Multi-view Performance Capture of Surface Details
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DOI: 10.1007/s11263-016-0979-1
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- Robertini, N., Casas, D., De Aguiar, E. et al. Int J Comput Vis (2017). doi:10.1007/s11263-016-0979-1
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Abstract
This paper presents a novel approach to recover true fine surface detail of deforming meshes reconstructed from multi-view video. Template-based methods for performance capture usually produce a coarse-to-medium scale detail 4D surface reconstruction which does not contain the real high-frequency geometric detail present in the original video footage. Fine scale deformation is often incorporated in a second pass by using stereo constraints, features, or shading-based refinement. In this paper, we propose an alternative solution to this second stage by formulating dense dynamic surface reconstruction as a global optimization problem of the densely deforming surface. Our main contribution is an implicit representation of a deformable mesh that uses a set of Gaussian functions on the surface to represent the initial coarse mesh, and a set of Gaussians for the images to represent the original captured multi-view images. We effectively find the fine scale deformations for all mesh vertices, which maximize photo-temporal-consistency, by densely optimizing our model-to-image consistency energy on all vertex positions. Our formulation yields a smooth closed form energy with implicit occlusion handling and analytic derivatives. Furthermore, it does not require error-prone correspondence finding or discrete sampling of surface displacement values. We demonstrate our approach on a variety of datasets of human subjects wearing loose clothing and performing different motions. We qualitatively and quantitatively demonstrate that our technique successfully reproduces finer detail than the input baseline geometry.
Keywords
Performance capture Surface detail Sums of Gaussian1 Introduction
Over the last decade, performance capture techniques have enabled the 3D reconstruction of the motion and the appearance of real-world scenes, such as human motion or facial expressions, using a multi-camera capture setup (de Aguiar et al. 2008; Gall et al. 2009a; Bradley et al. 2008; Vlasic et al. 2008). Captured sequences are reconstructed using a space-time coherent 3D mesh, also known as 4D model, that reproduce the original scene or motion. This can be done, for example, by deforming a mesh or a rigged template such that it aligns with the images (de Aguiar et al. 2008; Vlasic et al. 2008), or by a per-frame independent reconstruction of the scene (Starck and Hilton 2007a) followed by a surface alignment step to convert the temporally inconsistent mesh geometries to a coherent model (Budd et al. 2013). However, many reconstructing methods only produce a coarse-to-medium scale 4D model that does not reproduce the high-frequency geometry detail present in the original scene. Finer scale shape detail can be then added in a second refinement step.
For fine surface detail reconstruction, some methods have used photo-consistency constraints via stereo-based refinement (de Aguiar et al. 2008; Starck and Hilton 2007b). However, such methods often turn to discrete sampling of local displacements, since formulating dense stereo based refinement as a continuous optimization problem has been more challenging (Kolev et al. 2009). Other methods for mesh refinement align the surface to a combination of silhouette constraints and sparse image features (Gall et al. 2009a). But such approaches merely recover medium scale detail and may suffer from erroneous feature correspondences between images and shape. Recently, shading-based techniques such as shape-from-shading or photometric stereo (Wu et al. 2011a, b; Vlasic et al. 2008) have been also proposed to capture small-scale displacements. However, these methods either are limited to be used in controlled and calibrated lighting setups, or they require a complex inverse estimation of lighting and appearance when they are applied in uncontrolled recording conditions.
This paper extends the work presented in Robertini et al. (2014) and it describes an effective solution to the refinement step using multi-view photo-consistency constraints. As input, our method expects synchronized and calibrated multiple video of a scene and a reconstructed coarse mesh animation, as it can be obtained with previous methods from the literature. Background subtraction or image silhouettes are not required for refinement.
Our first contribution is a new shape representation that models the mesh surface with a dense collection of 3D Gaussian functions centered at each vertex, each having an associated color, which we referred to as Surface Gaussians. A similar decomposition into 2D Gaussian functions, which we referred to as Image Gaussians, is applied to each input video frame. Leveraging this new Gaussian-based representation, our second contribution is a new formulation for dense photo-consistency-based surface refinement, which we formulate as a global optimization problem in the position of each vertex on the surface. Unlike previous performance capture methods, we are able to formulate the model-to-image photo-consistency energy that guides the deformation as a closed form expression, and we can compute its analytic derivatives. This also enables implicit handling of occlusions, as well as spatial and temporal coherence constraints, while preserving a smooth consistency energy function. We can effectively minimize this function in terms of dense local surface displacements with standard gradient-based solvers. In addition to these advantages, unlike many previous methods, our framework does not require a potentially error-prone sparse set of feature correspondences or discrete sampling and testing of surface displacements, and thus provides a new way of continuous optimization of the dense surface deformation.
We used our approach for reconstructing full-body performances of human actors wearing loose clothing, and performing different motions. Input coarse geometry of the scenes were obtained using a template-based method (Gall et al. 2009a; de Aguiar et al. 2008), or a template-free method (Starck and Hilton 2007b) followed by a surface alignment step (Budd et al. 2013). We demonstrate (Sect. 6) that our approach is able to reconstruct more of the correct fine-scale detail that is present in the input video sequences, than both of the baseline methods (i.e. template-based and template-free), for instance the wrinkles in a skirt. We also demonstrate these improvements quantitatively.
In contrast to the original work presented in Robertini et al. (2014), we improved our energy function by adding an additional temporal smoothing term to avoid jittering and artifacts in the final results. We provide a comprehensive description of all components of our energy function as well as the complete derivation of the analytic derivatives. We also added an extensive set of results and experiments to validate the performance of our improved approach.
2 Related Work
Marker-less performance capture methods are able to reconstruct dense dynamic surface geometry of moving subjects from multi-view video, for instance of people in loose clothing, possibly along with pose parameters of an underlying kinematic skeleton (Theobalt et al. 2010). Most of them use data from dense multi-camera systems and are recorded under controlled studio environments. Some methods employ variants of shape-from-silhouette or active or passive stereo (Zitnick et al. 2004; Matusik et al. 2000; Starck and Hilton 2007b; Waschbüsch et al. 2005; Tung et al. 2009), which usually results in temporally incoherent reconstructions. A subsequent step, known as surface tracking or surface alignment, is used to convert the temporally incoherent geometry proxy to a coherent geometry that deforms over time to fit the reconstructed shapes. Cagniart et al. (2010) solve such free-form alignment sequentially by iterative close point matching of overlapping rigid-patches. Similarly, Budd et al. (2013) use a combination of geometric and photometric features in a non-sequential alignment framework. Volumentric constraints have also been used to formulate the surface tracking problem (Allain et al. 2015; de Aguiar et al. 2007). Nevertheless, regardless the accuracy of the method used for surface tracking, most of them result in a coarse-to-medium scale 4D model in which most of the high-frequency details are lost.
Space-time coherency is easier to achieve with model-based approaches that deform a static shape template (obtained by a laser scan or image-based reconstruction) such that it matches the subject, e.g. a person (Carranza et al. 2003; de Aguiar et al. 2008; Vlasic et al. 2008; Ballan and Cortelazzo 2008; Gall et al. 2009a) or a person’s apparel (Bradley et al. 2008). Some of them jointly track a skeleton and the non-rigidly deforming surface at the same time (Vlasic et al. 2008; Ballan and Cortelazzo 2008; Gall et al. 2009b); also multi-person reconstruction has been demonstrated (Liu et al. 2013). Other approaches use a generally deformable template without embedded skeleton to capture 4D models, e.g. an elastically deformable surface or volume (de Aguiar et al. 2008; Savoye 2013).
Most of the methods mentioned so far suffer from two main limitations: on one hand, the template-based methods that use an initial highly detailed scan model usually do not deform the fine-detail of the template according to the acquired per-frame imagery. This leads to incorrect surface detail on the final reconstructed meshes that does not match the captured dynamics (de Aguiar et al. 2008; Vlasic et al. 2008). On the other hand, methods that require the deformation of the per-frame reconstructed meshes to achieve temporal consistency (Starck and Hilton 2007b; Budd et al. 2013; Cagniart et al. 2010) tend to output a coarse reconstruction of the sequence. In both cases true fine detail needs to be recovered in a subsequent step. Medium scale non-rigid 4D surface detail can be estimated by using a combination of silhouette constraints and sparse feature correspondences (Gall et al. 2009a). Other approaches use stereo-based photo-consistency constraints in addition to silhouettes to achieve denser estimates of finer scale deformations (Starck and Hilton 2007b; de Aguiar et al. 2008). It is an involved problem to phrase dense stereo-based surface refinement as a continuous optimization problem, as it is done in variational approaches (Kolev et al. 2009). Thus, stereo-based refinement in performance capture often resorts to discrete surface displacement sampling which are less efficient, and with which globally smooth and coherent solutions are harder to achieve. Alternatively, fine-detail surface deformation caused by garment wrinkling has been investigated using cloth-specific approaches. Popa et al. (2009) enhance reconstructed surfaces using a wrinkle generation method based on the recorded shadows.
Fine-scale detail can be also recovered using active lighting methods, e.g. shape-from-shading or photometric stereo (Wu et al. 2011a). Many of these approaches require controlled and calibrated lighting (Hernandez et al. 2007; Vlasic et al. 2009a; Ahmed et al. 2008; Vlasic et al. 2009b), which reduces their applicability in real-world scenarios. For example, Hernandez et al. (2007) use red, green and blue lights from different directions to estimate surface normals with a photometric stereo approach. This enables detailed reconstruction of cloth, even untextured. Ahmed et al. (2008) estimate surface reflectance and time-varying normal fields on a coarse template mesh to incorporate garment details such as wrinkles and folds. Vlasic et al. (2009b) reproduce high-resolution geometric detail by capturing multi-view normal maps in a large lighting dome that provides a series of novel spherical lighting configurations. More recently, shading-based refinement of dynamic scenes captured under more general lighting was shown Wu et al. (2011b), but these approaches are computationally challenging as they require to solve an inverse rendering problem to obtain estimates of illumination, appearance and shape at the same time.
Our problem formulation is inspired by the work of Stoll et al. (2011) who used 2D and 3D Gaussian functions for marker-less skeletal pose estimation. The use of implicit surfaces for shape and motion estimation from multi-view video was originally proposed by Plankers and Fua (2003), who use smooth implicit surfaces attached to an articulated skeleton to approximate a human 3D shape. This representation allows to define a distance function of data points and models that is differentiable. Similarly, Ilic and Fua (2006) create implicit meshes by attaching triangular primitives to the faces of explicit 3D meshes. Surface reconstruction and refinement is performed by optimizing the implicit mesh (exploiting its attractive properties) and propagating the deformation to the explicit mesh for rendering.
3 Overview
Figure 1 shows an overview of our approach. The input to our system is a calibrated and synchronized multi-view video sequence showing images of the human performer and a spatio-temporally coherent set of coarse meshes, reconstructed from related approaches (Gall et al. 2009a; Starck and Hilton 2007b; de Aguiar et al. 2008; Budd et al. 2013).
Our algorithm refines the initial set of coarse geometries by incorporating fine dynamic surface details to the meshes. First, an implicit representation of the input mesh using a dense collection of 3D Gaussian functions, referred to as Surface Gaussians, with associated colors is created. The input images are also represented as a set of 2D Gaussian Functions, referred to as Image Gaussians, associated to image patches in each camera view. Thereafter, continuous optimization is performed to maximize the color consistency between the collection of Surface and Image Gaussians. The optimization displaces the Surface Gaussians along the associated vertex normal of the coarse mesh which yields the necessary vertex displacement. Our optimization method has a smooth energy function, that can be expressed in closed form and it allows us to analytically compute derivatives, enabling the possibility of using efficient gradient-based solvers.
4 Implicit Model
Our framework converts the input coarse animation and the input multi-view images into implicit representations using a collection of Gaussians: Surface Gaussians (i.e. 3D Gaussian functions) on the mesh surface with associated colors and Image Gaussians (i.e. 2D Gaussian functions), with associated colors, assigned to image patches in each camera view.
4.1 Surface Gaussians
We further assign a HSV color value \(\eta _s\) to each Surface Gaussian. In order to derive the colors we choose a reference frame where the initial coarse reconstruction is as close as possible to the real shape. This is typically the first frame in each sequence. For each vertex \(v_s\) of the input mesh, we chose the most direct camera, i.e. where vertex normal and camera viewing direction align best. The Surface Gaussian associated to \(v_s\) is projected to the image from the best camera view and the corresponding pixel color average is assigned as a color attribute.
4.2 Image Gaussians
Our implicit model for the input images I(c) of all cameras \(c \in \{0\dots n_c-1\}\), \(n_c\) being the number of cameras, is obtained by assigning Gaussian functions \(G_i(x)\), \(x \in \mathbb {R}^2\), which we referred to as Image Gaussians, to each image patch of all camera views. Similar to Stoll et al. (2011) we decompose each input frame into squared regions of coherent color by means of quad-tree decomposition. We set a maximum quad-tree depth \(T_{qt}\) and recursively split the image domain of each camera view in smaller regions. Then, we fuse together neighboring patches of similar color up to a color similarity threshold \(T_{fuse}\). The color similarity between two regions having colors \(\eta _1\) and \(\eta _2\) is computed by simply taking the Euclidean distance \(||\eta _1 - \eta _2||^2\).
4.3 Projection of Surface Gaussians
5 Surface Refinement
This hard constraint brings two main advantages: first, it forces the Surface Gaussians to maintain a regular distribution on the surface, and secondly it highly reduces the number of parameters to optimize for (single scalar displacements \(k_s\), instead of the 3 dimensions \([\hat{\mu }_s]_x, [\hat{\mu }_s]_y\) and \([\hat{\mu }_s]_z\)), resulting in higher performances as well as better-posed convergence.
5.1 Similarity Term
Derivative for\(E_{sim}\): In order to calculate the derivative of \(E_{sim}\), we note that most of its terms are constant with respect to \(k_s\), except the projected means \(\mu _{s}\) and the variances \(\sigma _{s}\), within the term \({\varPhi }_{i,s}\).
5.2 Regularization Term
User-defined parameters of the energy function \(\mathbf E \), together with their description, values interval and default value
Parameter | Description | Values int. | Def. |
---|---|---|---|
\(w_{reg}\) | Regularization weight | [0, 1] | \(5e^{-7}\) |
\(w_{temp}\) | Temporal weight | [0, 1] | \(1e^{-7}\) |
\(\sigma _s\) | Standard deviation, \(\forall G_s\) | \((0,\infty )\), [mm] | 5 |
\(T_{qt}\) | Quad-tree depth threshold | \([0,log_2(I^{W,H}) ]\) | 9 |
\(T_{fuse}\) | Color similarity for fusion threshold | \([0,\infty )\) | 0.05 |
\(T_{color}\) | Color similarity threshold | \([0,\infty )\) | 0.15 |
\(T_{dist}\) | Threshold on pixel distance | \([0,\infty )\), [px] | 30 |
\({\varDelta }_d\) | Max geodesic distance | \([0,\infty )\), \([\# edges]\) | 2 |
5.3 Temporal Smoothing Term
5.4 Optimization
Our energy function \(\mathbf {E}\) can be efficiently optimized using an iterative gradient-based approach. For each iteration t of the maximization process, we compute the derivative of \(\mathbf {E}^t\) with respect to each \(k_s, s \in \{0\dots n_s - 1\}\), as obtained summing-up all energy term derivatives together, following Eq. 6.
To improve computational efficiency, we evaluate the overlap \({\varPhi }_{i,s}\) only for visible Surface Gaussians from each camera view. Explicit visibility computation is performed only once at the beginning of each frame, by considering each Surface Gaussian as simple vertices. The implicit occlusion handling takes care of consistently handling new occlusions that might arise during optimization. The Gaussian overlap is then computed against visible projected Surface Gaussians and Image Gaussians in a local neighborhood, by considering only the closest Image Gaussians up to a distance threshold \(T_{dist}\) in number of pixels and a color distance threshold \(T_{color}\). Table 1 summarizes the main user defined parameters as well as their default values.
6 Results
In this Section we describe our datasets and provide detailed qualitative and quantitative evaluation of the results, as well as complexity and time performances of our approach. Finally, we discuss the main directions for a future work.
6.1 Datasets
Details for each sequence
Sequence | Skirt | Dance | Handstand | Wheel | Pop2lock | Synthetic | |||
Published by | Gall et al. (2009a) | Starck and Hilton (2007b) | Ours | ||||||
Cameras | 8 | 10 | |||||||
Frames | 640 | 574 | 150 | 120 | 250 | 1 | |||
Frame rate (Hz) | 40 | n/a | |||||||
Camera type | Phase space vision camera | Simulated | |||||||
Image resolution (px) | 1004 \(\times \) 1004 | \(1920\times 1080\) | \(1280\times 720\) | ||||||
Vertices | 12095 | 6002 | 13809 | 13809 | 10006 | 42 | |||
Used/total cameras | 7/8 | 10/10 | |||||||
3D Gaussians | 12095 | 3430 | 12095 | 12095 | 3880 | 42 | |||
2D Gaussians per frame | \(\approx \)9000 | \(\approx \)9800 | \(\approx \)15200 | \(\approx \)15200 | \(\approx \)13000 | Variable | |||
Ground truth | n/a, Flow displacement + silhouette error | Constructed | |||||||
Input meshes | Orig | Smooth | Orig | Smooth | Orig | Original | Orig | Smooth | Orig |
Timing (min/frame) | \(\approx \)20 | \(\approx \)3 | \(\approx \)30 | \(\approx \)30 | \(\approx \)3 | \(\approx \)0.04 |
In order to refine the input mesh sequences, we first subdivide the input coarse topology by inserting additional triangles and vertices. We then generate a collection of Gaussians on the surface, as described in Sect. 3. Since most of the fine-scale deformations happen on the clothing, we focus the refinement on those areas, generating Surface Gaussians only for the corresponding vertices. Table 2 shows the amount of Surface Gaussians created for each sequence.
To visually enhance the capabilities of our refinement approach in capturing fine-scale surface details, we have created 3 additional datasets by smoothing the input meshes of the sequences dance, skirt and pop2lock. By doing so we eliminated most of the baked-in surface details, and use the over-smoothed mesh animations as input to our system.
We additionally tested our method on a synthetic sequence, consisting on a 42-vertices sphere model with randomly generated colors. We generate three different scenarios: the unchanged sphere, the sphere with randomly displaced vertices along the corresponding vertex normals, and the sphere with randomly displaced vertices without any normal constraint. We then render each of the scenarios from 10 arbitrary view-points, which we use as the input multi-view images, and we use each scenario geometry as a ground truth. We run our approach in each scenario, using the unchanged geometry as input coarse mesh, and the rendered views as input multi-view images. Finally we compare the output of each scenario with the ground truth geometry. In this Section we show results of such experiment.
6.2 Computational Complexity and Efficiency
The complexity of our approach for each frame f and camera c depends on the number of iterations \(n_t\) performed by the gradient-based solver, as well as the number of Image Gaussians \(n_i^c\) and Surface Gaussians \(n_s\), \(O(n_t n_i^c n_s)\). These 3 quantities variate depending on the sequence, frame and specific camera. The order of complexity is obtained considering the maximum among the possible values for each sequence. For each frame, at most \(n_t = 1000\) iterations are performed (see Sect. 5.4), and at most each single pixel of each camera view is taken as Image Gaussian, resulting in \(n_i^c = 1004\times 1004\) for the sequences from Gall et al. (2009a) and \(n_i^c = 1920\times 1080\) for Starck and Hilton (2007b) (see Table 2). However, convergence to a solution is typically reached in less iterations. The number of Image Gaussians is also smaller than the number of image pixels for each camera, as for each Surface Gaussian we only take into account the closest Image Gaussians in terms of spacial and color distance, as described in Sect. 5.4. It can be easily shown that our approach has linear complexity with respect to varying Surface Gaussians.
We evaluated the performance of our system on an Intel Xeon Processor E5-1620, Quad-core with Hyperthreading and 16 GB of RAM. Table 2 summarizes the performances we obtained for the 6 tested sequences. The computation time can be further reduced by parallelizing orthogonal optimization steps, such as the evaluation of the overlap \({\varPhi }_{i,s}, \forall i,s\) (see Sect. 5.1). Our algorithm is particularly suited for implementation on GPU, which we believe has a strong impact on the time performances.
6.3 Evaluation
As shown by the graphs in Fig. 7 our method significantly decreases the average flow displacement error, leading to quantitatively more accurate results (up to \(11\%\) improvement). High percentage of flow error reduction between input and refined meshes is also obtained in the purposely smoothed datasets. This confirms that our approach successfully recovers true surface detail even when using very coarse geometry.
Our approach is able to capture the deformation dynamics happening on the clothing in all sequences, as well as fine-scale details, especially for highly textured regions, e.g. the skirt sequence. Details on the inner plain-colored regions, e.g. shirt folds in the dance sequence, are hard to reproduce with our approach, since they lack of sufficiently distinctive color information in the Surface Gaussians neighborhood.
Results demonstrate that our approach also improves the alignment of the reconstructed meshes with respect to the input image, without explicitly relying on any precomputed foreground segmentation. As opposite to most of the silhouette-based approaches (Starck and Hilton 2007a), our optimization approach smoothly and continuously improves the alignment to the silhouettes, without using error-prone correspondence finding and discrete optimization. We show silhouette reprojection improvement for all tested sequences in Figs. 8 and 9. The silhouette improvement per frame is computed by counting the false positive and false negative (i.e. wrong) pixels in the original and refined sequences, obtained by subtracting the ground-truth silhouette image pixels with the reprojected mesh pixels from an unused camera. Then, we take the difference between original and refined false pixels, which implicitly resembles the amount of newly found correct pixels. Notice, that our computations are performed using the available full-body silhouette images, although we just refine the clothing parts. We manually counted the average amount of true pixels in the clothing areas for each sequence as a reference, see the Figure caption 8. Follows detailed evaluation of each tested sequence.
6.3.1 Skirt Sequence
The skirt sequence is the most suited for our multi-view surface refinement technique thanks to its highly textured regions and accurate alignment with respect to the images. We focus on the refinement of the skirt surface region, which is highly textured. Textures lead to high-color variance of the Surface Gaussians, which in turn allow to estimate finer scale details and more accurate alignment, even in the inner parts away from the projected borders. As shown in Figs. 10 and 11, our method is able to capture additional fine-scale details for both the skirt and the smoothed skirt sequences using the default parameter settings depicted in Table 1. The incorporated details are best visible in the smoothed sequence version. The accompanying video shows some effects of the dynamics of the skirt being captured as well. See also the quantitative evaluation graphs in Fig. 7a, b, which report a consistent decrease in the flow error with respect to the input sequence. The deterioration in performance that happens around frame 400 is due to a temporal misalignment between the input skirt and the images, caused by an improperly captured full body rotation of the actor, that in turn voids the refinement attempts of the skirt surface. The alignment of the input sequence is stabilized in the following frames, allowing for correct refinement of the skirt motion. Although the original sequence presents highly accurate reprojection border alignment to the ground truth silhouettes, our quantitative evaluation shows that our approach captures the silhouette borders with even increased precision, particularly visible in the smoothed skirt sequence plots Fig. 8a.
6.3.2 Dance Sequence
6.3.3 Pop2lock Sequence
The pop2lock sequence (Starck and Hilton 2007b) is challenging to refine with our approach due to the large homogeneously colored regions on the subject’s shirt. On top of that, the input mesh presents severe misalignments with respect to the input multi-view images with several time-consistency artifacts throughout the video. Nevertheless, using the default parameters setting in Table 1 we quantitatively report a reduced average flow error displacement with respect to the input mesh animation, both for the original pop2lock and the smoothed version, see Fig. 7e, f. Our refinement approach is unable for this sequence to improve the silhouette overlap consistently due to the strong shirt deformations accompanied by complex folds forming during the motion, see Fig. 8c. The visible picks in the silhouette improvement graphs correspond to those time intervals where the shirt has the least deformations, because the actor stands upright (visible in the video). In those few frames our refinement approach performs adequately, improving the original alignment, especially for the smoothed input sequence. The overlap could be further improved by switching off our regularization term, allowing for more freedom in the surface displacements, at a cost of less smoothed surface areas.
6.3.4 Handstand and Wheel Sequences
6.3.5 Synthetic Sequence
We additionally evaluate the performances of our approach on a synthetic sequence. Figure 12 shows the input and output mesh for each scenario. For the first unchanged scenario we use the default parameter settings from Table 1, while for the remaining two we used \(w_{reg} = 0\) aiming at capturing the randomly generated displacements. For all scenarios we have used \(T_{dist} = 90\) pixels to capture also larger displacements. Since we dispone of the ground-truth for this sequence, we compute exact average Euclidean displacement error, by summing up the vertex displacement norm from the ground truth divided by the number of vertices. The average error is then estimate as a percentage of the total object volume size. Even with large distortions (i.e. severe displacement) applied to the original geometries, our method is able to recover the fine geometric details with average errors lower than \({\le }7\%\) of the volume size. We further validate the additional \(\epsilon \) we add to the optimized displacements of the Surface Gaussian (see Sect. 5.4), showing a quantitative improvement of the reconstructions, see Fig. 13. As we show in Fig. 14 the error increases when reducing the amount of views. This finding suggests that without providing more advanced surface smoothness term to handle missing depth, our approach is impractical for settings with few-cameras.
6.4 Parameters
In order to test the influence of the parameters used in our energy formulation, we run several experiments based on the synthetic dataset described at the end of Sect. 6.1. For each user-defined parameter, see Table 1, we change the parameter value within its pre-defined interval, while keeping the remaining parameters fixed, and run our refinement approach with this setting. The set of plots in Fig. 15 show the error and time performances obtained by varying each parameter.
As expected increasing \(w_{reg}\) improves the static results, deteriorating the randomly displaced ones. Since we do assume a certain amount of smoothness in our scenarios, we have chosen a proper default regularization finding the most suited trade-off between smoothness and amount of captured details.
By increasing the standard deviation \(\sigma _s\) size, we obtain slightly worse reconstructions. We believe this is due to the fact that the Gaussian color is taken from the corresponding vertex color of the ground-truth mesh, and not from the underlying pixel average, as we do for all the remaining sequences. In this case, the underlying pixel color poorly matches the Surface Gaussian color, leading to displacement errors.
The color similarity threshold \(T_{color}\) restricts the evaluation of our similarity function w.r.t. a Surface Gaussian to a subset of Image Gaussians with similar color properties. The larger the threshold the more Image Gaussians are included in the evaluation, leading to an increasing computational time. The distance threshold \(T_{dist}\) value highly depends on the sequence and mesh scale. Further increasing its value beyond its default (\(T_{dist} = 90\) pixels for the synthetic sequence) does not improve qualitatively the reconstruction, instead it slows down the performances. On the other hand the fusion threshold \(T_{fusion}\) and the quad-tree depth threshold \(T_{qt}\) only produce accurate reconstruction results for high values, which in turn decrease the time performances. The smoothing weight \(w_{temp}\) improves the stability of the refinement in time, without further improving the quality of the resulting surface. We show the effects of varying smoothing weight in the accompanying video.
6.5 Limitations and Future Work
In this subsection we state the main limitations of our approach and provide possible solutions to be considered as a future work. We assume the input mesh sequence to be sufficiently accurate, such that smaller details can be easily and correctly captured by simply displacing vertices along their corresponding vertex normals. In cases where the input reconstructed meshes present misalignments with respect to the images (e.g. pop2lock) or if it is necessary to reconstruct stronger deformations, then our method is unable to perform adequately. Our refinement might be reformulated allowing more complex displacements, e.g. without any normal constraint. However such weaker prior on vertices motion requires more complex regularization formulation in order to maintain smooth surface, also to handle unwanted self-intersections and collapsing vertices. On top of that the increased number of parameters to optimize for (i.e. 3 times more, when optimizing for all 3 vertices dimensions, x, y and z) would spoil computational efficiency and raise the probability of getting stuck in local maxima solutions. The risk of returning local maxima solutions is still high when employing local solvers (e.g. gradient ascent) on non-convex problems as in our case. A possible solution is to use more advanced solvers, e.g. global solvers, when computational efficiency is not a requirement.
We currently assign colors to the Surface Gaussians based on the underlying pixel average as seen from the best camera view. Among the available frames, we chose one of those where the input mesh is closely aligned to the input multi-view images in order to guarantee best color assignment. Apart from selecting the best frame, a way to improve the color assignment to the Surface Gaussian, taking into account the current parameters setting, is by solving our energy \(\mathbf E \) w.r.t. the colors of the Surface Gaussians, instead of averaging underlying pixels. By doing so, the obtained colors are best suited for the current setting as they take into account the actual Image Gaussians size, color and position that are later used for optimization.
The used temporal smoothing term analytically formulates smoothing in time based on a window of 3 frames, that might be insufficient to fully eliminate time inconsistencies, e.g. for videos captured at high frame rates. We would like to further investigate into the impact of a larger window size on the overall time consistency of the reconstruction. On top of that our temporal smoothing approach only keeps the optimized displacements smooth in time and cannot correct time inconsistencies in the original input geometry. This limitation is particularly visible in the sequence pop2lock.
7 Conclusions
Extending the original work presented in Robertini et al. (2014), we have presented an effective framework for performance capture of deforming meshes with fine-scale time-varying surface detail from multi-view video recordings. Our approach incorporates to the input coarse, over-smooth, meshes the fine-scale deformation present in the original video frames by deforming the input geometry to maximize photo-consistency on all vertex positions. We proposed a new model-to-image consistency energy function that uses an implicit representation of the deformable mesh using a collection of Gaussians for the surface and a set of Gaussians for the input images. The proposed formulation enables a smooth closed-form energy with implicit occlusion handling and analytic derivatives. We have extended the original optimization energy presented in Robertini et al. (2014) by adding an additional temporal smooth term that helps to avoid jittering and temporal inconsistencies in the output sequences.
We qualitatively and quantitatively evaluated our refinement strategy on 5 input sequences, initially obtained using 2 different state-of-the-art 3D reconstruction approaches: a template-free method, and template-based method. We demonstrate that in both cases the proposed method successfully recovers true fine-scale detailed geometry. Additionally, we have also shown the performance of our method on synthetic data, which we manually modeled and smoothed to create the ground truth. Results demonstrate that we successfully refine the distorted geometry to recover the original 3D model.
Acknowledgements
Open access funding provided by Max Planck Society. This research was funded by the ERC Starting Grant project CapReal (335545).
Supplementary material
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