Pierre Poulin

A survey of shadow algorithms

The various types of shadows are characterized. Most existing shadow algorithms are described, and their complexities, advantages, and shortcomings are discussed. Hard shadows, soft shadows, shadows of transparent objects, and shadows for complex modeling primitives are considered. For each type, shadow algorithms within various rendering techniques are examined. The aim is to provide readers with enough background and insight on the various methods to allow them to choose the algorithm best suited to their needs and to help identify the areas that need more research and point to possible solutions.<<ETX>>

Particle-based viscoelastic fluid simulation

We present a new particle-based method for viscoelastic fluid simulation. We achieve realistic small-scale behavior of substances such as paint or mud as they splash on moving objects. Incompressibility and particle anti-clustering are enforced with a double density relaxation procedure which updates particle positions according to two opposing pressure terms. From this process surface tension effects emerge, enabling drop and filament formation. Elastic and non-linear plastic effects are obtained by adding springs with varying rest length between particles. We also extend the technique to handle interaction between fluid and dynamic objects. Various simulation scenarios are presented including rain drops, fountains, clay manipulation, and floating objects. The method is robust and stable, and can animate splashing behavior at interactive framerates.

A model for anisotropic reflection

A reflection and refraction model for anisotropic surfaces is introduced. The anisotropy is simulated by small cylinders (added or subtracted) distributed on the anisotropic surface. Different levels of anisotropy are achieved by varying the distance between each cylinder and/or rising the cylinders more or less from the surface. Multidirectional anisotropy is modelled by orienting groups of cylinders in different direction. The intensity of the reflected light is computed by determining the visible and illuminated portion of the cylinders, taking self-blocking into account. We present two techniques to compute this in practice. In one the intensity is computed by sampling the surface of the cylinders. The other is an analytic solution. In the case of the diffuse component, the solution is exact. In the case of the specular component, an approximation is developed using a Chebyshev polynomial approximation of the specular term, and integrating the polynomial.This model can be implemented easily within most rendering system, given a suitable mechanism to define and alter surface tangents. The effectiveness of the model and the visual importance of anisotropy are illustrated with some pictures.

Visualization-based analysis of quality for large-scale software systems

We propose an approach for complex software analysis based on visualization. Our work is motivated by the fact that in spite of years of research and practice, software development and maintenance are still time and resource consuming, and high-risk activities. The most important reason in our opinion is the complexity of many phenomena related to software, such as its evolution and its reliability. In fact, there is very little theory explaining them. Today, we have a unique opportunity to empirically study these phenomena, thanks to large sets of software data available through open-source programs and open repositories. Automatic analysis techniques, such as statistics and machine learning, are usually limited when studying phenomena with unknown or poorly-understood influence factors. We claim that hybrid techniques that combine automatic analysis with human expertise through visualization are excellent alternatives to them. In this paper, we propose a visualization framework that supports quality analysis of large-scale software systems. We circumvent the problem of size by exploiting perception capabilities of the human visual system.

Motion-motif graphs

We present a technique to automatically distill a motion-motif graph from an arbitrary collection of motion capture data. Motion motifs represent clusters of similar motions and together with their encompassing motion graph they lend understandable structure to the contents and connectivity of large motion datasets. They can be used in support of motion compression, the removal of redundant motions, and the creation of blend spaces. This paper develops a string-based motif-finding algorithm which allows for a user-controlled compromise between motif length and the number of motions in a motif. It allows for time warps within motifs and assigns the majority of the input data to relevant motifs. Results are demonstrated for large datasets (more than 100,000 frames) with computation times of tens of minutes.

Interactive Rendering of Trees with Shading and Shadows

The goal of this paper is the interactive rendering of 3D trees covering a landscape, with shading and shadows consistent with the lighting conditions. We propose a new IBR representation, consisting of a hierarchy of Bidirectional Textures, which resemble 6D lightfields. A hierarchy of visibility cube-maps is associated to this representation to improve the performance of shadow calculations. An example of hierarchy for a given tree can be a small branch plus its leaves (or needles), a larger branch, and the entire tree. A Bidirectional Texture (BT) provides a billboard image of a shaded object for each pair of view and light directions. We associate a BT for each level of the hierarchy. When rendering, the appropriate level of detail is selected depending on the distance of the tree from the viewpoint. The illumination reaching each level is evaluated using a visibility cube-map. Thus, we very efficiently obtain the shaded rendering of a tree with shadows without loosing details, contrary to mesh simplification methods. We achieved 7 to 20 fps fly-throughs of a scene with 1000 trees.

Lights from highlights and shadows

Designing the illumination of a scene is a di cult task because one needs to render the whole scene in order to look at the result. Obtaining the correct lighting e ects may require a long sequence of modeling/rendering steps. We propose to use directly the highlights and shadows in the modeling process. By creating and altering these lighting e ects, the lights themselves are indirectly modi ed. We believe this new technique to design lighting is more intuitive and can lead to a reduction of the number of modeling/rendering steps required to obtain the desired image. CR

Visual Detection of Design Anomalies

Design anomalies, introduced during software evolution, are frequent causes of low maintainability and low flexibility to future changes. Because of the required knowledge, an important subset of design anomalies is difficult to detect automatically, and therefore, the code of anomaly candidates must be inspected manually to validate them. However, this task is time- and resource-consuming. We propose a visualization-based approach to detect design anomalies for cases where the detection effort already includes the validation of candidates. We introduce a general detection strategy that we apply to three types of design anomaly. These strategies are illustrated on concrete examples. Finally we evaluate our approach through a case study. It shows that performance variability against manual detection is reduced and that our semi-automatic detection has good recall for some anomaly types.

Sketching shadows and highlights to position lights

In inverse shading, a user provides information about the desired shading as she/he would like it to appear in the final image. The computer then interprets this information to identify the best values for the various shading parameters that would lead to the desired visual effect. The authors introduce an approach based on sketching in order to position light sources. Point light sources are positioned by sketches of shadows or highlights. Extended light sources are positioned by sketches of umbra or penumbra. The resulting system allows one to quickly position light sources and to refine their positions interactively and more intuitively.

Interactive virtual relighting and remodeling of real scenes

Lighting design is often tedious due to the required physical manipulation of real light sources and objects. As an alternative, we present an interactive system to virtually modify the lighting and geometry of scenes with both real and synthetic objects, including mixed real/virtual lighting and shadows. In our method, real scene geometry is first approximately reconstructed from photographs. Additional images are taken from a single viewpoint with a real light in different positions to estimate reflectance. A filtering process is used to compensate for inaccuracies, and per image reflectances are averaged to generate an approximate reflectance image for the given viewpoint, removing shadows in the process. This estimate is used to initialise a global illumination hierarchical radiosity system, representing real-world secondary illumination; the system is optimized for interactive updates. Direct illumination from lights is calculated separately using ray-casting and a table for efficient reuse of data where appropriate. Our system allows interactive modification of light emission and object positions, all with mixed real/virtual illumination effects. Real objects can also be virtually removed using texture-filling algorithms for reflectance estimation.