiment, they applied a range of styles (stippling, line art, painterly rendering, and blur) to a set of images with different affective content from the International Affective Picture System (IAPS), and analyzed people's feeling of arousal, valence, dominance, and aesthetics. Stylization generally muted participants' emotional responses toward a neutral point, yet emotions were never completely suppressed. Their negative stimuli (e. g., a gun pointed at camera, or a cemetery), however, did not have the repulsive potency that surgery photos can have. This study thus inspires our own, but we specifically target surgery pictures that many people cannot easily look at.

We note that researchers also have examined the opposite path: changing the stylization of images based on emotions detected in a video feed. [START_REF] Shugrina | Empathic painting: Interactive stylization through observed emotional state[END_REF], for example, presented their "empathic painting" technique that recognizes a person's emotional state based on features of their facial expression, which they then use to adjust the parameters of a painterly rendering technique. Here Shugrina et al. borrow from the psychological literature and created a mapping from the detected emotional state to rendering parameters such as stroke path and color. Yet, it is not clear if the resulting images also change the emotional state of the viewer, or if so in what way this process can be controlled.

Human Response to Surgery Imagery

For several decades, researchers have studied human response to repellent images in order to uncover the physiological and psychological mechanisms involved. Studies have used various types of aversive stimuli such as homicide scenes [START_REF] Hare | Autonomic responses to affective visual stimulation[END_REF], spiders [START_REF] Tolin | Disgust and disgust sensitivity in blood-injection-injury and spider phobia[END_REF]], vomit [Olatunji et al. 2008], maggots, cadavers, and dirty toilets [START_REF] Schienle | The insula is not specifically involved in disgust processing: An fMRI study[END_REF]. Many studies have examined responses to scenes depicting a body envelope violated by an injury or a surgery. Examples include photos of body mutilation (e.g., [START_REF] Rafael Klorman | Individual differences in fear and autonomic reactions to affective stimulation[END_REF]), of surgery procedures (e.g., [START_REF] Sawchuk | Emotional responding to fearful and disgusting stimuli in specific phobics[END_REF][START_REF] Tolin | Disgust and disgust sensitivity in blood-injection-injury and spider phobia[END_REF]), and videos of medical interventions such as blood draw [START_REF] Gilchrist | The effects of blood-draw and injection stimuli on the vasovagal response[END_REF], open-heart surgery [START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF], or surgical amputation [START_REF] Rohrmann | Cardiovascular indicators of disgust[END_REF]. Studies have involved both ordinary subjects (e.g., [START_REF] Hare | Autonomic responses to affective visual stimulation[END_REF]), BIIphobic1 subjects (e.g., [Öst et al. 1984]), and often a combination of both (e.g., [START_REF] Haberkamp | Enhanced visuomotor processing of phobic images in blood-injury-injection fear[END_REF]).

Various measurements have been employed to quantify subject reactions, the most common being heart rate (e.g., [START_REF] Rafael Klorman | Individual differences in fear and autonomic reactions to affective stimulation[END_REF][START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF]). Others include facial expression (using videotaping [START_REF] Lumley | Blood phobics and nonphobics: Psychological differences and affect during exposure[END_REF] or electromyography [START_REF] Lang | Looking at pictures: Affective, facial, visceral, and behavioral reactions[END_REF][START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF]), skin conductance [START_REF] Lang | Looking at pictures: Affective, facial, visceral, and behavioral reactions[END_REF]], neural activation using fMRI [START_REF] Schienle | The insula is not specifically involved in disgust processing: An fMRI study[END_REF], attentional avoidance using eye tracking [START_REF] Armstrong | Attentional bias in injection phobia: Overt components, time course, and relation to behavior[END_REF], and visuomotor processing using a response priming task [START_REF] Haberkamp | Enhanced visuomotor processing of phobic images in blood-injury-injection fear[END_REF]. Subjective measures were also used, where subjects were asked to report to what extent they felt fear and disgust [START_REF] Sawchuk | Emotional responding to fearful and disgusting stimuli in specific phobics[END_REF][START_REF] Tolin | Disgust and disgust sensitivity in blood-injection-injury and spider phobia[END_REF]], avoided watching [START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF]], or experienced vasovagal (i.e., pre-fainting) symptoms [START_REF] Gilchrist | The effects of blood-draw and injection stimuli on the vasovagal response[END_REF]. A strong reaction to a body injury depiction is often marked by a decrease in heart rate, or an increase followed by a rapid decrease called "diphasic response" [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF]]. It also often involves activation of the corrugator supercilii (the "frowning muscle") and the levator labii (responsible for lifting the upper lip) [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF]]. However, studies are inconsistent and there appears to be no perfectly reliable measure that can consistently elicit the same response [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF].

Most of these studies were conducted in order to untangle the emotions involved when people witness surgeries or injuries, sometimes in the hope of better treating BII phobia. This has proven hard to study, as reactions seem to involve various emotions such as anxiety, fear, disgust, and vicarious pain [START_REF] Benuzzi | Does it look painful or disgusting? Ask your parietal and cingulate cortex[END_REF][START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF]]. In particular, the relative role of fear vs. disgust has long been a subject of debate, although now the consensus seems to be that disgust is the main emotion involved [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF][START_REF] Bunmi | Is disgust associated with psychopathology? Emerging research in the anxiety disorders[END_REF]]. To understand why, it helps to recall that fear has evolved for organisms to run away from threats such as spiders, but for static content like body injuries, no such response is necessary [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF]]. More likely, body injuries are experienced as repellent in order to prevent risks of disease or contagion following physical contact, which requires a disgust response [START_REF] Cisler | Disgust, fear, and the anxiety disorders: A critical review[END_REF]. [START_REF] Hanah | Understanding disgust[END_REF] introduced a taxonomy of disgust where blood-injury disgust is a subtype of physical disgust, and whose evolutionary function is to avoid infection. [START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF], however, distinguishes contamination disgust from animal-reminder disgust, with animal-reminder disgust being elicited by "attitudes and practices surrounding sex, injury to the body or violations of its outer envelope, and death" which all act as "reminders of our own mortality and inherent animalistic nature" [START_REF] Bunmi | Core, animal reminder, and contamination disgust: Three kinds of disgust with distinct personality, behavioral, physiological, and clinical correlates[END_REF].

Despite all this previous work, human reactions to the sight of surgery scenes remains poorly understood. Our goal is not to further this understanding, but simply to find out whether processing surgery photos can dampen their affective potency. As far as we know, all studies on blood-injury disgust have either assessed aversive stimuli in isolation or compared them with neutral stimuli, and none of them has studied the effect of processing aversive stimuli using filters or stylization. When conducting our study, we drew from the experience accumulated in this research area, but simplified the methods to directly answer our research question.

In parallel to this body of work focusing on blood-injury disgust, there has been work in psychology and neurosciences where various types of emotionally-salient stimuli were used to study emotion and cognition. Such stimuli were used, for example, to study cultural differences in emotion processing [START_REF] Wrase | Gender differences in the processing of standardized emotional visual stimuli in humans: A functional magnetic resonance imaging study[END_REF]], and emotion regulation [START_REF] Eippert | Regulation of emotional responses elicited by threat-related stimuli[END_REF]]. Some of the stimuli involved surgery and injury photos but again, affective neutralization through image processing has not been a focus. Nevertheless, this area of research has produced standardized stimuli sets which we will use for our own study, as explained in Section 5.1.

Picture Censorship Practices

On a societal level, offensive imagery has been addressed in two major ways: legal censorship and de facto (or self) censorship. While there appears to be close to no legal restriction on what visual content can be published in newspapers [START_REF] Tooth | Graphic content: When photographs of carnage are too upsetting to publish[END_REF]] or in Wikipedia [Wikipedia Contributors 2010a,b], films and videos games are usually regulated by rating systems to classify the media with regard to its suitability for different audiences. While movies cannot be easily customized, the video game industry has explored a wide range of "adjustable censorship" techniques. Some old video games had violent and sexual content disabled by default, while giving the option to reactivate it through the use of secret codes. More elaborate adjustable censorship techniques were also developed: some video games (e.g., Silent Hill, Resident Evil, House Of The Dead, later release of Ocarina of Time) offer the option to change the color of blood to various tones such as blue, dark, or green depending on the game [TVTropes Contributors 2018a;ZeldaWiki Contributors 2018]. While most mangas feature black blood due to the constraints of black & white printing, some color animes and animated films employ a different blood color to suit all audiences (e.g., Dragon Ball Kai, Pokémon, Bleach, The little mermaid) [TVTropes Contributors 2018b]. Similarly, in movies, black and white has been occasionally used to censor scenes with excessive bloodshed [Kill Bill Wiki Contributors 2017]. All such practices suggest that blood is considered to epitomize violence, but once deprived from its characteristic red color it seems to suddenly become inoffensive in people's minds. These practices provide motivation for considering simple color manipulation techniques in our work.

CHOICE OF PROCESSING TECHNIQUES

In this article we use the term image processing technique or simply technique to refer to any procedure that transforms an image into another image, while keeping it recognizable. We considered four classes of techniques of varying complexity: color manipulation, edge-preserving smoothing, edge detection and enhancement, and image-based artistic rendering. We first outline relevant work and provide rationales for the techniques we retained, and then provide parameter settings yielding reasonable levels of abstraction. The 13 techniques and their settings are summarized in Table 1 and illustrated in Figure 1 and Figure 2 with a 1024x768 photo.2

Color Manipulation

We considered two approaches for changing blood color: decolorization and recolorization [Pratt 2007].

Decolorization. Grayscale conversion is a popular method for image decolorization, where the main challenge is to preserve and make use of the chrominance components so that perceptual image features are retained [START_REF] Čadík | Perceptual evaluation of color-to-grayscale image conversions[END_REF][START_REF] Lumley | Blood phobics and nonphobics: Psychological differences and affect during exposure[END_REF]. Most algorithms transform the problem into optimization to preserve salient features, e. g., by quantifying color differences between image locations [START_REF] Gooch | Color2Gray: Salience-preserving color removal[END_REF] or prevailing chromatic contrasts [START_REF] Grundland | Decolorize: Fast, contrast enhancing, color to grayscale conversion[END_REF], optimizing color and luminance contrasts [START_REF] Neumann | An efficient perceptionbased adaptive color to gray transformation[END_REF], or considering the Helmholtz-Kohlrausch color appearance effect [START_REF] Smith | Apparent greyscale: A simple and fast conversion to perceptually accurate images and video[END_REF]]. The latter localized apparent greyscale algorithm performed best in a previous experiment [Čadík 2008] and we thus retained it and named it ApparentGrey in this article. However, the method may suffer non-homogeneity artifacts near region boundaries, which can be addressed with a global mapping scheme [START_REF] Kim | Robust color-to-gray via nonlinear global mapping[END_REF]].

Recolorization.

A simple yet effective method of recolorization is to alter all color chrominances by a hue shift in the HSV space. We consider a uniform hue shift which makes blood green to which we refer to as HueShift. The hue shift shown in Figure 1 uses different settings and we discuss it in Section 5.2. A more sophisticated approach could involve color transfer between source and target images or color palettes, which typically relies on image statistics to globally and locally control color distributions [START_REF] Sheikh Faridul | A survey of color mapping and its applications[END_REF]].

Edge-preserving Image Smoothing

While color manipulation might reduce the emotional impact carried by blood, it preserves the details of the original photo. A blackand-white photo, in particular, may still appear too crude. Therefore, we consider other types of filters, starting with edge-aware image smoothing as a building block for abstraction, artistic stylization and tone mapping. Numerous filter-based techniques have been proposed to approach these applications in an automated way, most of them deriving local image structures for feature-aware processing.

Bilateral filter. The bilteral filter is a popular choice to approximate an anisotropic diffusion, which works by weight averaging pixel colors in a local neighborhood based on their distances in space and range [START_REF] Tomasi | Bilateral filtering for gray and color images[END_REF]]. It weights pixels with a high difference in intensity less than a Gaussian filter to preserve image structures at a better scale. We retain it and refer to it as Bilateral. Most relevant applications apply the bilateral filter in a multi-stage process for real-time rendering with a cartoon look [START_REF] Winnemöller | Real-time video abstraction[END_REF]], and enhance it by flow-based implementations adapted to the local image structure [START_REF] Kang | Flow-based image abstraction[END_REF][START_REF] Eric | Image abstraction by structure adaptive filtering[END_REF]. Providing smooth outputs at curved boundaries of delicate structures, we thus consider the flow-based variant [START_REF] Eric | Image abstraction by structure adaptive filtering[END_REF], and name it FlowAbs. As a generalized variant, the guided filter [START_REF] He | Guided image filtering[END_REF]] may provide similar characteristics with reduced unwanted gradient reversal artifacts, but only provides a non-feature-aligned implementation.

Mean-shift.

A mean-shift is a popular approach for edge-preserving smoothing [START_REF] Comaniciu | Mean shift: A robust approach toward feature space analysis[END_REF]] and saliency-guided image abstraction [START_REF] Decarlo | Stylization and abstraction of photographs[END_REF]. It provides a nonparametric filter that estimates probability density functions by iteratively shifting color values to averaged color values of a local neighborhood. However, the approach typically produces rough boundaries that is more suited to image segmentation. Kuwahara filter. A popular approach that works accurately even with high-contrast images-contrary to the bilateral filter-and provides smoothed outputs at curved boundaries, is the Kuwahara filter [START_REF] Kuwahara | Processing of RIangiocardiographic images[END_REF]] and its generalized [START_REF] Papari | Artistic edge and corner enhancing smoothing[END_REF] and anisotropic [Kyprianidis 2011;[START_REF] Kyprianidis | Image and video abstraction by anisotropic Kuwahara filtering[END_REF] variants. The kernel of the anisotropic Kuwahara filter is divided into shapealigned overlapping subregions, where the response is defined as the mean of the subregion with minimal variance. We retain the multiscale variant [Kyprianidis 2011] and refer to it as Kuwahara. It maintains a uniform level of abstraction due to local area flattening and can scale with the image resolution.

Shock, morphological, and geodesic filters. Contrary to previous filters, additional categories weight colors across feature boundaries for higher levels of abstraction, for which we retain methods with shock filtering, i. e., in conjunction with a constrained mean curvature flow [START_REF] Kang | Shape-simplifying image abstraction[END_REF] (ShapeSimpl) and diffusion tensors for coherence-enhancing abstraction [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF]] (CoherenceEnh). Morphological filtering based on dilation and erosion, and geodesic filtering using distance transforms are also popular choices to obtain results of high abstraction [START_REF] Criminisi | Geodesic image and video editing[END_REF][START_REF] Mould | Texture-preserving abstraction[END_REF]], but were found to require local control to effectively adjust the level of abstraction.

ApparentGrey [START_REF] Smith | Apparent greyscale: A simple and fast conversion to perceptually accurate images and video[END_REF] HueShift Bilateral [START_REF] Tomasi | Bilateral filtering for gray and color images[END_REF]]

Kuwahara [Kyprianidis 2011] ShapeSimpl [START_REF] Kang | Shape-simplifying image abstraction[END_REF] CoherenceEnh [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF] XDoG [START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF] Oilpaint [START_REF] Semmo | Image stylization by interactive oil paint filtering[END_REF] Watercolor [START_REF] Bousseau | Interactive watercolor rendering with temporal coherence and abstraction[END_REF][START_REF] Wang | Towards photo watercolorization with artistic verisimilitude[END_REF] BrushStrokes [START_REF] Hertzmann | Painterly rendering with curved brush strokes of multiple sizes[END_REF]] Hatching [START_REF] Praun | Real-time hatching[END_REF] Stippling [START_REF] Martín | Example-based stippling using a scale-dependent grayscale process[END_REF][START_REF] Martín | Scaledependent and example-based stippling[END_REF] Figure 2: Image processing techniques used in the interviews with surgeons, together with FlowAbs shown in Figure 1. [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF] N = 4, σ d = 1, τ r = 0.002, σ t = 6, σ i = 0, σ д = 1.5, r = 2, τ s = 0.005, σ a = 1.5 XDoG [START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF]] σ c = 2.28, σ e = 1, σ m = 4.4, p = 99.0, φ = 100.0, ϵ = 0.65, σ a = 1.0

ρ = 2, n e = 1, n a = 3, σ d = 6, σ r = 5.25%, σ e = 1, τ = 0.99, ϵ = 0, φ e = 2, σ m = 3, q = 8, φ q = 2 Kuwahara [Kyprianidis 2011] N = 8, ρ = 2.0, r = 6, q = 8, α = 1, τ w = 0.02, p s = 0.5, p d = 1.25, τ v = 0.1 ShapeSimpl [Kang and Lee 2008] N = 8, k = 8, r = 1, etf halfw = 3, etf N = 4, σ shock = 1, τ shock = 1 CoherenceEnh [
Stylization Oilpaint [START_REF] Semmo | Image stylization by interactive oil paint filtering[END_REF] σ s = 12.0, n e = 0, σ b = 12.0, k scale = 5.0, k specular = 0.8, k shininess = 12.0 Watercolor [START_REF] Bousseau | Interactive watercolor rendering with temporal coherence and abstraction[END_REF][START_REF] Wang | Towards photo watercolorization with artistic verisimilitude[END_REF] Effects of [START_REF] Bousseau | Interactive watercolor rendering with temporal coherence and abstraction[END_REF][START_REF] Wang | Towards photo watercolorization with artistic verisimilitude[END_REF]] using FlowAbs: ρ = 2.0, σ d = 4.0, σ r = 15.00% BrushStrokes [START_REF] Hertzmann | Painterly rendering with curved brush strokes of multiple sizes[END_REF]]

T = 200, size max = 8, f c = 1, f σ = 0.5, j {h,s,v } = 0, j {r,д,b } = 0.3, f д = 1, ∼ = 0.5, length = [4, 16]
Hatching [START_REF] Praun | Real-time hatching[END_REF]] Art map of [START_REF] Praun | Real-time hatching[END_REF]] scaled at 0.6, linear mapping with luminance, edge settings of FlowAbs Stippling [START_REF] Martín | Scaledependent and example-based stippling[END_REF] r es 0 = 1200 ppi, f p = 2.0, placement randomness = 25%, distribution = normal, colors = b&w, τ = 127

Filters using global optimizations. Many filters focus on image decompositions by solving optimization problems to separate detail from base information, e. g., based on weighted least squares [START_REF] Farbman | Edgepreserving decompositions for multi-scale tone and detail manipulation[END_REF], histograms [START_REF] Kass | Smoothed local histogram filters[END_REF], and gradient minimization [START_REF] Xu | Image smoothing via L 0 gradient minimization[END_REF]]. However, they are found to have strengths in applications requiring complementary global optimizations, such as tone mapping and colorization. Moreover, they are typically not suited for interactive applications. [START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF] distinguish between gradient-based edge detection that thresholds the gradient magnitude of an image and Laplacian-based edge detection that identifies zero-crossings in the second derivative. Popular gradient-domain approaches identify image gradients with high magnitudes by using convolution filters, such as the Prewitt and Sobel filter [Pratt 2007], with subsequent thresholding of the magnitude. The approach is popular with medical images to ease object recognition, however produces results that are sensitive to noise. The Canny edge detector [START_REF] Canny | A computational approach to edge detection[END_REF]] as a multi-stage algorithm provides several enhancements by combining smoothing and differentiation operators. However, although popular with MRI and CT images, it is more directed to semantic segmentation and may produce disconnected edge segments.

Edge Detection and Enhancement

A real-time approach that is less sensitive to noise is to approximate the Laplacian of Gaussian [START_REF] Marr | Theory of edge detection[END_REF] using difference of Gaussians (DoG). The approach has shown to provide smooth edges of delicate structures, e. g., with respect to human faces [START_REF] Gooch | Human facial illustrations: Creation and psychophysical evaluation[END_REF]]. Therefore, we use the enhanced separable flow-based implementations of the DoG [START_REF] Kang | Coherent line drawing[END_REF][START_REF] Eric | Image abstraction by structure adaptive filtering[END_REF][START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF] for FlowAbs, since they are adapted to the local orientation of an input image to create smooth coherent outputs for line and curve segments. We also retain the XDoG filter [START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF]] as a generalized approach that is able to obtain two tone black-and-white images, which relates to drawings found in illustrative visualization.

Image-based Artistic Rendering

Artistic image stylization has been suggested to dampen emotional responses [START_REF] Mandryk | Evaluation of emotional response to non-photorealistic images[END_REF]]. We consider stylization techniques that simulate traditional media and painting techniques found in illustrative visualization, i.e., watercolor, oil paint, pen-and-ink, and stippling, for which we strive for classical or state-of-the-art methods that cover the taxonomy proposed by [START_REF] Kyprianidis | State of the "art": A taxonomy of artistic stylization techniques for images and video[END_REF].

Image filters are prominently used as building blocks of complex stylization effects, such as the bilateral filter and DoG to obtain toon renderings (FlowAbs), and flow-based Gaussian smoothing for more abstract filtering that simulates oil paint [START_REF] Semmo | Image stylization by interactive oil paint filtering[END_REF]] (Oilpaint). In addition, we use a Watercolor technique that simulates effects such as pigment density variation, edge darkening, wet-in-wet, and wobbling [START_REF] Bousseau | Interactive watercolor rendering with temporal coherence and abstraction[END_REF][START_REF] Wang | Towards photo watercolorization with artistic verisimilitude[END_REF]]. For stroke-based rendering, a popular method is to iteratively align brush strokes of varying color, size, and orientation according to the input image, for which we use the classical approach of [START_REF] Hertzmann | Painterly rendering with curved brush strokes of multiple sizes[END_REF] (BrushStrokes). Techniques for tonal depiction typically direct tonal art maps based on luminance, for which we use a 2D hatching implementation that borrows from [START_REF] Praun | Real-time hatching[END_REF] coupled with a DoG-based edge (Hatching). Finally, we consider the example-based stippling technique described by [START_REF] Martín | Scaledependent and example-based stippling[END_REF] (Stippling), as it is able to provide scale-dependent results.

Parameters and Configurations

The parameters and configurations summarized in Table 1 are based on reported results and presets of the respective works, and experiments with medical images at a resolution of 1024 × 768 pixels.

Filters. We set the ApparentGrey filter to use default settings with uniform spatial control of four subbands to locally adjust local chromatic contrasts. For the HueShift, we use a shift of -120.0 degrees on the hue channel mapped to the HSL color wheel. We align the Bilateral filter to obtain a soft Gaussian smoothing with a spatial distance σ d = 3.0 using additional filtering in the CIE-Lab color space using an increased weight (σ r = 4.25%). For FlowAbs, we use default parameters for edge enhancement with a doubled distance for bilateral filtering to compromise with the 512×512 pixel images used by [START_REF] Eric | Image abstraction by structure adaptive filtering[END_REF]. We set the Kuwahara filter to use a typical configuration with a radius of six pixels aligned to eight sectors, a slightly smoothed structure tensor, and multi-scale estimation [Kyprianidis 2011]. For deliberate smoothing across shape boundaries using ShapeSimpl and CoherenceEnh, we configure these filters to perform a single step of shock filtering after every iteration of mean curvature flow-i. e., four [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF]) and eight [START_REF] Kang | Shape-simplifying image abstraction[END_REF] steps in total respectively. Finally, we set the edge enhancement using XDoG to output fine coherent lines with high details and a two-tone thresholding to sparsely obtain negative edges [START_REF] Winnemöller | XDoG: An extended difference-of-Gaussians compendium including advanced image stylization[END_REF]].

Artistic Styles. Here we mainly seek to replicate the level of abstraction targeted by the respective works: For Oilpaint, smoothing parameters with a light paint texture that fall into the medium range as described by [START_REF] Semmo | Image stylization by interactive oil paint filtering[END_REF]; An implementation for Watercolor using flow-based bilateral filtering of FlowAbs but with wider filter kernels to achieve a similar level of abstraction as done by [START_REF] Bousseau | Interactive watercolor rendering with temporal coherence and abstraction[END_REF] and [START_REF] Wang | Towards photo watercolorization with artistic verisimilitude[END_REF]; The "colorist wash" preset of BrushStrokes defined by [START_REF] Hertzmann | Painterly rendering with curved brush strokes of multiple sizes[END_REF] to produce semi-transparent layered brush strokes; The default art map used by [START_REF] Praun | Real-time hatching[END_REF] that is linearly mapped to the luminance in CIE-Lab color space; And default parameters for Stippling described by [START_REF] Martín | Scaledependent and example-based stippling[END_REF] at highest resolution, with a normal distribution and black-and-white thresholding.

INTERVIEWS WITH SURGEONS

To find out which of these 13 techniques are usable in practice, we interviewed four surgeons: two otolaryngologists (respectively 13 and 35 years of experience), one orthopaedic surgeon, and one reconstructive surgeon (both 10 years of experience). Although all four surgeons are co-authors of this article, none of them was involved in the research at the time of the interviews.

We asked each surgeon to send us three of their own surgery photos that could help them explain a specific procedure to non-experts. We cropped (when necessary) and resized them to 1600 × 1200 pixels before processing them with the configurations detailed in Section 3.5. We then printed each processed image on a separate A4 sheet. At the beginning of each session, we asked the surgeon to compare and classify the processed images by making piles based on how useful the image would be as a support for communication and explanation, especially in terms of how much important information is preserved. Two images perceived as equally useful would go into the same pile. This procedure was repeated three times, once for each photo. Each surgeon saw 3 photos × 13 techniques = 39 images in addition to the 3 original photos. To limit ordering effects, we randomized the order in which processed images were presented. After the classification, we let surgeons further comment on the techniques and inquired them about possible applications.

Reported Preferences

We processed self-reported preferences as follows: first, for each combination of surgeon × photo, we assigned a number to each technique according to the pile it was in: 1 for the leftmost (most preferred) pile, 2 for the second pile, and so on. We then normalized these ranks using the halfway accumulative distribution [START_REF] Jin | A study of methods for normalizing user ratings in collaborative filtering[END_REF]. This method gives each rank a score between 0 and 1 that corrects for possible differences in the way ranks are assigned (e.g., when some surgeons make more piles than others). We then derived preference scores by inverting the normalized ranks (y = 1x). All preference scores are shown in Figure 3. Finally, we averaged preference scores across pictures and surgeons to derive a single aggregated preference score per filter. These scores are also shown in Figure 3, on top. As we can see, CoherenceEnh was the most preferred technique, followed by FlowAbs, Kuwahara and Bilateral. We discuss the surgeons' comments in the following.

Qualitative feedback

We encouraged surgeons to voice comments both during the classification and in the debriefing interview. 3 In general, they reported that some of our processed images could be used for textbooks or classes (× 2 surgeons), to communicate with patients (× 2), and surprisingly even to communicate with other experts (× 1), as "drawings can be augmented" with notes for instance. One surgeon reported they could be particularly useful for plastic surgery and that it would be interesting to see how automatic processing could help communicate with children patients, as they are more sensitive to surgery images. We now report on the comments that were made for more than one photo and/or by more than one surgeon.

To begin with, the Bilateral technique was found to be usable for patients or in a book (× 2 photos, 1 surgeon). In addition to its good ranking, the FlowAbs technique's ressemblance to cartoons or comic strips was pointed out, and with it the possibility to remove a bit of "violence" from the photo (× 2, 1). It was also praised for the high visibility it gave to contours (× 3, 2). In contrast, the Hatching technique was reported to remove too many details (× 5, 3). Similarly, while HueShift only manipulated colors, it was reported to cause loss of information (× 3, 1), and made it hard to find anatomical correspondences (× 2, 1). The Oilpaint technique generated mixed reactions. On the one hand, it was praised for it artistic look (× 3, 2) and could potentially be used in books or with patients (× 2, 1). On the other hand, it was reported to remove useful information (× 2, 2). BrushStrokes was also reported to be artistic and possibly useful with patients (× 2, 2). The same qualities were reported for the ShapeSimpl technique. Stippling was explicitly reported as not usable (× 3, 1) and causing too much loss of information (× 7, 3). The Watercolor technique also removed too many details (× 5, 2). Finally, XDoG was also found to cause too much information loss (× 2, 1) as it makes it difficult to distinguish colors and contours (× 5, 2).

3 Interview notes are available at osf.io/4pfes/.

EXPERIMENT WITH LAY PEOPLE

Our interview with surgeons helped us understand which processing techniques preserve key information from surgery photos. However, it is hard for surgeons to accurately predict the affective impact of processed photos on lay people. Thus, we conducted an experiment where we presented surgery photos to 30 participants, both unprocessed and processed, and asked them to rate them according to how repulsive they are. This experiment was approved by Inria's ethics committee (COERLE, approval number 2017-015).

Pictures

Throughout this section, we use the term picture to refer to an original photo, and the term stimulus to refer to a processed or unprocessed picture that is meant to be presented to participants. We selected five pictures from two research catalogs: the Nencki Affective Picture System (NAPS) [START_REF] Marchewka | The Nencki Affective Picture System (NAPS): Introduction to a novel, standardized, wide-range, high-quality, realistic picture database[END_REF]] and the International Affective Picture System (IAPS) [START_REF] Lang | International affective picture system (IAPS): Technical manual and affective ratings[END_REF]]. These catalogs contain a range of emotionally-evocative photos that have been validated to elicit a positive, neutral, or negative affect. We selected five surgery photos among the negative pictures:4

• People 202 (NAPS)-a leg surgery,

• People 216 (NAPS)-a leg surgery or autopsy,

• People 221 (NAPS)-a surgery in the eye area,

• 3212 (IAPS)-a surgery performed on a dog, and • 3213 (IAPS)-a finger surgery. In addition to those five surgery pictures, we selected five neutral pictures from the NAPS catalog, all with consistent ratings of 1 on the disgust scale. These photos include, e.g., a surfer riding a wave and a man walking on the beach with his son.

Picture resolution is 1600 × 1200 for NAPS and 1024 × 768 for IAPS. For consistency, we rescaled all NAPS pictures to 1024 × 768. Thus, the effect of spatial filters will be slightly stronger than for the interview sessions, which used 1600 × 1200 pictures.

Techniques

We first selected the five most usable techniques according to surgeons (see Figure 3). We observed that Kuwahara and Bilateral yielded almost identical results on our experimental stimuli, and were therefore likely to elicit the same response. Thus, in order to save experimental conditions, we decided to remove Bilateral, since it is already used as a building block of FlowAbs. We further decided to add HueShift in order to cover a broader spectrum of approaches, even though it was ranked poorly. Looking back at surgeon pictures, we realized that HueShift often gives skin and blood the same green tones. This might be the reason why surgeons found that HueShift suppressed important information. Thus, we changed the shift angle from 120°to -120°in the HSL space, which maps blood and skin to blue/purple tones where color discrimination is superior [START_REF] Bachy | Hue discrimination, unique hues and naming[END_REF]. We refer to this technique as HueShift2. Finally, by examining results on experimental stimuli, we realized that the higher number of iterations used for ShapeSimpl eliminated significantly more details than other techniques. We therefore tuned the settings in order to get more comparable levels of abstraction. We refer to this technique as ShapeSimpl2. This leaves us with 6 techniques, summarized in Table 2.

Metrics

A variety of psychophysiological measures exist to quantify emotional response (see Section 2.3), but they tend to be noisy and they require specialized equipment. We thus decided to simply measure self-reported subjective experience. Psychology has adopted standardized scales to assess affect, such as valence, arousal and disgust scales [START_REF] Lang | International affective picture system (IAPS): Technical manual and affective ratings[END_REF][START_REF] Marchewka | The Nencki Affective Picture System (NAPS): Introduction to a novel, standardized, wide-range, high-quality, realistic picture database[END_REF]]. However, the difficulty of looking at a repellent surgery image may not directly map to either valence, arousal or disgust as they are understood by participants. Therefore, we chose to ask a more direct question, i.e., "how difficult was it for you to look at this image?", on a 9-point scale from very easy to very difficult (see Figure 4). The question was framed in the past tense because, as we will see later on, participants did not see the stimulus when they answered the question.

As a complement to the expert interview, we also asked participants to estimate to what extent the content of the scene has been obfuscated by the filter. The question was "how difficult was it to recognize the image's content?", again on a 9-point scale from very easy to very difficult. The meaning of both questions was explained in preliminary instructions, with examples.

Design and Procedure

Because between-subject designs typically suffer from low statistical power [START_REF] Bellemare | Statistical power of within and between-subjects designs in economic experiments[END_REF], we used a within-subject design. Each participant saw all combinations of picture and technique. With 10 pictures (5 surgery, 5 neutral) and 7 techniques (6 + unfiltered), a total of 70 stimuli were presented to each participant.

We expected strong ordering effects, as a participant is likely to become less sensitive to surgery pictures after repeated exposure. Furthermore, a participant is more likely to recognize the content of a picture if it had been presented before unfiltered or with a weaker filter. We addressed this in four ways:

• The order of stimulus presentation was fully randomized across participants, with the constraint that two presentations of the same picture had to be separated by at least two other pictures. • Experiment instructions warned participants that they would see the same picture multiple times, but asked them to try to answer questions as if they saw each picture for the first time.

• Each stimulus was presented for two seconds only, after which a mask was displayed and the participant was invited to answer the two questions (see Figure 4). Limiting exposure to each picture was expected to slow down habituation. At the same time, pilot studies confirmed that a two-second exposure was more than enough to be able to fully scan and recognize a picture. • Surgery pictures were interleaved with neutral pictures, which we expected would further slow down habituation, and incite participants to stay focused. Since the alternance of surgery and neutral pictures was purely random, participants could not predict what would come next.

Responses to neutral images were also used to screen participants with poor data. With Likert items, a common and damaging mistake is inversion (e.g., replying "easy" instead of "hard"). Although instructions explicitly warned against this mistake, we i) considered a response of 5 or more to the first question for a neutral image as an obvious inversion, and ii) discarded the data from all participants who made two or more obvious inversions. This exclusion rule was decided before collecting the data.

The experiment unfolded as follows: first, participants were given an information sheet and a consent form to sign. 5 The information sheet warned participants about the surgery images, with an example, and asked them not to participate if they thought they were hypersensitive. It also informed them that they would be free to stop at any time, should they feel too uncomfortable. Then, participants read instructions on a computer (MacBook Pro 2015 with a 2880 × 1800 retina display and a mouse) and completed the 70 trials. Finally, they were given a research debriefing sheet, a brief questionnaire, and were invited to comment on the experiment. The entire experiment lasted between 15 and 20 minutes.

Participants

We recruited 30 unpaid participants (9 females, age 21-49, mean = 29, med = 26, SD = 8.6), in conformity with our planned sample size. One additional participant made two obvious inversions as defined in Section 5.4 and was therefore discarded from the analysis. Participants were recruited by email posting to our institution and to students we give classes to, as well as word of mouth to neighbor institutions. Four participants were left-handed and one was ambidextrous. All had normal or corrected-to-normal vision, none were colorblind and they all had higher-education diplomas. Twenty-four reported seeing surgery images or videos before (TV, internet, books), including real surgeries or traumatic injuries (× 3). Eighteen participants participated in a perception study before. Reported sensitivity to surgery images was on average 4.1 (SD = 2.1) on a scale from 1 (not sensitive at all) to 9 (extremely sensitive). 5 All documents and the experiment software are available at osf.io/4pfes.

Results

We report and interpret all our results using interval estimation instead of p-values [START_REF] Cumming | The new statistics: Why and how[END_REF][START_REF] Dragicevic | Fair statistical communication in HCI[END_REF]. All analyses were planned before collecting the data and were preregistered [START_REF] Cockburn | HARK no more: On the preregistration of CHI experiments[END_REF] with the Open Science Framework. 6We report on two dependent variables: repulsiveness, which is the response to the first question in Figure 4, and recognizability, which is the complement (y = 10-x) of the reponse to the second question. For each participant and technique, we averaged responses across all 5 surgery pictures (neutral pictures were not analyzed). Then, for each technique, we derived a point estimate using the mean response across participants, and an interval estimate using the 95% BCa bootstrap confidence interval [START_REF] Kirby | BootES: An R package for bootstrap confidence intervals on effect sizes[END_REF].

Figure 5-left shows point estimates (dots) and interval estimates (gray boxes) within the full space of possible responses. Roughly speaking, each interval indicates the range of plausible values for the population mean, with the point estimate being about seven times more plausible than the interval endpoints [START_REF] Cumming | Understanding the new statistics: Effect sizes, confidence intervals, and meta-analysis[END_REF]]. The mean response for unfiltered surgery images is about 5 on the repulsiveness scale and 8 on the recognizability scale. There is strong evidence that all 6 techniques i) yield smaller average recognizability, and ii) yield smaller repulsiveness except for Ap-parentGrey and HueShift2. Unsurprisingly, repulsiveness tends to correlate with recognizability. An ideal processing technique would be an outlier located on the top left of the regression line, but the figure provides no conclusive evidence for such an outlier.

This being a within-subject design, it is more informative to examine average within-subject reduction in repulsiveness and in recognizability [START_REF] Cumming | Understanding the new statistics: Effect sizes, confidence intervals, and meta-analysis[END_REF]], summarized in Figure 5-right. The same results are shown in Figure 6, with a separate plot for each dependent variable. There is overwhelming evidence that all techniques have a within-subject effect on repulsiveness and on recognizability, on average. In terms of repulsiveness reduction, the overlap between error bars [START_REF] Krzywinski | Points of significance: Error bars[END_REF] suggests that ApparentGrey has the weakest effect, followed by HueShift2, followed by the remaining four. In terms of recognizability reduction the results are less clear, but it is likely that ApparentGrey yields more recognizability than the other techniques.

Overall, all six techniques are effective, but color manipulation appears to be outperformed by space-domain filtering (ShapeSimpl2, CoherenceEnh, Kuwahara, FlowAbs). HueShift2, in particular, is not as effective at making the surgery pictures easier to look at, despite being comparable in terms of preserving informational content. ApparentGrey, on the other hand, simply appears to be a weaker filter: while it does not reduce repulsiveness dramatically, it better preserves image legibility. Among all techniques, FlowAbs and CoherenceEnh offer the best trade-offs if we only consider point estimates, but given the large overlaps in interval estimates the evidence is very weak.

Participant Feedback

Among the ones who offered feedback, two found FlowAbs to be the best Several participants mentioned that it looked like a cartoon or comic-strip (× 8) making surgery photos easier to look at (× 8). A single participant found that it could make it harder because of the more salient contours, while three participants said that it makes content easier to recognize. HueShift2 was said to make photos easier to look at (× 3) but the content harder to recognize (× 3). Four participants found that it made content appear "unnatural" or "disturbing", while one mentioned that it makes patients look like aliens. The ApparentGrey was deemed easier to look at (× 3) and easier to recognize (× 1), while two participants reported it was harder to recognize the content without colors. Additionally, one participant thought that the ApparentGrey's effectiveness depends on the input picture. This was also reflected by another participant who mentioned that the best technique could be picture-dependent. CoherenceEnh, Kuwahara, and ShapeSimpl2 were reported to be the best technique by one participant each, and to make content harder to recognize by three participants.

DISCUSSION AND CONCLUSIONS

In this section, we discuss our results, the limitations of our study, and conclude with future work.

Summary of findings

We found that all six tested techniques make it easier to look at surgery photos as judged by lay people, even if the effects are moderate and come at a slight cost in legibility. Combining quantitative and qualitative results from both studies, two particularly successful techniques are FlowAbs (structure-adaptive filtering [START_REF] Eric | Image abstraction by structure adaptive filtering[END_REF]) and CoherenceEnh (coherence-enhancing filtering [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF]). FlowAbs is the only technique integrating edge enhancement, which seems to provide a double benefit: it may increase legibility and recognizability by emphasizing contours, while at the same time producing a cartoon appearance that may make photos look less "real" and therefore less disturbing. Concerning CoherenceEnh and the almost equally successful Kuwahara, ShapeSimpl, and Bilateral, they all provide some form of nonlinear/coherence-enhancing diffusion [START_REF] Weickert | Coherence-enhancing diffusion filtering[END_REF] or anisotropic filtering in order to obtain a kind of a painterly look, although not as explicit as Oilpaint. This subtle painterly look may also make the photos look less real, without compromising too much in legibility. CoherenceEnh was found to be the most legible by surgeons, perhaps because compared to ShapeSimpl, for example, it preserves "the shape by using a curvature preserving smoothing method that enhances coherence" [START_REF] Eric | Image and video abstraction by coherenceenhancing filtering[END_REF] and is more resistant to noise.

The finding that color manipulation was less effective came to us as a surprise, given the emotional force blood appears to carry, and the tradition of altering its color in video games and Japanese animation. Not only HueShift was found to discard information despite the absence of spatial filtering, it was also found to be weaker at affective dampening compared to the approaches mentioned before. ApparentGrey may still have useful applications as a weak surgery filter, as it was found to be relatively usable by surgeons.

Almost all artistic stylization techniques (Watercolor, Brush-Strokes, Hatching and Stippling) were found by surgeons to discard too much information to be useful. However, Oilpaint was relatively well received, even if it did not make it in our final experiment. This, together with the fact that FlowAbs is the filter that comes closest to stylization, suggest that stylization still holds promise and deserves to be investigated further.

Limitations

There are several clear limitations to our experiment. First, our measures are self-reported and may not truly reflect participants' experiences. In particular, we cannot rule out the possibility that responses were tainted by a social desirability bias [START_REF] Fisher | Social desirability bias and the validity of indirect questioning[END_REF]] or, worse, by a good-subject effect [START_REF] Nichols | The good-subject effect: Investigating participant demand characteristics[END_REF]]. This could be improved in the future by adopting between-subject designs and/or objective measurements (Section 2.3), but it is unclear whether such methods would provide sufficient sensitivity for populations that are not hypersensitive or BII-phobic. Second, all our results only apply to the surgery pictures we tested. To draw general inferences about the effectiveness of the techniques, one would need to design a study where experimental pictures are randomly drawn from a large collection of representative surgery photos [START_REF] Judd | Treating stimuli as a random factor in social psychology: A new and comprehensive solution to a pervasive but largely ignored problem[END_REF]. A third important limitation is that we only compared techniques with specific settings, so our findings do not necessarily apply to the techniques in general. Solutions to this include testing many settings, or devising a systematic calibration method that can determine parameters ensuring fair comparisons.

Finally, we only examined 13 techniques and many others remain to be tested. More sophisticated (and possibly more effective) variants exist, especially in the area of color manipulation. Combinations of techniques also merit consideration, such as using edge enhancement from FlowAbs with other techniques, or combining color manipulation with spatial filters. As future work, we also plan to investigate how neural style transfer [START_REF] Gatys | Image style transfer using convolutional neural networks[END_REF][START_REF] Johnson | Perceptual losses for real-time style transfer and super-resolution[END_REF][START_REF] Semmo | Neural style transfer: A paradigm shift for image-based artistic rendering?[END_REF]] can be used to replicate illustrations found in medical textbooks. Such techniques can be quite promising in our context, although their outcome is still hard to control.

Other Future Work

This article is an initial investigation but opens up exciting avenues for future research. These include supporting surgery videos, other types of medical images than open surgery (e. g., skin diseases), as well as disturbing imagery outside the medical domain, such as offensive user-generated content that can psychologically impact professionals who monitor it [START_REF] Feinstein | Witnessing images of extreme violence: a psychological study of journalists in the newsroom[END_REF]. This topic also raises interesting human-computer interaction questions and opportunities for design, such as the design of adjustable picture censorship in web pages (e. g., Wikipedia) or browsers, or the emotional dampening of live scenes through a "diminished reality" paradigm [START_REF] Azuma | Recent advances in augmented reality[END_REF], which if successful could allow general audiences to witness real surgery procedures.

A pending challenge is the use of a standardized picture set for future studies. Even though developments are underway to specify benchmark pictures for expressive rendering [Mould andRosin 2016, 2017;[START_REF] Rosin | Benchmarking non-photorealistic rendering of portraits[END_REF], we had to resort to the NAPS and IAPS catalogs, yet even in these sources only few pictures met our requirements. While the selected pictures are available for future work through the named sources, it may be useful to specify a set of standard pictures to study the impact of stylization on affect.

Figure 4 :

 4 Figure 4: Experiment screen after stimulus presentation.

Figure 5 :Figure 6 :

 56 Figure 5: Mean repulsiveness and recognizability ratings for each technique (left); Mean within-subject reduction in repulsiveness and recognizability (right). Boxes are 95% CIs.

Table 1 :

 1 Overview of techniques with their correspondent parameters.

		Abbreviation	Based on works	Parameter settings
		ApparentGrey	[Smith et al. 2008]	N = 4, p i = 0.5, k i = 0.5
		HueShift	custom, see Section 3.5	α = 120.0
	Filters	Bilateral FlowAbs	[Tomasi and Manduchi 1998] [Kyprianidis and Döllner 2008]	σ d = 3.0, σ r = 4.25%

Table 2 :

 2 Techniques tested in the experiment. ShapeSimpl2 Same as Table 1 except N = 4, k = 4 CoherenceEnh Same as Table 1 Kuwahara Same as Table 1 FlowAbs Same as Table 1

	Abbreviation	Parameter settings
	ApparentGrey	Same as Table 1
	HueShift2	α = -120.0

Blood Injection and Injury phobia (BII phobia) refers to "an extreme and irrational fear of blood, injuries, or of receiving an injection or an invasive medical procedure" which affects about 3.5% of the population[Haberkamp and Schmidt

2014].

Our custom image processing software is available at https://osf.io/4pfes/ .

Usage restrictions do not allow us to reproduce or distribute the pictures. These can be obtained from the Nencki Institute of Experimental Biology and CSEA Media.

Preregistered analysis available at osf.io/34vzj.

ACKNOWLEDGMENTS

Thanks to Steve Haroz, Yvonne Jansen, Wesley Willett and Pascal Guitton for their help and feedback. This research was partly funded by the Federal Ministry of Education and Research (BMBF), Germany, for the AVA project 01IS15041B.