Grégoire Dupont De Dinechin
email: gregoire.dupontdedinechin@mines-paristech.fr

Alexis Paljic
email: alexis.paljic@mines-paristech.fr

Cinematic Virtual Reality With Motion Parallax From a Single Monoscopic Omnidirectional Image

Complementary advances in the fields of virtual reality (VR) and reality capture have led to a growing demand for VR experiences that enable users to convincingly move around in an environment created from a real-world scene. Most methods address this issue by first acquiring a large number of image samples from different viewpoints. However, this is often costly in both time and hardware requirements, and is incompatible with the growing selection of existing, casually-acquired 360degree images available online. In this paper, we present a novel solution for cinematic VR with motion parallax that instead only uses a single monoscopic omnidirectional image as input. We provide new insights on how to convert such an image into a scene mesh, and discuss potential uses of this representation. We notably propose using a VR interface to manually generate a 360-degree depth map, visualized as a 3D mesh and modified by the operator in real-time. We applied our method to different real-world scenes, and conducted a user study comparing meshes created from depth maps of different levels of accuracy. The results show that our method enables perceptually comfortable VR viewing when users move around in the scene.

I. INTRODUCTION

Using the growing online collection of 360-degree images to generate VR environments representing real-world scenes is tempting for a great number of applications, such as virtual reality tours and immersive cultural heritage experiences. These real-world-based applications are often grouped under the name cinematic VR. Such experiences are all the more convincing if they provide a sense of motion parallax, i.e. if, when users move around in the environment (translation, not just head rotation), novel viewpoints are rendered in which the different objects in the scene are shifted differently depending on their respective depths.

Representing a scene with motion parallax using a single image as input, however, is not an easy task. One notably cannot estimate depth information using standard methods based on feature detection and matching in multiple stereo pairs. Nor does one have access to color or structure information for objects occluded or not seen in the original view. A single image with a restricted field of view offers in fact very limited information on the scene.

This has led many in the literature to discard the problem of generating a scene from a single image, and focus instead on the less-constrained problem of using multiple images taken from different viewpoints. We argue, however, that the complementary development of consumer VR HMDs and low-cost 360-degree cameras breathes new life into the issue. One can now acquire images containing full 360-degree information on a scene as easily as as one would acquire a regular photo, i.e. instantaneously, with a simple click of a button. Consumers now also easily have access to immersive devices for viewing virtual environments. We believe that these developments are game-changing, and that studying how to create 3D environments for VR from a single 360-degree image is a research avenue with great potential: it would open up the possibility of viewing any such image found online in VR with full motion parallax. To our knowledge, this is a question that has not yet been addressed in the literature.

Our contributions to the problem are threefold. Firstly, we present a novel method for manually generating a depth map from a 360-degree image, a process we propose to do interactively in VR to enhance spatial visualization. The problem of recovering depth from a single image is frequently examined in the literature, yet few works study the specific case of omnidirectional images, and to our knowledge none propose to do so using a VR interface. We provide insights on the specific strengths of 360-degree images that facilitate the depth assignment process, and explain the advantages of doing so in VR. Secondly, we provide a detailed account of the strengths and limits of the monoscopic panorama plus depth representation, which creates a scene mesh from a single 360degree depth-color image pair. Few papers have examined this representation in detail despite the growing number of images acquired by consumer monoscopic 360-degree cameras, and fewer still have proposed using this representation to enable VR viewing with motion parallax. Thirdly, we conduct and discuss a user study comparing, for different real-world scenes, how different meshes (i.e. different depth maps) of the scene are perceived in terms of user comfort. The results of this study show that different levels of depth map accuracy are perceived as providing significantly different levels of comfort, and are not perceived in the same way under different viewing conditions. We conclude by discussing the implications of these results, as well as potential paths for future work.

II. RELATED WORK

A. Capturing Real-World Scenes for VR Applications

Many recent works in the field of image-based modeling and rendering have examined the question of rendering novel views from a set of image samples for viewing in VR. We

Solution for 6-DOF VR?

Depth Map Generation

Pixel Grid Meshing

User Study

Fig. 1. A summary of our approach.

discuss in the following paragraphs how our approach can be compared with these state-of-the-art methods.

A first group of methods seeks to render novel views without recovering explicit geometry for the scene. This includes light field rendering techniques, seen as having great potential for VR content creation [START_REF] Yu | A light-field journey to virtual reality[END_REF] and recently showcased in Google's Welcome to Light Fields VR experience [START_REF]Welcome to light fields on steam[END_REF]. Although they also enable convincing VR viewing with motion parallax in a given area, these works require the use of expensive camera arrays and are based on very large sets of image samples, all in which they thus differ from our approach. Other such works seek instead to generate omnistereo images and videos, presenting innovative camera rigs and novel algorithms to obtain high-quality outputs at lower costs [3][4]. Although some of these works are also based on the use of consumer hardware, they only seek to provide a sense of binocular parallax, whereas our approach aims to enable motion parallax.

Another popular group of techniques explicitly reconstructs textured 3D meshes. This includes methods referred to as photogrammetry or videogrammetry, based on structure-frommotion and multi-view stereo algorithms, and aiming to create 3D meshes or volumetric videos. Many recent works thus present the use of state-of-the-art methods to explicitly generate meshes for static scenes [START_REF] Hedman | Casual 3d photography[END_REF][6] and dynamic objects [START_REF] Orts-Escolano | Holoportation: Virtual 3d teleportation in real-time[END_REF] [START_REF] Ebner | Multiview reconstruction of dynamic real-world objects and their integration in augmented and virtual reality applications[END_REF] for VR applications. Our approach differs from these works in that we seek to use only a single image for our reconstruction, not a large dataset. Contrarily to approaches requiring the use of converging multi-camera rigs, we also only require the use of a consumer monoscopic 360-degree camera. That being said, our approach resembles these works in that we generate a textured 3D mesh as output. A strong resemblance can notably be drawn with the approach presented in [START_REF] Hedman | Casual 3d photography[END_REF], as we both use low-cost, consumer hardware to produce textured meshes of real-world scenes from recovered depth maps.

A related group of methods is concerned with representing environments by recovering a dense depth map for each input view, enabling the creation of a point cloud which can then be reprojected or used as an image warping proxy to generate novel views. The method presented in [START_REF] Huang | 6-dof vr videos with a single 360-camera[END_REF] thus renders VR video with motion parallax using a single moving consumer monoscopic omnidirectional camera. A similar lowcost approach is presented in [START_REF] Pathak | Virtual reality with motion parallax by dense optical flow-based depth generation from two spherical images[END_REF], based on computing optical flow between two displaced monoscopic 360-degree cameras to enable VR motion parallax in a small zone. Our work resembles these approaches by the use of consumer hardware and recovered depth information to enable VR viewing with motion parallax. However, we do not rely on multiple views of the same scene, nor do we use image warping to produce novel views.

A final set of papers we believe is important to cite as a related work is the work of J. Thatte, concerned with developing novel scene representations that enable motion parallax in a given area with only a small number of omnistereo panoramas plus depth [START_REF] Thatte | Depth augmented stereo panorama for cinematic virtual reality with head-motion parallax[END_REF] [START_REF] Thatte | Stacked omnistereo for virtual reality with six degrees of freedom[END_REF]. Although these works focus on reducing visual artifacts, whereas our constraint is using only a single image, our approaches converge in seeking to enable VR viewing with motion parallax using an omnidirectional color-depth image pair. Moreover, [START_REF] Thatte | Depth augmented stereo panorama for cinematic virtual reality with head-motion parallax[END_REF] presents an interesting comparison with the monoscopic panorama plus depth representation, a representation we expand on in this paper.

B. Manual 2D-to-3D Conversion

Another related field of work is concerned with developing methods that leverage human perception of monocular depth cues to add depth to existing color images. The typical manual workflow consists in three steps: segmentation/rotoscoping, depth assignment, and inpainting [START_REF] Smolic | Three-dimensional video postproduction and processing[END_REF]. Each step can be automated to some degree and facilitated by the user interface, yet by definition leverages human input at some point, often for reasons of accuracy. Our approach recovers depth based on such a manual pipeline, with the particularity of using a VR user interface for interactive real-time 3D visualization. We believe that such a user interface is particularly adapted for the 2D-to-3D conversion of omnidirectional images.

Note that many recent works instead present semi-automatic methods, typically using human input in the form of sparse scribbles on a color image [START_REF] Phan | Robust semi-automatic depth map generation in unconstrained images and video sequences for 2d to stereoscopic 3d conversion[END_REF][15] or a pre-segmentation of said image [START_REF] Zeng | Hallucinating stereoscopy from a single image[END_REF] to help guide depth generation algorithms. Our approach may be augmented to integrate such methods in future work, if they prove to generate sufficiently convincing results for viewing in VR. The same can be said for methods that generate depth maps using deep convolutional neural networks [START_REF] Liu | Learning depth from single monocular images using deep convolutional neural fields[END_REF] [START_REF] Cao | Estimating depth from monocular images as classification using deep fully convolutional residual networks[END_REF], which may be used to complement our approach provided the necessary datasets are available.

III. OUR APPROACH

A. Enabling Motion Parallax in VR from a Single Image

The standard solution for displaying a single 360-degree image in a VR HMD is to project the image on an inside-facing sphere of large radius surrounding the user (we refer to this as the standard spherical representation). A drawback of this method is that it fails to provide a sense of motion parallax. This causes an uncomfortable sensation when users perform translations, due to the incoherence between the respective stimuli received by the visual and vestibular systems.

In this paper, we propose to solve this problem by generating novel information in the form of a dense depth map for the image, i.e. a per-pixel encoding of the distance between elements in the scene and the central viewpoint. The meshed model created from the obtained depth-color image pair is referred to as the monoscopic panorama plus depth representation. It can be seen as an extension of the standard spherical representation, where the uniform depth map is replaced by a more accurate one, deforming the sphere in a radial fashion. This paper intends to show the relevance of creating such a mesh from a single 360-degree image, in the sense that this process (1) requires a single image as input, (2) entails little to no additional cost, and (3) creates a representation significantly more comfortable than the standard spherical alternative.

B. Depth Map Generation With Human Input

For the representation to be relevant, one must therefore be able to generate perceptually convincing depth maps rapidly, at a low cost, and based solely on a single omnidirectional image. Our approach aims to solve that problem by leveraging the strengths of both 360-degree panoramas and VR interfaces.

An efficient pipeline can indeed be defined for any 360degree image containing a planar surface of easily-estimated depth, e.g. the ground. Namely, one can take the following steps (visualized in Figure 2):

1) The operator defines a maximum depth, used to initialize a uniform depth map. This creates a sphere in 3D space.

2) The operator estimates the distance between ground and acquisition camera center. The bottom of the sphere is horizontally flattened at the corresponding depth.

3) The operator enters VR. The sphere created in (1) and

(2) is visualized as a static 3D mesh, around which the operator can move. On the wall of the virtual room is also an image representation of the depth map. 4) Using the controllers, the operator selects an object in the image (segmented beforehand) to enhance in depth.

The operator determines where the object is in 3D space, by looking at the intersection in the image of the object and the ground (placed in 3D space in (2)). 5) Using the controllers, the operator takes a 3D primitive in hand. The operator places the primitive inside the sphere at the position determined in (4). Upon release of the operator's hand, the program computes the projection of the segmented part of sphere onto the primitive. The depth map and 3D visualization (deformed sphere) are updated in real-time. 6) The operator iterates steps (4)-( 5) for all objects segmented in the image. The operator then exits VR and saves the output depth map. Note that our implementation projects the image based on computed in-engine depth for each pixel, without the need for a parameterization of the projection primitives. We can thus estimate every object in the scene using planes (billboard representation), but can also use any other primitive, and even 3D models if need be. For simplicity, the pre-segmentation part of the process was not done in VR, but using consumer image editing software, as it would have been time-consuming to obtain similar results in our prototype.

Also note that one simply has to provide a maximum depth (in step ( 1)) and an estimated distance from the ground (in step ( 2)) to subsequently obtain metric position information for all objects in the scene, provided they are connected to the ground (at least transitively, i.e. provided an unbroken path to the ground can be drawn in the image).

The choice of implementing this process as a VR application allows for a very intuitive positioning of primitives in 3D space. Using our implementation, an operator simply has to select a segment of the image, choose a projection primitive, and manually place the primitive in 3D space at the estimated position for the real-world object. Since the operator can see the applied changes in real-time and in VR, and given that the underlying goal is to use the reconstructed environment for viewing in VR, this creation tool thus generates a what-yousee-is-what-you-get output, which we believe is particularly relevant. The operator can also choose to activate an inside view of the scene for additional precision.

As an output of this depth generation step, we thus obtain an omnidirectional depth map corresponding to the input color image. With our implementation, this comes at the cost of only about twenty minutes for an experienced operator on a moderately complex scene. This paper does not aim to evaluate our particular prototype implementation, which was developed only as an illustration of the potential of these methods and for the purpose of the subsequent user study. As discussed in Section II-B, semi-automatic approaches are likely to be quicker, and may be examined in future work. We do however also encourage the development of novel tools centered on the specificities of omnidirectional images and VR interfaces described above, and hope that our insights on the subject can help developers adapt their tools when relevant.

We now give more details on how we transform the obtained color-depth image pair into a textured mesh, and outline the strengths and limits of this representation.

C. The Monoscopic Panorama Plus Depth Representation

Monoscopic panoramas plus depth are very practical on multiple levels. Concerning data storage and manipulation, they are composed of images, a format which is compact, easy to modify using consumer-available editing tools, and already well-integrated as a data format in modern game engines. As for acquisition, monoscopic color panoramas are easy to acquire at a low cost using modern consumer pocket-sized cameras. As shown in Section III-B, the complementary perpixel depth information can then be generated post-acquisition with relative ease. Another strength of this representation is that its conversion to the 3D space is conceptually simple. In the equirectangular format for example, an omnidirectional depth map can be interpreted directly as a 3D point cloud, where each (w, h) pixel (0 ≤ w, h ≤ 1) of 8-bit value v in the depth map (where v encodes the real-world depth d) is a 3D point defined by the spherical coordinates {d, 2πw, πh}. Note that if the depth map contains only 256 values, a staircase effect may appear, very visible in VR. In our implementation, we thus encoded the depth maps such that d = R + 0.01G + 0.0001B (in meters). The colored maps shown in this paper, where red is close and blue is far, are only used for better visual representation, not mesh creation.

Going even further, monoscopic panoramas plus depth provide a natural mesh for the point cloud, based on the pixel grid. Indeed, one can intuitively form a 3D mesh by defining each pixel in the image as a mesh vertex and each quad of pixels as a pair of mesh triangles. Projecting each vertex into 3D space can then be done efficiently on GPU using vertex shaders. Mesh deformation can thus be done in real-time, meaning that movement in the scene can be accounted for without the need for any animation software, and that our approach can immediately be transposed to video panoramas. Note that the large number of vertices (e.g. 7372800 for a 3840x1920 panorama) may require building multiple meshes, but can easily be handled by standard game engines provided one has the modern hardware required to run a roomscale VR head-mounted display (e.g. in our case, HTC Vive in Unity with NVIDIA GeForce GTX 1070 GPU), and thus does not hinder real-time performance (e.g. above 120 FPS with our implementation).

A consequence of this is that monoscopic panoramas plus depth easily enable user interaction, in particular moving around in the scene. Indeed, since rendering is done using the conventional graphics pipeline, users can move in this virtual environment exactly as they would in a computer-generated one, within the boundaries defined by the maximum depth encoded in the depth map. Moreover, real-time interactive elements such as shading from virtual objects and collision handling can immediately be accounted for (see Figure 3). This means that this representation has strong potential for interactive cinematic VR experiences.

D. Visual Artifacts: a Potential Source of Discomfort

Why are monoscopic panoramas plus depth not more popular then? A blatant weakness of this representation is the large amount of visual artifacts it produces at the edges of occluding objects. Indeed, since all the available color information is contained in the single monoscopic panorama, no hole-filling step can be performed to recover occluded objects. One therefore has to make do with the interpolation naturally performed by the fragment shader between distant vertices, causing a stretching effect. This effect can be observed in other recent works demonstrating depth-based mesh reconstruction [START_REF] Hedman | Casual 3d photography[END_REF], and was referred to in the seminal work of T. Kanade as phantom surfaces [START_REF] Kanade | Virtualized reality: Constructing virtual worlds from real scenes[END_REF]. This representation therefore does not provide the most visually faithful representation of the scene, and gains in relevance only when one wishes to use a single image as input: typical use cases could be immersing friends in a 360-degree photo casually taken during a tourist trip (e.g. oneself in front of a historic monument), or visualizing an omnidirectional image found online in a VR HMD. Those are the typical use cases we target in this paper.

In order to determine to what extent our method is able to enhance users' sense of comfort despite the presence of visual artifacts, we thus led a user study, measuring reactions to different levels of depth map accuracy under different viewing conditions.

IV. USER STUDY

A. Independent and Dependent Variables

A first independent variable for the study was the type of scene representation, i.e. the type of depth map used to represent the scene. Four levels of this variable were considered:

• SCAN: the omnidirectional color image and depth map were acquired by LiDAR; this served as a ground truth reference (ideal depth map), and provided the color images for the other representations.

• NONE: no depth was recovered; the depth map was uniform at 10m, creating a standard spherical projection.

• MMIN: depth was added manually but without the timeexpensive segmentation step; the depth map thus accounted only for the walls and ground in the scene.

• MMAX: a segmentation step was performed before the manual depth assignment step; the depth map thus accounted for all close objects in the scene, including walls and ground. A second independent variable was the viewing condition, which we separated into two levels, VIEW and MOVE. This variable was chosen as a between-subjects factor to avoid a learning effect due to users understanding the difference between the distinct scene representations. Two user groups were thus formed before the study. The VIEW group conducted the experiment sitting on a rotating chair, able only to rotate body and head, without translation. The MOVE group on the other hand conducted the experiment with room-scale movement enabled, being encouraged to naturally walk around the 2.5x2.5m testing space.

As a dependent variable, we chose to measure users' sense of comfort on a 5-point Likert scale. Users were immersed in a scene representation, tasked either with observing or moving in the space, and asked to describe how comfortable they felt whenever ready to do so. Users were told that 1 corresponded to feeling very uncomfortable / disturbed by the representation, and 5 to feeling very comfortable / able to spend much more time within it. Note that our user study thus derives results based on user responses to a subjective Likert-scale question, and is therefore to be interpreted as seeking to determine general trends, not individually meaningful values [START_REF] Slater | The use of questionnaire data in presence studies: do not seriously likert[END_REF].

B. Hypotheses

Our hypotheses for the experiment were as follows:

• Hypothesis 1 (H1): In the room-scale viewing condition, users will feel most comfortable when there is added depth (depth-augmented representations). Comfort is likely to be linked to how close the depth map is to the ground truth depth map, and the ground truth is likely to have the highest comfort scores.

• Hypothesis 2 (H2): In the seated viewing condition, users will feel most comfortable when there is no added depth (spherical representation). Indeed, seated users are likely to notice depth-induced visual artifacts, but not the absence of motion parallax.

C. Experimental Setup

The user study was conducted on 25 participants (6 female). The median age was 25 (standard deviation: 3.36).

Four different omnidirectional images were used. These images were captured using a FARO laser scanner, to obtain the ground truth depth maps. The captured images had a resolution of 8192x3414 representing a 360x150 field-of-view, which we padded with black pixels at the nadir to obtain 8192x4096 equirectangular images representing a 360x180 field-of-view. The scenes were chosen to be diverse in terms of location, size and illumination: one was taken in a sunlit garden with benches, one in a snow-covered garden with a table, one in a small room with a person and two bookshelves, and one in a museum room filled with various minerals.

The images were depth-augmented by the experimenter in a pre-processing phase. For simplicity, depth assignment was done by projecting exclusively on plane primitives. Two depth maps were thus manually created for each of the four image, in addition to the ground truth and uniform depth maps. Each participant therefore gave comfort scores to 4 * 4 = 16 different environments. The order in which the environments were presented to users was randomized, to prevent biases (due to user fatigue for example). In total, 16 * 25 = 400 observations were thus recorded. Of the 25 participants, 13 were tasked with moving around the room, and 12 with observing from a seated position. 208 observations were thus recorded in the MOVE condition and 192 in the VIEW condition. The experiment lasted on average 463 seconds, i.e. 7-8 minutes.

D. Results

Our results were obtained by conducting an ANOVA on the comfort scores given by the participants, with scene representation as a within-subjects factor and viewing condition as a between-subjects factor. Statistical testing was computed in R.

Interaction between viewing condition and scene representation was found to be a significant factor (p < 0.001). We refined this result by conducting Tukey's HSD test to determine significantly different means for comfort scores under the different conditions. In this way, under the MOVE viewing condition, user appreciation was found to be significantly different for the scene representations NONE and SCAN (p < 0.01) and NONE and MMAX (p < 0.01). Additionally, the NONE scene representation was appreciated in a significantly different manner based on whether the viewing condition was VIEW or MOVE (p < 0.05). No other result was found to be of statistical significance. Means and 95% confidence intervals for the comfort scores can be found in Table I.

E. Hypothesis Validation

H1 was proven for the SCAN and MMAX depth-augmented representations, as compared to the NONE spherical representation. Although the same cannot be said for MMIN, note that it also benefited from higher comfort scores than NONE in our sample group. These results validate the idea that the monoscopic panorama plus depth representation is a relevant choice when users are encouraged to perform room-scale movement. They are also consistent with the idea that user comfort is increased as the depth map is refined to grow closer to the ground truth (although this statement cannot be validated statistically beyond our sample group).

H2 cannot be confirmed with this user study, since no statistical significance was established between scene representations under the VIEW condition. That being said, the spherical representation was established to be significantly more appreciated under this condition than when users could perform full room-scale movement. In a non-statistically significant manner, this representation also obtained the highest comfort score in our sample user group. The H2 hypothesis is therefore consistent with the results of our user study.

V. DISCUSSION

A. Viewing Conditions

Under the constraint of creating an environment from a single omnidirectional image, these results validate the use of monoscopic panoramas plus depth for applications where users are to move around in the scene. The output scene representation enables perceptually comfortable viewing, with increasing levels of comfort as the depth map grows increasingly accurate. Depth-augmented meshes are preferred in that sense to the standard spherical representation.

However, our results encourage us to recommend to content creators not to use this representation when the VR application does not require enabling more than just head rotation. Indeed, depth assignment remains an operation with non-null cost, and our user study fails to show that it improves user comfort when users cannot translate in the scene. In fact, users in our sample group preferred when no depth was added, since in that case the lack of motion parallax was less a factor of discomfort than the presence of visual artifacts. The standard spherical projection may therefore be more relevant if the environment is to be viewed in a seated position or without positional tracking.

B. Depth Map Accuracy

If the monoscopic panorama plus depth representation is used, we recommend performing a segmentation step before manually assigning depth. Indeed, estimating depth for only the walls and ground seems to enhance user comfort much less than if other objects are depth-augmented as well. The cubic representation for a walled room for example is therefore probably not the best solution if there are also other visible objects in the scene. On the other hand, depth-enhancing segmented elements seems to produce results perceptually close to those obtained when working with the ground truth. Our manual depth maps, which were created using only planes as the projection primitives, thus obtained comfort scores similar to those given to the ground truth depth map. This validates the idea of generating depth in a post-acquisition step: precise depth values (i.e. close to the ground truth) are not required to create a comfortable VR experience.

C. Extending Our Approach

A first possible extension of this paper could consist in exploring semi-automatic depth assignment methods. Deep convolutional neural networks notably have been a popular approach in past years, yielding good results when trained correctly on the right datasets. Implementing these methods could thus be interesting, provided we have access to large datasets of 360-degree depth-color image pairs for different types of scenes. Whether or not depth maps generated by such methods are convincing during VR viewing is to be studied.

A second path for future work could be extension to video. If one moves the monoscopic 360-degree camera during filming, approaches such as [START_REF] Huang | 6-dof vr videos with a single 360-camera[END_REF] offer hole-filling solutions, and are thus to be preferred. If the video is taken with a fixed camera however, the constraints are similar to those considered in this paper, in which case the approach presented here is relevant as well. Extending our current approach and implementation for video seems quite feasible, e.g. by replacing segmentation with rotoscoping, and by interpolating the depth assignment step between a set of keyframes.

VI. CONCLUSION

In this paper, we presented a low-cost approach enabling cinematic VR experiences with motion parallax from a single monoscopic omnidirectional image. Specifically, we presented a method enabling the generation of a 360-degree depth map by projection on 3D primitives manually placed in VR by a human operator, and demonstrated the generation of a scene mesh from the omnidirectional color-depth image pair. We validated the strength of this scene representation in terms of viewing comfort by means of a comparative user study. We believe our approach can be used by content creators of all kinds to enhance novel or existing omnidirectional images for viewing in virtual reality with motion parallax.

Fig. 2 .

 2 Fig. 2. Main steps of the manual creation process, demonstrated here using our prototype implementation. The numbering corresponds to the steps described in the text of Section III-B.

Fig. 3 .

 3 Fig. 3. The monoscopic panorama plus depth mesh immediately enables effects such as collision handling and shading.

Fig. 4 .

 4 Fig. 4. The four different levels of depth map accuracy, defining four levels of scene representation. From left to right, top to bottom: NONE, MMIN, MMAX, SCAN. Note that the ground truth depth map was vertically filled where the laser scanner was unable to acquire depth values.

Fig. 5 .

 5 Fig. 5. The four environments used in the user study. From left to right, top to bottom: Garden, Bookshelves, Snow, Museum. Each environment was depth-augmented using four different depth maps.

TABLE I CONFIDENCE

 I INTERVALS -COMFORT SCORES

	Viewing	Depth	Mean	CI (95%)
	MOVE	SCAN	3.48	[3.26; 3.70]
	MOVE	MMAX	3.42	[3.15; 3.70]
	MOVE	MMIN	2.94	[2.66; 3.22]
	MOVE	NONE	2.58	[2.24; 2.92]
	VIEW	SCAN	3.23	[2.89; 3.56]
	VIEW	MMAX	3.00	[2.68; 3.32]
	VIEW	MMIN	3.42	[3.14; 3.70]
	VIEW	NONE	3.52	[3.24; 3.80]


Maxime Leroy

Jérôme Sawtschuk
email: jerome.sawtschuk@univ-brest.fr

Frédéric Bioret

Sébastien Gallet

Maxime Le Roy

Toward a

Keywords: goals, maritime clifftop, restoration methods, restoration monitoring, social-ecological systems, stakeholders

come

Introduction

In 2004, the Society for Ecological Restoration defined the characteristics of restored ecosystems for their potential use as objectives in restoration plans. In most cases, restoration projects aim to reach a reference state or to restore indigenous species, and objectives are defined on the basis of an ecosystem structure and composition. Goals are also commonly related to ecosystem function and social values [START_REF] Hallett | Do we practice what we preach? Goal setting for ecological restoration[END_REF]). However, it appears that this planning phase is often underestimated during restoration projects [START_REF] Clewell | Motivations for the restoration of ecosystems[END_REF].

In terms of achieving restoration goals, passive and active restoration methods are the two possible options in a restoration project (McIver & Starr 2001). The first uses the resilience capacity of an ecosystem where restoration action is reduced to the limitation or removal of a feature that is preventing natural restoration (McIver & Starr 2001), such as the use of fencing to limit the impacts of human trampling. The second is achieved through technical measures that improve or replace natural processes [START_REF] Prach | Spontaneous succession versus technical reclamation in the restoration of disturbed sites[END_REF]. Active restoration can be implemented through management methods (e.g. mowing), Author contributions: MLR, JS conceived and wrote the manuscript; FB, SG provided feedback to improve different versions of the manuscript; all authors edited the manuscript.

A c c e p t e d m a n u s c r i t p t ecological engineering methods (e.g. soil removal), or civil engineering methods (e.g. building removal).

Ecological issues are not the only factors that should be considered. Restoration sites should also be considered as social-ecological systems [START_REF] Rissman | Where are ecology and biodiversity in social-ecological systems research? A review of research methods and applied recommendations[END_REF]. In other words, it is important to take into account both the social and ecological issues and the interactions between them [START_REF] Hobbs | Setting effective and realistic restoration goals: key directions for research[END_REF][START_REF] Gunderson | Assessing resilience in social-ecological systems: workbook for practitioners[END_REF]) in order to understand all the issues related to a site [START_REF] Lowe | Ecology and the social sciences[END_REF][START_REF] Wyborn | Negotiating multiple motivations in the science and practice of ecological restoration[END_REF]. Many studies incorporating the social aspect have done so from the perspective of ecosystem functions restoration [START_REF] Rumps | Stream restoration in the Pacific Northwest: analysis of interviews with project managers[END_REF]Fernández-Manjarrés et al. 2018), which does not take into consideration all the social aspects necessary for restoration success. For instance, the requirement for social acceptance might be met through the use of effective communication [START_REF] Metcalf | The role of trust in restoration success: public engagement and temporal and spatial scale in a complex social-ecological system[END_REF]. A good understanding of each stakeholder's goals and values is also essential [START_REF] Guerrero | Using structured decision-making to set restoration objectives when multiple values and preferences exist[END_REF]) when planning restoration projects. More broadly, human goals and values might also be considered as part of an ecosystem [START_REF] Wiens | Integrating conservation and restoration in a changing world[END_REF] when planning restoration projects. However, it is still rare to see all these issues taken into account in restoration contexts [START_REF] Perring | Advances in restoration ecology: rising to the challenges of the coming decades[END_REF] due to a lack of consideration of social issues [START_REF] Aronson | Are socioeconomic benefits of restoration adequately quantified? A meta-analysis of recent papers (2000-2008) in restoration ecology and 12 other scientific journals[END_REF].

This article focuses on the specific case of the maritime clifftops in Brittany, France, which have been followed by scientists for the past three decades [START_REF] Bioret | Contribution à l'étude de la flore et de la végétation de quelques îles et archipels ouest et sud armoricains[END_REF][START_REF] Bioret | Fréquentation humaine et protection des espaces naturels littoraux: un exemple de diagnostic en vue d'une stratégie d'aménagement intégrée[END_REF][START_REF] Gallet | Responses of three heathland shrubs to single or repeated experimental trampling[END_REF][START_REF] Gallet | Quelles méthodes pour le suivi et l'évaluation des opérations de restauration écologique? Exemple de la Côte Sauvage de Quiberon[END_REF][START_REF] Sawtschuk | Restauration écologique des pelouses et des landes des falaises littorales atlantiques: analyse des trajectoires successionnelles en environnement contraint[END_REF][START_REF] Sawtschuk | Spontaneous succession as a restoration tool for maritime cliff-top vegetation in Brittany, France[END_REF][START_REF] Gallet | La restauration des végétations des hauts de falaises du littoral atlantique, vers une évaluation globale[END_REF][START_REF] Sawtschuk | Evaluation of the most common engineering methods for maritime cliff-top vegetation restoration[END_REF][START_REF] Sawtschuk | Évaluation écologique de la restauration et de la gestion des végétations littorales sur le territoire de Lorient Agglomération[END_REF]. These clifftops present particular conditions for flora and fauna due to their strong environmental stresses, mainly involving wind and salt deposition [START_REF] Malloch | Salt-spray deposition on the maritime cliffs of the Lizard Peninsula[END_REF][START_REF] Sawtschuk | Restauration écologique des pelouses et des landes des falaises littorales atlantiques: analyse des trajectoires successionnelles en environnement contraint[END_REF]. The unusual conditions have led to the establishment of maritime grasslands (e.g. characterized by Festuca rubra ssp. pruinosa) and coastal heathlands (e.g. characterized by Erica cinerea, Ulex gallii var. humilis), both of which are protected by European legislation. These cliffs are also important for maritime bird conservation because of the nesting sites they offer (Le [START_REF] Corre | Le dérangement de l'avifaune sur les sites naturels protégés de Bretagne: état des lieux, enjeux et réflexions autour d'un outil d'étude des interactions hommes/oiseaux[END_REF].

At some sites, these ecological stakes can be threatened by the colonization of invasive vegetal species (e.g. Carpobrotus edulis) [START_REF] Sawtschuk | Evaluation of the most common engineering methods for maritime cliff-top vegetation restoration[END_REF], which can compete with the native vegetation (D 'Antonio & Meyerson 2002). However, the main threat to this system comes from human practices, particularly those related to tourism. Tourism activities can develop out of different motivations [START_REF] Arnegger | Toward a product-based typology for nature-based tourism: a conceptual framework[END_REF], such as hedonism or sport and adventure practices. All of these practices have led to the creation of a dense network of paths in the clifftop vegetation (Fig. 1). Hedonistic tourists were present, mainly during summer, in all the sites studied [START_REF] Gallet | Responses of three heathland shrubs to single or repeated experimental trampling[END_REF]. Some maritime cliffs, like the Pointe du Raz and the Cap Fréhel (Le Fur 2013), can attract more than 1 million tourists annually. In addition to these regular visitors, nautical events may increase visitor numbers even further (e.g. 50,000 visitors, in 1 day, to the Cap Fréhel every 4 years for the departure of a popular boat race). This high trampling pressure affects both vegetation composition and soil structure [START_REF] Liddle | A selective review of the ecological effects of human trampling on natural ecosystems[END_REF]) over areas up to 100 ha. Furthermore, to support demand from these high visitor numbers and generate income, the construction of hotels, restaurants, and shops (Le Fur 2013) has caused the complete destruction of vegetation and soil [START_REF] Gallet | Resistance of Atlantic Heathlands to trampling in Brittany (France): influence of vegetation type, season and weather conditions[END_REF].

The first restoration projects were set up in the 1980s in response to such cases of vegetation degradation. Since then, however, there has been no review of maritime clifftop restoration practices such as those that exist for other types of ecosystems, such as peatlands [START_REF] Andersen | An overview of the progress and challenges of peatland restoration in Western Europe[END_REF][START_REF] Chimner | An overview of peatland restoration in North America: where are we after 25 years?[END_REF], forests [START_REF] Halme | Challenges of ecological restoration: lessons from forests in northern Europe[END_REF], and rivers [START_REF] Kondolf | Two decades of river restoration in California: what can we learn?[END_REF]). These reviews are essential to focus future research, allowing the most effective restoration strategies to be applied in future projects.

In this study, we considered maritime clifftops to be social-ecological systems in which the natural ecosystem and human activities coexist. We identified the main developments in maritime clifftop restoration over the past 30 years. The social (e.g. stakeholder interactions, knowledge sharing) and ecological (e.g. the restoration and monitoring methods used) aspects were compared with current knowledge and theory in the field of restoration ecology. Our aim was to describe the current state of maritime clifftop restoration in order to highlight its main strengths and weaknesses. We thus focused on three research questions pertaining to these restorations: (1) are both social and ecological issues considered in restoration projects? (2) Are we able to assess the effectiveness of restoration methods? (3) Has the restoration ecology knowledge produced in our early projects been used for more recent ecological restoration projects?

Methods

Study Sites

The study was based on 36 maritime clifftop sites along Brittany's coastline (western France) that either had been or were currently subject to a restoration project (Fig. 2). Heathlands (dominated by dwarf shrubs) and maritime grasslands (dominated by Festuca rubra ssp. pruinosa) were the typical vegetation on the sites we studied. We considered a "site" to be one geographical unit that had been restored through at least one restoration project and a "restoration project" to be composed of one or several restoration methods performed on the same site over a particular period (i.e. <1 year). A single restoration program (e.g. research program, funding program) could present several ecological restoration projects that had been adapted to different degradation levels or different ecosystems. Prior to restoration, these sites had all been impacted by human trampling and/or infrastructure creation.

Data Collection

Data collection was based on an inventory compiled in 2007 (Lebras 2007) and 2016 of maritime clifftop restoration projects. First, we identified the restoration sites through a survey of a large variety of stakeholders and a review of the gray literature. We then met with the different project managers to gain a better understanding of their site characteristics (e.g. ecology, landowner, site manager, protection statutes) and the restoration and assessment methods used. From this inventory, we identified and contacted the managers and scientists involved in ecological monitoring (i.e. covering at least one monitoring plot that was inventoried with the same protocol for at least 2 years). We asked them to locate monitoring plots, and we collected raw data from these plots. All datasets were compiled in a geographic information system using QGIS 2.8.8 Wien. Three layers were created: the "site" layer, which described all the restored sites in terms of their different attributes (e.g. landowner, manager, and vegetation type); the "project" layer, which described all the restoration projects conducted (e.g. funding, implementation date, and restoration methods); and the "monitoring" layer, which described all the inventoried monitoring plots (e.g. location, monitoring method, and author).

Recorded, semistructured interviews, which are part of a larger ongoing study, were conducted with 19 restoration stakeholders who were identified with the aim of covering as many profiles (five landowners, three scientists, four managers, five Natura 2000 coordinators, one naturalist, and one nature conservation officer) and as many restoration contexts as possible (with/without scientific involvement, initiated and managed by different entities, different localities, with/without known monitoring). We asked open-ended questions (see Appendix S1, Supporting Information) to allow discussion of any relevant topic. We focused on the interviewee's (1) restoration goal; (2) technical choices (restoration and monitoring methods); and (3) involvement in the project.

Data Analysis

For this database, we summarized the human uses of the clifftops, the number of restoration projects, the restoration and monitoring methods used in every 5-year period between 1980 and 2015, the number of new monitoring plots per year, and the number of different types of landowners (e.g. private, conservation agency) and site managers (local authority, nonprofit organization).

Data on restoration goals and assessment methods were extracted from the interviews. The goals were classified into five categories: "habitat restoration," "habitat conservation," "social goal," "landscape goal," or "species-scale goal." For each goal, A c c e p t e d m a n u s c r i t p t we indicated whether a clear objective (e.g. restoring a specific species composition, maintaining low vegetation) or only guidelines (e.g. restoring heathland, improving landscape quality) had been set out. The different ways in which the interviewees spoke about restoration assessment were also classified into three categories: "visual assessment" (i.e. expert opinion), assessment based on photographs, and scientific assessment (i.e. based on monitoring).

Results

Three Decades of Active and Passive Maritime Clifftop Restoration

The first restoration projects were implemented at the beginning of the 1980s on sites such as La Pointe du Grouin and Cap d'Erquy in northern Brittany. Since then, at least 36 sites have been restored through one or several restoration projects (Fig. 2). Funding has come from a wide range of public bodies (e.g. European Union, local government organizations).

The types of stakeholders on the sites varied, and often multiple stakeholders were involved with one site. Of the 36 sites identified, 14 were owned or part-owned by their respective town councils and France's coastal conservation agency (Conservatoire du Littoral). The remaining ownership was made up of county (départemental) councils, private buyers, and the French navy, owning or part-owning 13, 8, and 4 sites, respectively. The managers responsible for the restoration projects were either local government organizations (31 sites) or nonprofit organizations (5 sites). External stakeholders, such as researchers and consultancies, were also occasionally involved in restoration projects.

Habitat restoration and landscape improvement were the goals most frequently cited by our interviewees. Half of the stakeholders wished to restore habitat but did not specify which types of habitat. For example, very few interviewees mentioned the specific type of heathland they wished to restore or a precise reference habitat in the local area. These goals were usually linked to the desire to restore a landscape's aesthetic value through the revegetation of degraded areas. While a quarter of the interviewees gave precise objectives for improving landscape quality (e.g. keeping heathland low, reducing path widths), another quarter just wanted to improve the landscape's aesthetic value, mainly for hedonistic tourism. This goal of improving the tourist's experience, although not always mentioned during our interviews with the stakeholders, was very common. Indeed, 29 of the 36 restored sites were managed by the installation of fencing (29 sites) and the creation of car parks (7 sites) for this very purpose. Only a few interviewees mentioned that they aimed to remove invasive species or "weeds."

While restoration projects also had to be managed for local activities like rock climbing and fishing (activities that spread out across the whole maritime cliff area), such activities were mentioned by less than a quarter of the stakeholders. Hiking was an omnipresent activity on the sites we studied and was integrated into restoration plans in the same way that hedonistic tourism was. Rock climbing was practiced in at least a quarter of the restoration sites. The sites generally contained fewer than 50 climbing routes, although the most famous climbing site, La Pointe de Pen-Hir, had over 130. Amateur fishing was practiced on more than half of the restoration sites. Because of the lack of any controlling structure, we were not able to estimate the exact number of fishers on these sites but they did not exceed a few dozen per day per site.

Pedestrian and vehicular reorganizations were not readily accepted by the public. For example, some climbers and fishers were still trespassing on the newly restricted areas but this was tolerated by the managers, who were not acting to enforce the ban. More forceful protests at the new layouts were evidenced in the sabotaged barriers and fences on at least seven different sites, especially where vehicular access was forbidden.

Evolution of Restoration Methods

A total of 76 restoration projects were listed for the 36 sites identified. From the beginning of the 1990s until 2005, the number of new projects increased from 4 to 24 (Table 1). From 2005 onward, there was a rapid decline from 24 new projects between 2001 and 2005 to only 6 between 2011 and 2015. The distribution of restoration projects among the sites was not homogenous. In most cases, only one project had been implemented at a site, while some sites had benefited from three or more restoration projects (Fig. 2).

The most common restoration method was fencing, which was used in nearly two-thirds of the restoration projects (Table 1). Fences were installed to channel visitors onto a few well-designed paths and steer them away from the main part of the restoration site (Fig. 3A). The most frequent active ecological engineering method used (found in more than a quarter of the restoration projects) was biodegradable geotextile, which was laid on bare ground in order to protect seedlings (Fig. 3C). Fascines, generally made of heather, were used in a quarter of the projects to limit soil erosion on slopes. In about half the cases, these active methods were used in combination with fencing. The civil engineering methods used included nine infrastructure deconstructions (e.g. buildings, landfill sites) (Fig. 4D) and, mainly prior to 1995, seven car park constructions to limit disorganized vehicular access. Management methods, such as the removal of invasive plants (e.g. Carpobrotus spp.) and mowing, were used in 11 of 76 restoration projects (Table 1).

There seems to have been no clear change in the restoration methods used since the implementation of the first restoration projects in 1980. The proportion of passive and active methods has remained constant (Table 1). Around one-third of the restoration methods in every 5-year period were passive, and half the projects were composed of ecological engineering methods. Moreover, there was no change in the proportion of ecological engineering methods used since the 1980s (Table 1).

Evolution of Assessment and Monitoring Methods

In all, there were 465 monitoring plots in the maritime cliff restoration projects identified in Brittany. The longest 1980-1985 1986-1990 1991-1995 1996-2000 2001-2005 2006- Period 1980-19851986-19901991[START_REF] Rozé | Restauration des landes littorales. Exemple: le Cap d'Erquy (Bretagne 22 F)[END_REF]1996[START_REF] Ehrenfeld | Defining the limits of restoration: the need for realistic goals[END_REF]2001-2005[START_REF] Kerbiriou | Impact des changements d'usage sur la viabilité d'une population menacée dans un espace multi-protégé: Le Crave à bec rouge (Pyrrhocorax pyrrhocorax) sur l'île d'Ouessant[END_REF][START_REF] Sawtschuk | Restauration écologique des pelouses et des landes des falaises littorales atlantiques: analyse des trajectoires successionnelles en environnement contraint[END_REF][START_REF] Collier | Incorporating socio-economic factors into restoration: implications from industrially harvested peatlands[END_REF]-2015 Total Point All these major restoration programs (Fig. 4) were conducted with the involvement of academic scientists. Only one of the monitoring programs was launched exclusively with the aim of experimental restoration. A second monitoring program included both experimental and applied restoration. Both of these were designed and conducted by academic and nonacademic scientists. The other four monitoring programs were implemented during applied (i.e. nonexperimental) restoration projects. All the programs were designed and conducted through manager-scientist collaborations, and all followed clear protocols, with controls and replicates allowing comparisons to be made between them. Once the experiments and monitoring programs had started, most of the responsibility for data collection fell to the researchers and very rarely to the managers.

The results of these experiments have been published in specialized scientific journals [START_REF] Rozé | Restauration des landes littorales. Exemple: le Cap d'Erquy (Bretagne 22 F)[END_REF][START_REF] Bioret | Révision phytosociologique des végétations halophiles des falaises littorales atlantiques françaises[END_REF][START_REF] Gallet | La phytosociologie, outil pertinent pour le suivi de la restauration écologique?[END_REF][START_REF] Sawtschuk | Spontaneous succession as a restoration tool for maritime cliff-top vegetation in Brittany, France[END_REF][START_REF] Sawtschuk | Evaluation of the most common engineering methods for maritime cliff-top vegetation restoration[END_REF] as well as in managers' handbooks for the nonscientist stakeholders [START_REF] Enoul | Restaurer nos espaces naturels : bilan d'un savoir-faire en région Bretagne. Institut régional du patrimoine de Bretagne, Rennes, France Fernández-Manjarrés JF, Roturier S, Bilhaut A-G[END_REF][START_REF] Bioret | Restauration des végétations des hauts de falaises du littoral Manche-Atlantique[END_REF]. Numerous articles have also been published in nonindexed journals [START_REF] Bioret | La flore et la végétation de l'île de Groix[END_REF][START_REF] Gallet | Quelles méthodes pour le suivi et l'évaluation des opérations de restauration écologique? Exemple de la Côte Sauvage de Quiberon[END_REF][START_REF] Gallet | La restauration des végétations des hauts de falaises du littoral atlantique, vers une évaluation globale[END_REF][START_REF] Sawtschuk | Évaluation écologique de la restauration et de la gestion des végétations littorales sur le territoire de Lorient Agglomération[END_REF]. The results of the experiments and restoration projects conducted over large areas exclusively by nonscientist stakeholders have, in contrast, been transferred by site managers mostly orally and through photographs. There was only very scant written evidence and information about the restoration methods used or the effectiveness of those restoration actions.

The three monitoring methods used in the majority of cases, both by the scientists and the managers, were the quadrat (i.e. where the percentage cover of each species present in a fixed area from 0.5 to 4 m 2 is recorded), point contact (i.e. where the species present every 10 cm along a 10 m transect are recorded if in contact with a vertical pin), and phytosociological methods (i.e. where Braun-Blanquet's abundance/dominance scores of each species present in a fixed area from 1 to 25 m 2 are recorded) (Table 2). The point contact method has been used since 1991 in Pointe du Raz. Its use peaked between 2001 and 2005 in the restoration experiments conducted after the Erika oil spill. While the point contact method subsequently fell away, the use of the phytosociological method has remained common. Quadrats have been used over the past decade with the development of research projects on ecological restoration [START_REF] Sawtschuk | Restauration écologique des pelouses et des landes des falaises littorales atlantiques: analyse des trajectoires successionnelles en environnement contraint[END_REF][START_REF] Bioret | Restauration des végétations des hauts de falaises du littoral Manche-Atlantique[END_REF] and have now also been applied to more recent restoration projects. The only study to have compared two monitoring methods used in maritime clifftop vegetation restoration was that of [START_REF] Gallet | La phytosociologie, outil pertinent pour le suivi de la restauration écologique?[END_REF] who demonstrated that no reading variation existed between the point contact monitoring and the monitoring performed using phytosociological method.

Ecological monitoring data were used to assess restoration success by a third of the nonscientist stakeholders interviewed.

A c c e p t e d m a n u s c r i t p t

Most of them considered that visual assessment was an adequate measure for assessing the effectiveness of a restoration project with the use of a temporal series of photographs. Two of the site managers conducted sociological surveys but the results are unpublished. One of these consisted of interviews with the public and workshops with local inhabitants with the aim of understanding restoration project perceptions and assessing the effectiveness of knowledge sharing to local inhabitants (T. Delatouche 2017, N2000 coordinator for Guidel-Ploemeur littoral and Groix island, personal communication). The second focused on path visits in order to understand how people used the different pathways before they reconfigured the site (G. Duthion 2017, Manager in the Natural Areas Department of the county council of Ille et Vilaine, personal communication).

Discussion Balance Between Social and Ecological Outcomes

As with many other natural environments, ecological issues concerning maritime clifftops have only been taken into consideration since the 1990s (Le Fur 2013). For example, in Iceland, nature conservation on birch woodland, heathlands, and grasslands has only really been considered since the 1990s [START_REF] Aradóttir | Drivers of ecological restoration: lessons from a century of restoration in Iceland[END_REF], and, in Europe, the restoration of peatlands also began around this time [START_REF] Andersen | An overview of the progress and challenges of peatland restoration in Western Europe[END_REF]). In the maritime clifftop context, restoration projects have mostly aimed to restore vegetation and landscape quality while facilitating hedonistic tourism by channeling foot and vehicle traffic. However, our interview data showed that these goals were usually set out as restoration guidelines rather than the precise and quantifiable objectives needed to assess restoration success [START_REF] Mcdonald | International standards for the practice of ecological restoration-including principles and key concepts[END_REF]).

The answer to our question "Are both social and ecological issues considered in restoration projects?" is yes. A balance does exist but it does not include all social practices. Apart from tourism, all other human uses (e.g. fishing, car access) are excluded from most targeted social-ecological systems. The reason for this is their incompatibility with vegetation restoration because maintaining these social activities means keeping the paths open, while vegetation restoration means closing them. This common policy, which seems to be accepted by most restoration stakeholders, has led to a similar restoration pattern all over Brittany's maritime clifftops. Every practice that leads to ecological degradation is treated in the same way (i.e. hiking and sightseeing are controlled through fencing, and other activities are usually banned) in order to achieve the same level of social, ecological, or landscape impact (Table 3). This pattern may lead to a homogenization of restored sites, which are all presenting delimited paths and are all free from other activities such as tourism (Le Fur 2013).

From the stakeholder point of view (but with no supporting statistical data), this policy seemed to be accepted by most tourists. However, no information was given on other users' perceptions. In order to verify and supplement the stakeholder point of view, quantitative and qualitative studies of the human uses and perceptions [START_REF] Guerrero | Using structured decision-making to set restoration objectives when multiple values and preferences exist[END_REF] should therefore be carried out on these social-ecological systems. These studies should be accompanied by good public communication in order to gain public acceptance [START_REF] Barthélémy | A comparison of social processes at three sites of the French Rhône River subjected to ecological restoration[END_REF][START_REF] Metcalf | The role of trust in restoration success: public engagement and temporal and spatial scale in a complex social-ecological system[END_REF], particularly from the fishers and climbers, who are the users most impacted by these projects.

However, as has been pointed out in other studies, some practices are not compatible with vegetation restoration goals [START_REF] Miller | Habitat restoration-do we know what we're doing?[END_REF]. This has been shown, for example, for grassland restoration and boreal forests [START_REF] Bullock | Restoration of ecosystem services and biodiversity: conflicts and opportunities[END_REF]). However, this does not mean these conflicting practices (e.g. fishing, climbing) have to be fully excluded from maritime clifftops. Restoration should be thought out at landscape scale [START_REF] Weinstein | Linking restoration ecology and ecological restoration in estuarine landscapes[END_REF], with some sites dedicated to vegetation restoration and tourism acceptance and other sites left unrestored for people who prefer no restrictions because of their practices or their personal choices [START_REF] Nordstrom | Perceptions of the value of natural and restored beach and dune characteristics by high school students in New Jersey, USA[END_REF].

Restoration Methods Assessment

Long-term surveys conducted on passive restoration projects have shown that typical clifftop vegetation is able to spontaneously recolonize bare soil following vegetation degradation as long as the soil has not been too damaged [START_REF] Sawtschuk | Spontaneous succession as a restoration tool for maritime cliff-top vegetation in Brittany, France[END_REF]. However, the effectiveness of this method after heavy degradation, such as building deconstruction or high human trampling pressure, has not been assessed in the literature. It is possible that this degradation will have crossed biotic and abiotic thresholds [START_REF] Hobbs | Restoration ecology: repairing the Earth's ecosystems in the new millennium[END_REF], making passive restoration ineffective. This conclusion has been reached for some types of heavily damaged ecosystems, such as the riparian systems and the sage-steppe communities (McIver & Starr 2001). However, passive restoration has been found to be effective at restoring some heavily damaged ecosystems, such as forests (Prach et al. 2014).

Recently, active methods have been tested in maritime clifftop areas to accelerate revegetation. Methods like harvested biomass transfer, geotextile use, and litter transfer have already proven their effectiveness in accelerating the short-term restoration process [START_REF] Sawtschuk | Evaluation of the most common engineering methods for maritime cliff-top vegetation restoration[END_REF], although Kerambrun and Ragot (unpublished data) showed contrasting results on the effectiveness of active methods in their experiment. These studies are, however, based on short-term assessments (<5 years), which can lead to invalid interpretations [START_REF] Saccone | Extrapolating multi-decadal plant community changes based on medium-term experiments can be risky: evidence from high-latitude tundra[END_REF]. Moreover, as in many other restoration projects [START_REF] Hallett | Do we practice what we preach? Goal setting for ecological restoration[END_REF][START_REF] Wortley | Evaluating ecological restoration success: a review of the literature[END_REF], most of the analyses did not monitor progress against a reference ecosystem as recommended [START_REF] Aronson | On the need to select an ecosystem of reference, however imperfect: a reply to Pickett and Parker[END_REF][START_REF] Ser | The SER international primer on ecological restoration[END_REF]) but instead used only controls (i.e. degraded plots without active restoration).

Such mixed conclusions prove the need for a new analytical framework incorporating longer-term monitoring [START_REF] Bell | The value of long-term assessment of restoration: support from a seagrass investigation[END_REF] and defining reference ecosystems from surrounding nondegraded vegetation [START_REF] Ser | The SER international primer on ecological restoration[END_REF] using key attributes [START_REF] Mcdonald | International standards for the practice of ecological restoration-including principles and key concepts[END_REF]. Moreover, the conclusions drawn from the experimental plots in the current studies should be compared with those from larger restoration projects in order to confirm that the ecological processes observed in small experimental A c c e p t e d m a n u s c r i t p t Table 3. Summary of the impact of human practices on the ecological system before restoration and the impact of restoration measures on the ecological system, landscape, and social system. Estimations of restoration impact: ++, very positive; +, positive; 0, no impact; -, negative; --, very negative; +/-, depends on the restoration context; and ?, not determined. areas can be transposed to larger areas, which are generally only monitored through photographs. However, these photographic data could give valuable information about vegetation cover evolution [START_REF] Baxendale | Can digital image classification be used as a standardised method for surveying peatland vegetation cover?[END_REF]) and about some species' composition changes [START_REF] Michel | Spatial analysis of oblique photo-point images for quantifying spatio-temporal changes in plant communities[END_REF].

Ecological Impact of Human

Hence, the choice between active or passive methods, particularly for highly impacted sites (which seem quite common in ecological restoration), is still not clear [START_REF] Zahawi | Hidden costs of passive restoration[END_REF][START_REF] Zahawi | Passive restoration can be an effective strategy: a reply to Prach and del Moral (2015)[END_REF][START_REF] Prach | Passive restoration is often quite effective: response to Zahawi et al[END_REF]. However, the intensely monitored diversity of restoration methods and sites in maritime clifftop restoration should help to fill this gap because it will allow restoration to be adapted to different types of degradation and to different targeted reference ecosystem attributes (e.g. species composition, functionality) [START_REF] Mcdonald | International standards for the practice of ecological restoration-including principles and key concepts[END_REF], as is the case with other ecosystem types [START_REF] Andersen | An overview of the progress and challenges of peatland restoration in Western Europe[END_REF][START_REF] Chimner | An overview of peatland restoration in North America: where are we after 25 years?[END_REF]. Given this large, diversified database, new monitoring might not be essential, but new analysis using more recent data should be carried out. More importantly, the current monitoring must be continued to allow long-term assessments, in particular to determine the existence of passive restoration thresholds [START_REF] Hobbs | Restoration ecology: repairing the Earth's ecosystems in the new millennium[END_REF], which prevent passive restoration success in areas that are too degraded.

Link Between Ecological Restoration and Restoration Ecology

The importance of linking ecological restoration and restoration ecology through active collaborations between researchers and managers has already been shown [START_REF] Roux | Bridging the science-management divide: moving from unidirectional knowledge transfer to knowledge interfacing and sharing[END_REF][START_REF] Burbidge | Linking science and practice in ecological research and management: how can we do it better?[END_REF][START_REF] Gallet | L'écologie de la restauration en France. Dynamique actuelle et rôle d'un réseau multi-acteurs[END_REF]. Such collaborations are win-win situations because, on the one hand, the restoration projects provide sites and specific contexts for the scientists to study ecological processes and, on the other, restoration ecology enhances the different restoration practices used by the managers [START_REF] Palmer | Ecological theory and restoration ecology[END_REF].

In maritime clifftop restoration, knowledge is often coproduced by scientists and practitioners, and this appears to be the best way of creating knowledge [START_REF] Roux | Bridging the science-management divide: moving from unidirectional knowledge transfer to knowledge interfacing and sharing[END_REF][START_REF] Gonzalo-Turpin | Rethinking partnerships with the aim of producing knowledge with practical relevance: a case study in the field of ecological restoration[END_REF]). Nevertheless, this seems to have had little impact on the restoration methods used given that there has been no change in the restoration methods used for the past two decades nor any adaptive management when restoration is not working. Different hypotheses can be put forward to explain this situation. One is that the paucity of long-term restoration analyses has led to a lack of knowledge about restoration method effectiveness. Another is the lack of interest among managers in monitoring data collection and analysis. Hence, even if numerous nonscientific articles and handbooks were published, it is possible that a deficiency in knowledge sharing would still exist, as has been noted by some authors [START_REF] Dettman | Lessons learned about research and management: a case study from a Midwest Lowland Savanna[END_REF]. It may therefore not be enough just to rely on these publications; it may also be necessary to hold workshops or provide training for decision-makers [START_REF] Burbidge | Linking science and practice in ecological research and management: how can we do it better?[END_REF].

We did, however, notice a shift in monitoring methods from the point contact method to the quadrat method. There is no explanation for this shift in the literature. No scientific studies have proven that quadrat use is more valuable than point contact use. For this reason, this change seems more likely to be based on personal preference or custom than on any scientific motive. The time required for the methods did not seem to impact this choice either. Indeed, while managers should be more disposed to using less time-consuming methods than scientists who usually seek more precise data, all monitoring methods were equally used by both managers and scientists. However, the fact that very few studies compared monitoring protocols is a generalized issue in ecological science [START_REF] Godínez-Alvarez | Comparison of three vegetation monitoring methods: their relative utility for ecological assessment and monitoring[END_REF]. Monitoring method choices should be based on monitoring objectives and adapted according to different criteria, such as vegetation type [START_REF] Leis | Comparison of vegetation sampling procedures in a disturbed mixed-grass prairie[END_REF], time allowed [START_REF] Baxendale | Can digital image classification be used as a standardised method for surveying peatland vegetation cover?[END_REF], and the skills needed for data collection and analysis [START_REF] Gallet | La restauration des végétations des hauts de falaises du littoral atlantique, vers une évaluation globale[END_REF].

Hence, the analyses of restoration methods and monitoring methods seem to show that neither scientists nor practitioners learn enough from past experiences. As already mentioned, new data analyses might therefore be necessary. However, stronger collaborations also seem essential to allow a better two-way A c c e p t e d m a n u s c r i t p t knowledge transfer between practitioners and scientists. This could be particularly true for large-scale restoration projects, which there are currently very little data on.

New Challenges for Maritime Clifftop Restoration

Although socioeconomic knowledge has been cited as being key to the success of a restoration project [START_REF] Carr | Talking frogs: the role of communication in ecological research on private land[END_REF][START_REF] Collier | Incorporating socio-economic factors into restoration: implications from industrially harvested peatlands[END_REF][START_REF] Hallett | Do we practice what we preach? Goal setting for ecological restoration[END_REF], we identified a lack of socioeconomic monitoring and assessment in maritime clifftop restoration. For instance, the paths created in the vegetation by the climbers, fishers, and hikers could not be easily distinguished from those created by the hedonistic tourists, even with photointerpretation. This makes it difficult to fully consider the different sources of damage in restoration plans without specific studies. A better understanding of the social system is also needed to be able to foster social acceptance of the project [START_REF] Petursdottir | An evaluation of the short-term progress of restoration combining ecological assessment and public perception[END_REF] and avoid conflict, which can potentially lead to the destruction of restoration layouts. This lack of social consideration is generalized throughout ecological restoration projects [START_REF] Aronson | Are socioeconomic benefits of restoration adequately quantified? A meta-analysis of recent papers (2000-2008) in restoration ecology and 12 other scientific journals[END_REF]. Links between the social and ecological systems should therefore be strengthened, in particular by studying and considering every holder point of view through surveys and workshops [START_REF] Guerrero | Using structured decision-making to set restoration objectives when multiple values and preferences exist[END_REF].

There should also be research conducted on ecosystem services, which are often used in the literature as restoration goals [START_REF] Ehrenfeld | Defining the limits of restoration: the need for realistic goals[END_REF][START_REF] Hallett | Do we practice what we preach? Goal setting for ecological restoration[END_REF]). In the present context, two particular ecosystem services were considered, namely cultural services (e.g. improving landscape quality) and the limitation of soil erosion through fencing. However, we found no study or report dealing with ecosystem services showing an urgent need for such research.

Most of the monitoring plots in this study concerned vegetation. As a result, we are not yet able to assess the impact of high human trampling pressure on other ecosystem elements, such as soil properties. Furthermore, no data are available on invertebrate communities restoration despite the fact it has been identified as a potentially good restoration indicator [START_REF] Gerlach | Terrestrial invertebrates as bioindicators: an overview of available taxonomic groups[END_REF]. Although the bird disturbance has been identified [START_REF] Kerbiriou | Impact des changements d'usage sur la viabilité d'une population menacée dans un espace multi-protégé: Le Crave à bec rouge (Pyrrhocorax pyrrhocorax) sur l'île d'Ouessant[END_REF], it is not possible to quantify its impact. Hence, our focus here on the vegetation community should be widened through studies of these other groups. Finally, vegetation could be studied at the species level to look for easy-to-use indicators of the good health of restored ecosystems [START_REF] Gerlach | Terrestrial invertebrates as bioindicators: an overview of available taxonomic groups[END_REF][START_REF] González | Can indicator species predict restoration outcomes early in the monitoring process? A case study with peatlands[END_REF]) in order to make assessment easier.

Figure 1 .

 1 Figure 1. Aerial photograph of Pointe de Pen-Hir showing a dense network of paths created by hedonistic tourists, fishers, rock climbers, and hikers. Source, BD ORTHO ® IGN 2012.

Figure 2 .

 2 Figure 2. Location and number of restoration projects on Breton maritime clifftops. Projects with at least one monitoring plot are shaded black. Small dots: one restoration project; medium dots: two restoration projects; and large dots: three or more restoration projects.

Figure 3 .

 3 Figure 3. Different restoration methods. Passive method: (A) fence; ecological engineering method: (B) harvested biomass transfer; (C) biodegradable geotextile; civil engineering method: (D) building destruction. (Photos A, B, and C by J. Sawtschuk; Photo D by J. Froger)

Figure 4 .

 4 Figure 4. The evolution in the number of new maritime clifftop ecological restoration monitoring plots. The main programs are written above their corresponding periods.

Table 1 .

 1 Evolution of the number of restoration projects and number of methods used on maritime clifftop restoration projects since 1980. Bold text indicate totals.

Table 2 .

 2 Evolution of monitoring methods used in the maritime clifftop restoration context.

Acknowledgments

The authors would like to express their gratitude for all the discussions they had with the different stakeholders, too many to be named individually here, who provided essential information and opinions on maritime clifftop restoration for this article. We would also like to thank the Fondation de France for its financial support of this program and the Ministère de l'Écologie du développement durable et de l'Énergie, which funded a preliminary assessment of maritime clifftop restoration. We also gratefully acknowledge H. Martin-Brelot's contribution to the design of the semistructured interview schedule.

Supporting Information

The following information may be found in the online version of this article: