EG 2016 - Full Papers - CGF 35-Issue 2

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https://diglib.eg.org/handle/10.2312/15064

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Generalized As-Similar-As-Possible Warping with Applications in Digital Photography
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Chen, Renjie; Gotsman, Craig; Joaquim Jorge and Ming Lin
Discrete conformal mappings of planar triangle meshes, also known as the As-Similar-As-Possible (ASAP) mapping, involve the minimization of a quadratic energy function, thus are very easy to generate and are popular in image warping scenarios. We generalize this classical mapping to the case of quad meshes, taking into account the mapping of the interior of the quad, and analyze in detail the most common case - the unit grid mesh. We show that the generalization, when combined with barycentric coordinate mappings between the source and target polygons, spawns an entire family of new mappings governed by quadratic energy functions, which allow to control quite precisely various effects of the mapping. This approach is quite general and applies also to arbitrary planar polygon meshes. As an application of generalized ASAP mappings of the unit grid mesh, we demonstrate how they can be used to warp digital photographs to achieve a variety of effects. One such effect is modifying the perspective of the camera that took a given photograph (without moving the camera). A related, but more challenging, effect is re-photography - warping a contemporary photograph in order to reproduce the camera view present in a vintage photograph of the same scene - taken many years before with a different camera from a different viewpoint. We apply the generalized ASAP mapping to these images, discretized to a unit grid. Using a quad mesh (as opposed to a triangle mesh) permits biasing towards affine maps of the unit squares. This allows the introduction of an As-Affine-As-Possible (AAAP) mapping for a good approximation of the homographies present in these warps, achieving quite accurate results. We demonstrate the advantages of the AAAP mapping on a variety of synthetic and real-world examples.
Data-guided Model Predictive Control Based on Smoothed Contact Dynamics
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Han, Daseong; Eom, Haegwang; Noh, Junyong; Shin, Joseph S. (formerly Sung Yong); Joaquim Jorge and Ming Lin
In this paper, we propose an efficient data-guided method based on Model Predictive Control (MPC) to synthesize a full-body motion. Guided by a reference motion, our method repeatedly plans the full-body motion to produce an optimal control policy for predictive control while sliding the fixed-span window along the time axis. Based on this policy, the method computes the joint torques of a character at every time step. Together with contact forces and external perturbations if there are any, the joint torques are used to update the state of the character. Without including the contact forces in the control vector, our formulation of the trajectory optimization problem enables automatic adjustment of contact timings and positions for balancing in response to environmental changes and external perturbations. For efficiency, we adopt derivative-based trajectory optimization on top of state-of-the-art smoothed contact dynamics. Use of derivatives enables our method to run much faster than the existing sampling-based methods. In order to further accelerate the performance of MPC, we propose efficient numerical differentiation of the system dynamics of a full-body character based on two schemes: data reuse and data interpolation. The former scheme exploits data dependency to reuse physical quantities of the system dynamics at near-by time points. The latter scheme allows the use of derivatives at sparse sample points to interpolate those at other time points in the window. We further accelerate evaluation of the system dynamics by exploiting the sparsity of physical quantities such as Jacobian matrix resulting from the tree-like structure of the articulated body. Through experiments, we show that the proposed method efficiently can synthesize realistic motions such as locomotion, dancing, gymnastic motions, and martial arts at interactive rates using moderate computing resources.
Multi-Resolution Meshes for Feature-Aware Hardware Tessellation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Lambert, Thibaud; Bénard, Pierre; Guennebaud, Gaël; Joaquim Jorge and Ming Lin
Hardware tessellation is de facto the preferred mechanism to adaptively control mesh resolution with maximal performances. However, owing to its fixed and uniform pattern, leveraging tessellation for feature-aware LOD rendering remains a challenging problem. We relax this fundamental constraint by introducing a new spatial and temporal blending mechanism of tessellation levels, which is built on top of a novel hierarchical representation of multi-resolution meshes. This mechanism allows to finely control topological changes so that vertices can be removed or added at the most appropriate location to preserve geometric features in a continuous and artifact-free manner. We then show how to extend edge-collapse based decimation methods to build feature-aware multi-resolution meshes that match the tessellation patterns. Our approach is fully compatible with current hardware tessellators and only adds a small overhead on memory consumption and tessellation cost.
A Practical Method for High-Resolution Embedded Liquid Surfaces
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Goldade, Ryan; Batty, Christopher; Wojtan, Chris; Joaquim Jorge and Ming Lin
Combining high-resolution level set surface tracking with lower resolution physics is an inexpensive method for achieving highly detailed liquid animations. Unfortunately, the inherent resolution mismatch introduces several types of disturbing visual artifacts. We identify the primary sources of these artifacts and present simple, efficient, and practical solutions to address them. First, we propose an unconditionally stable filtering method that selectively removes sub-grid surface artifacts not seen by the fluid physics, while preserving fine detail in dynamic splashing regions. It provides comparable results to recent error-correction techniques at lower cost, without substepping, and with better scaling behavior. Second, we show how a modified narrow-band scheme can ensure accurate free surface boundary conditions in the presence of large resolution mismatches. Our scheme preserves the efficiency of the narrow-band methodology, while eliminating objectionable stairstep artifacts observed in prior work. Third, we demonstrate that the use of linear interpolation of velocity during advection of the high-resolution level set surface is responsible for visible grid-aligned kinks; we therefore advocate higher-order velocity interpolation, and show that it dramatically reduces this artifact. While these three contributions are orthogonal, our results demonstrate that taken together they efficiently address the dominant sources of visual artifacts arising with high-resolution embedded liquid surfaces; the proposed approach offers improved visual quality, a straightforward implementation, and substantially greater scalability than competing methods.
Anisotropic Diffusion Descriptors
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Boscaini, Davide; Masci, Jonathan; Rodolà, Emanuele; Bronstein, Michael M.; Cremers, Daniel; Joaquim Jorge and Ming Lin
Spectral methods have recently gained popularity in many domains of computer graphics and geometry processing, especially shape processing, computation of shape descriptors, distances, and correspondence. Spectral geometric structures are intrinsic and thus invariant to isometric deformations, are efficiently computed, and can be constructed on shapes in different representations. A notable drawback of these constructions, however, is that they are isotropic, i.e., insensitive to direction. In this paper, we show how to construct direction-sensitive spectral feature descriptors using anisotropic diffusion on meshes and point clouds. The core of our construction are directed local kernels acting similarly to steerable filters, which are learned in a task-specific manner. Remarkably, while being intrinsic, our descriptors allow to disambiguate reflection symmetries. We show the application of anisotropic descriptors for problems of shape correspondence on meshes and point clouds, achieving results significantly better than state-of-the-art methods.
Space-Time Co-Segmentation of Articulated Point Cloud Sequences
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Yuan, Qing; Li, Guiqing; Xu, Kai; Chen, Xudong; Huang, Hui; Joaquim Jorge and Ming Lin
Consistent segmentation is to the center of many applications based on dynamic geometric data. Directly segmenting a raw 3D point cloud sequence is a challenging task due to the low data quality and large inter-frame variation across the whole sequence. We propose a local-to-global approach to co-segment point cloud sequences of articulated objects into near-rigid moving parts. Our method starts from a per-frame point clustering, derived from a robust voting-based trajectory analysis. The local segments are then progressively propagated to the neighboring frames with a cut propagation operation, and further merged through all frames using a novel space-time segment grouping technqiue, leading to a globally consistent and compact segmentation of the entire articulated point cloud sequence. Such progressive propagating and merging, in both space and time dimensions, makes our co-segmentation algorithm especially robust in handling noise, occlusions and pose/view variations that are usually associated with raw scan data.
SAH Guided Spatial Split Partitioning for Fast BVH Construction
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Ganestam, Per; Doggett, Michael; Joaquim Jorge and Ming Lin
We present a new SAH guided approach to subdividing triangles as the scene is coarsely partitioned into smaller sets of spatially coherent triangles. Our triangle split approach is integrated into the partitioning stage of a fast BVH construction algorithm, but may as well be used as a stand alone pre-split pass. Our algorithm significantly reduces the number of split triangles compared to previous methods, while at the same time improving ray tracing performance compared to competing fast BVH construction techniques. We compare performance on Intel's Embree ray tracer and show that BVH construction with our splitting algorithm is always faster than Embree's pre-split construction algorithm. We also show that our algorithm builds significantly improved quality trees that deliver higher ray tracing performance. Our algorithm is implemented into Embree's open source ray tracing framework, and the source code will be released late 2015.
Learning 3D Deformation of Animals from 2D Images
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Kanazawa, Angjoo; Kovalsky, Shahar; Basri, Ronen; Jacobs, David; Joaquim Jorge and Ming Lin
Understanding how an animal can deform and articulate is essential for a realistic modification of its 3D model. In this paper, we show that such information can be learned from user-clicked 2D images and a template 3D model of the target animal. We present a volumetric deformation framework that produces a set of new 3D models by deforming a template 3D model according to a set of user-clicked images. Our framework is based on a novel locally-bounded deformation energy, where every local region has its own stiffness value that bounds how much distortion is allowed at that location. We jointly learn the local stiffness bounds as we deform the template 3D mesh to match each user-clicked image. We show that this seemingly complex task can be solved as a sequence of convex optimization problems. We demonstrate the effectiveness of our approach on cats and horses, which are highly deformable and articulated animals. Our framework produces new 3D models of animals that are significantly more plausible than methods without learned stiffness.
Single Image Weathering via Exemplar Propagation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Iizuka, Satoshi; Endo, Yuki; Kanamori, Yoshihiro; Mitani, Jun; Joaquim Jorge and Ming Lin
This paper presents an efficient approach for generating weathering effects with detailed appearance variations in a single image. Previous approaches merely change chroma or reflectance of weathered objects, which is not sufficient for materials with detailed shading and texture variations, such as growing moss and peeling plaster. Our method propagates such detailed features via seamless patch-based synthesis driven by weathering degree distribution. Unlike previous methods, the weathering degrees are calculated efficiently using Radial Basis Functions even for materials with wide color variations. We use graph cut-based optimization to identify the most weathered region as a ''weathering exemplar'', from which we sample weathering patches. We demonstrate our method enables us to generate various types of detailed weathering effects interactively.
Dexterous Manipulation of Cloth
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Bai, Yunfei; Yu, Wenhao; Liu, C. Karen; Joaquim Jorge and Ming Lin
This paper introduces a new technique to synthesize dexterous manipulation of cloth. Given a simple description of the desired cloth motion, our algorithm computes appropriate joint torques for physically simulated hands, such that, via contact forces, the result of cloth simulation follows the desired motion. Instead of optimizing the hand control forces directly, we formulate an optimization problem that solves for the commanding forces from the hands to the cloth, which have more direct impact on the dynamic state of the hands and that of the cloth. The solution of the optimization provides commanding forces that achieve the desired cloth motion described by the user, while respecting the kinematic constraints of the hands. These commanding forces are then used to guide the joint torques of the hands. To balance between the effectiveness of control and computational costs, we formulate a model-predictive-control problem as a quadratic program at each time step. We demonstrate our technique on a set of cloth manipulation tasks in daily activities, including folding laundry, wringing a towel, and putting on a scarf.
EUROGRAPHICS 2016: CGF 35-2 Frontmatter
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Joaquim Jorge; Ming Lin;
CAMA: Contact-Aware Matrix Assembly with Unified Collision Handling for GPU-based Cloth Simulation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Tang, Min; Wang, Huamin; Tang, Le; Tong, Ruofeng; Manocha, Dinesh; Joaquim Jorge and Ming Lin
We present a novel GPU-based approach to robustly and efficiently simulate high-resolution and complexly layered cloth. The key component of our formulation is a parallelized matrix assembly algorithm that can quickly build a large and sparse matrix in a compressed format and accurately solve linear systems on GPUs. We also present a fast and integrated solution for parallel collision handling, including collision detection and response computations, which utilizes spatio-temporal coherence. We combine these algorithms as part of a new cloth simulation pipeline that incorporates contact forces into implicit time integration for collision avoidance. The entire pipeline is implemented on GPUs, and we evaluate its performance on complex benchmarks consisting of 100 - 300K triangles. In practice, our system takes a few seconds to simulate one frame of a complex cloth scene, which represents significant speedups over prior CPU and GPU-based cloth simulation systems.
BlendForces: A Dynamic Framework for Facial Animation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Barrielle, Vincent; Stoiber, Nicolas; Cagniart, Cédric; Joaquim Jorge and Ming Lin
In this paper we present a new paradigm for the generation and retargeting of facial animation. Like a vast majority of the approaches that have adressed these topics, our formalism is built on blendshapes. However, where prior works have generally encoded facial geometry using a low dimensional basis of these blendshapes, we propose to encode facial dynamics by looking at blendshapes as a basis of forces rather than a basis of shapes. We develop this idea into a dynamic model that naturally combines the blendshapes paradigm with physics-based techniques for the simulation of deforming meshes. Because it escapes the linear span of the shape basis through time-integration and physics-inspired simulation, this approach has a wider expres- sive range than previous blendshape-based methods. Its inherent physically-based formulation also enables the simulation of more advanced physical interactions, such as collision responses on lip contacts.
Motion Grammars for Character Animation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Hyun, Kyunglyul; Lee, Kyungho; Lee, Jehee; Joaquim Jorge and Ming Lin
The behavioral structure of human movements is imposed by multiple sources, such as rules, regulations, choreography, habits, and emotion. Our goal is to identify the behavioral structure in a specific application domain and create a novel sequence of movements that abide by structure-building rules. To do so, we exploit the ideas from formal language, such as rewriting rules and grammar parsing, and adapted those ideas to synthesize the three-dimensional animation of multiple characters. The structured motion synthesis using motion grammars is formulated in two layers. The upper layer is a symbolic description that relates the semantics of each individual's movements and the interaction among them. The lower layer provides spatial and temporal contexts to the animation. Our multi-level MCMC (Markov Chain Monte Carlo) algorithm deals with the syntax, semantics, and spatiotemporal context of human motion to produce highly-structured, animated scenes. The power and effectiveness of motion grammars are demonstrated in animating basketball games from drawings on a tactic board. Our system allows the user to position players and draw out tactical plans, which are animated automatically in virtual environments with three-dimensional, full-body characters.
Adapting Feature Curve Networks to a Prescribed Scale
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Gehre, Anne; Lim, Isaak; Kobbelt, Leif; Joaquim Jorge and Ming Lin
Feature curves on surface meshes are usually defined solely based on local shape properties such as dihedral angles and principal curvatures. From the application perspective, however, the meaningfulness of a network of feature curves also depends on a global scale parameter that takes the distance between feature curves into account, i.e., on a coarse scale, nearby feature curves should be merged or suppressed if the surface region between them is not representable at the given scale/resolution. In this paper, we propose a computational approach to the intuitive notion of scale conforming feature curve networks where the density of feature curves on the surface adapts to a global scale parameter. We present a constrained global optimization algorithm that computes scale conforming feature curve networks by eliminating curve segments that represent surface features, which are not compatible to the prescribed scale. To demonstrate the usefulness of our approach we apply isotropic and anisotropic remeshing schemes that take our feature curve networks as input. For a number of example meshes, we thus generate high quality shape approximations at various levels of detail.
Convolutional Sparse Coding for High Dynamic Range Imaging
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Serrano, Ana; Heide, Felix; Gutierrez, Diego; Wetzstein, Gordon; Masia, Belen; Joaquim Jorge and Ming Lin
Current HDR acquisition techniques are based on either (i) fusing multibracketed, low dynamic range (LDR) images, (ii) modifying existing hardware and capturing different exposures simultaneously with multiple sensors, or (iii) reconstructing a single image with spatially-varying pixel exposures. In this paper, we propose a novel algorithm to recover high-quality HDRI images from a single, coded exposure. The proposed reconstruction method builds on recently-introduced ideas of convolutional sparse coding (CSC); this paper demonstrates how to make CSC practical for HDR imaging. We demonstrate that the proposed algorithm achieves higher-quality reconstructions than alternative methods, we evaluate optical coding schemes, analyze algorithmic parameters, and build a prototype coded HDR camera that demonstrates the utility of convolutional sparse HDRI coding with a custom hardware platform.
Interactive Videos: Plausible Video Editing using Sparse Structure Points
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Chang, Chia-Sheng; Chu, Hung-Kuo; Mitra, Niloy J.; Joaquim Jorge and Ming Lin
Video remains the method of choice for capturing temporal events. However, without access to the underlying 3D scene models, it remains difficult to make object level edits in a single video or across multiple videos. While it may be possible to explicitly reconstruct the 3D geometries to facilitate these edits, such a workflow is cumbersome, expensive, and tedious. In this work, we present a much simpler workflow to create plausible editing and mixing of raw video footage using only sparse structure points (SSP) directly recovered from the raw sequences. First, we utilize user-scribbles to structure the point representations obtained using structure-from-motion on the input videos. The resultant structure points, even when noisy and sparse, are then used to enable various video edits in 3D, including view perturbation, keyframe animation, object duplication and transfer across videos, etc. Specifically, we describe how to synthesize object images from new views adopting a novel image-based rendering technique using the SSPs as proxy for the missing 3D scene information. We propose a structure-preserving image warping on multiple input frames adaptively selected from object video, followed by a spatio-temporally coherent image stitching to compose the final object image. Simple planar shadows and depth maps are synthesized for objects to generate plausible video sequence mimicking real-world interactions. We demonstrate our system on a variety of input videos to produce complex edits, which are otherwise difficult to achieve.
Structure-adaptive Shape Editing for Man-made Objects
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Fu, Qiang; Chen, Xiaowu; Su, Xiaoyu; Li, Jia; Fu, Hongbo; Joaquim Jorge and Ming Lin
One of the challenging problems for shape editing is to adapt shapes with diversified structures for various editing needs. In this paper we introduce a shape editing approach that automatically adapts the structure of a shape being edited with respect to user inputs. Given a category of shapes, our approach first classifies them into groups based on the constituent parts. The group-sensitive priors, including both inter-group and intra-group priors, are then learned through statistical structure analysis and multivariate regression. By using these priors, the inherent characteristics and typical variations of shape structures can be well captured. Based on such group-sensitive priors, we propose a framework for real-time shape editing, which adapts the structure of shape to continuous user editing operations. Experimental results show that the proposed approach is capable of both structure-preserving and structure-varying shape editing.
Compressed Multiresolution Hierarchies for High-Quality Precomputed Shadows
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Scandolo, Leonardo; Bauszat, Pablo; Eisemann, Elmar; Joaquim Jorge and Ming Lin
The quality of shadow mapping is traditionally limited by texture resolution. We present a novel lossless compression scheme for high-resolution shadow maps based on precomputed multiresolution hierarchies. Traditional multiresolution trees can compactly represent homogeneous regions of shadow maps at coarser levels, but require many nodes for fine details. By conservatively adapting the depth map, we can significantly reduce the tree complexity. Our proposed method offers high compression rates, avoids quantization errors, exploits coherency along all data dimensions, and is well-suited for GPU architectures. Our approach can be applied for coherent shadow maps as well, enabling several applications, including high-quality soft shadows and dynamic lights moving on fixed-trajectories.
Fast and Robust Inversion-Free Shape Manipulation
(The Eurographics Association and John Wiley & Sons Ltd., 2016) Liu, Tiantian; Gao, Ming; Zhu, Lifeng; Sifakis, Eftychios; Kavan, Ladislav; Joaquim Jorge and Ming Lin
We present a shape manipulation technique capable of producing deformations of 2D and 3D meshes, guaranteeing that no elements will be inverted. We achieve this by augmenting the quadratic ex-rotated elastic energy with additional convex terms that penalize the presence of inverted elements. Using a schedule of increasing penalty coefficients, we efficiently and robustly converge to an inversion free state by solving a sequence of unconstrained convex minimization problems. This process can be interpreted as a special purpose Semi-Definite Programming (SDP) solver. We demonstrate that our method outperforms solvers used in previous work, including commercial-grade SDP software (MOSEK). As an additional benefit, our method also converges to the solution via a more intuitive path, which can be used for quick preview. We demonstrate the efficacy of our scheme in a number of 2D and 3D shapes undergoing moderate to drastic deformation.

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