Jiří Žára

Geometric skinning with approximate dual quaternion blending

Skinning of skeletally deformable models is extensively used for real-time animation of characters, creatures and similar objects. The standard solution, linear blend skinning, has some serious drawbacks that require artist intervention. Therefore, a number of alternatives have been proposed in recent years. All of them successfully combat some of the artifacts, but none challenge the simplicity and efficiency of linear blend skinning. As a result, linear blend skinning is still the number one choice for the majority of developers. In this article, we present a novel skinning algorithm based on linear combination of dual quaternions. Even though our proposed method is approximate, it does not exhibit any of the artifacts inherent in previous methods and still permits an efficient GPU implementation. Upgrading an existing animation system from linear to dual quaternion skinning is very easy and has a relatively minor impact on runtime performance.

Skinning with dual quaternions

Skinning of skeletally deformable models is extensively used for real-time animation of characters, creatures and similar objects. The standard solution, linear blend skinning, has some serious drawbacks that require artist intervention. Therefore, a number of alternatives have been proposed in recent years. All of them successfully combat some of the artifacts, but none challenge the simplicity and efficiency of linear blend skinning. As a result, linear blend skinning is still the number one choice for the majority of developers. In this paper, we present a novel GPU-friendly skinning algorithm based on dual quaternions. We show that this approach solves the artifacts of linear blend skinning at minimal additional cost. Upgrading an existing animation system (e.g., in a videogame) from linear to dual quaternion skinning is very easy and has negligible impact on run-time performance.

Spherical blend skinning: a real-time deformation of articulated models

Skin deformation based on an underlying skeleton is a common method to animate believable organic models. The most widely used skeletal animation algorithm, linear blend skinning, is also known as skeleton subspace deformation, vertex blending, or enveloping. It runs in real-time even on a low-end hardware but it is also notorious for its failures, such as the collapsing-joints artifacts. We present a new algorithm which removes these shortcomings while maintaining almost the same time and memory complexity as the linear blend skinning. Unlike other approaches, our method works with exactly the same input data as the popular linear version. This minimizes the cost of upgrade from linear to spherical blend skinning in many existing applications: the data structures and models need no change at all. The paper discusses also theoretical properties of rotation interpolation, essential to spherical blend skinning.

Unsupervised colorization of black-and-white cartoons

We present a novel color-by-example technique which combines image segmentation, patch-based sampling and probabilistic reasoning. This method is able to automate colorization when new color information is applied on the already designed black-and-white cartoon. Our technique is especially suitable for cartoons digitized from classical celluloid films, which were originally produced by a paper or cel based method. In this case, the background is usually a static image and only the dynamic foreground needs to be colored frame-by-frame. We also assume that objects in the foreground layer consist of several well visible outlines which will emphasize the shape of homogeneous regions.

Polypostors: 2D polygonal impostors for 3D crowds

Various methods have been proposed to animate and render large crowds of humans in real time for applications such as games and interactive walkthroughs. Recent methods have been developed to render large numbers of pre-computed image-based human representations (Impostors) by exploiting commodity graphics hardware, thus achieving very high frame-rates while maintaining visual fidelity. Unfortunately, these images consume a lot of texture memory, no in-betweening is possible, and the variety of animations that can be shown is severely restricted. This paper proposes an alternative method that significantly improves upon pre-computed impostors: automatically generated 2D polygonal characters (or Polypostors). When compared with image-based crowd rendering systems, Polypostors exhibit a similarly high level of rendering efficiency and visual fidelity, with considerably lower memory requirements (up to a factor of 30 in our test cases). Furthermore, Polypostors enable simple in-betweening and can thus deliver a greater variety of animations at any required level of smoothness with almost no overhead.

Fast Collision Detection for Skeletally Deformable Models

We present a new method of collision detection for models deformed by linear blend skinning. The linear blend skinning (also known as skeleton-subspace deformation, vertex-blending, or enveloping) is a popular method to animate believable organic models. We consider an exact collision detection based on a hierarchy of bounding spheres. The main problem with this approach is the update of bounding volumes – they must follow the current deformation of the model. We introduce a new fast method to refit the bounding spheres, which can be executed on spheres in any order. Thanks to this on-demand refitting operation we obtain a collision detection algorithm with speed comparable to the standard rigid body collision detection. The algorithm was tested on a variety of practical situations, including an animated crowd. According to these experiments, the proposed approach is considerably faster than the previous method.

Making radiance and irradiance caching practical: adaptive caching and neighbor clamping

Radiance and irradiance caching are efficient global illumination algorithms based on interpolating indirect illumination from a sparse set of cached values. In this paper we propose an adaptive algorithm for guiding spatial density of the cached values in radiance and irradiance caching. The density is adapted to the rate of change of indirect illumination in order to avoid visible interpolation artifacts and produce smooth interpolated illumination. In addition, we discuss some practical problems arising in the implementation of radiance and irradiance caching, and propose techniques for solving those problems. Namely, the neighbor clamping heuristic is proposed as a robust means for detecting small sources of indirect illumination and for dealing with problems caused by ray leaking through small gaps between adjacent polygons.

Fast robust BSP tree traversal algorithm for ray tracing

An orthogonal BSP (binary space partitioning) tree is a commonly used spatial subdivision data structure for ray-tracing acceleration. While the construction of a BSP tree takes a relatively short time, the efficiency of a traversal algorithm significantly influences the overall rendering time. We propose a new fast traversal algorithm based on statistical evaluation of all possible cases occuring during the traversal of a BSP tree. More frequent cases are handled simply, while less frequent ones are more computationally expensive. The proposed traversal algorithm handles all singularities correctly, and saves between 30% and 50% of traversal time compared with the commonly-known Sung and Arvo algorithms.

Skinning arbitrary deformations

Matrix palette skinning (also known as skeletal subspace deformation) is a very popular real-time animation technique. So far, it has only been applied to the class of quasi-articulated objects, such as moving human or animal figures. In this paper, we demonstrate how to automatically construct skinning approximations of arbitrary precomputed animations, such as those of cloth or elastic materials. In contrast to previous approaches, our method is particularly well suited to input animations without rigid components. Our transformation fitting algorithm finds optimal skinning transformations (in a least-squares sense) and therefore achieves considerably higher accuracy for non-quasi-articulated objects than previous methods. This allows the advantages of skinned animations (e.g., efficient rendering, rest-pose editing and fast collision detection) to be exploited for arbitrary deformations.

Segmentation of black and white cartoons

We introduce novel semi-automatic, fast and accurate segmentation technique that allow us to simplify color transfer to the old black and white cartoons produced by classical paper or foil technology, where foreground parts are represented by homogeneous regions with constant grey-scale intensity surrounded by bold dark contours. We assume that original analogue material has been converted to the sequence of digital grey-scale images with PAL resolution suitable for TV broadcasting.