Section: New Results

Texturing and rendereing

Participants : Ismael Garcia, Anass Lasram, Sylvain Lefebvre, Bruno Lévy, Vincent Nivoliers, Nicolas Ray, Dmitry Sokolov.

Texture synthesis

We developped new texturing tools (material space texturing) in cooperation with Greg Turk (Georgia Tech). Many objects have patterns that vary in appearance at different surface locations. We say that these are differences in materials, and we present a material-space approach for interactively designing such textures. At the heart of our approach is a new method to pre-calculate and use a 3D texture tile that is periodic in the spatial dimensions (s,t) and that also has a material axis along which the materials change smoothly. Given two textures and their feature masks, our algorithm produces such a tile in two steps. The first step resolves the features morphing by a level set advection approach, improved to ensure convergence. The second step performs the texture synthesis at each slice in material-space, constrained by the morphed feature masks. With such tiles, our system lets a user interactively place and edit textures on a surface, and in particular, allows the user to specify which material appears at given positions on the object. Additional operations include changing the scale and orientation of the texture. We support these operations by using a global surface parameterization that is closely related to quad re-meshing. Re-parameterization is performed on-the-fly whenever the user's constraints are modified. We published this result in Computer Graphics Forum [14] .

Matthaus Chajdas was an intern within the REVES team from April to September 2009. He was supervised by Sylvain Lefebvre and worked on an algorithm to help modelers texture large virtual environments. Modelers typically manually select a texture from a database of materials for each and every surface of a virtual environment. Our algorithm automatically propagates user input throughout the entire environment as the user is applying textures to it. After choosing textures for only a small subset if the surface, the entire scene is textured. This work was accepted by I3D 2010.

Subdivision and fractals

Boundary Controlled IFS is a new layer of control over traditional IFS, allowing to create a wide variety of shapes. Recently we have demonstrated how subdivision surfaces with extraordinary points may be generated by Boundary Controlled Iterated Function Systems means, as well as how we may get beyond the traditional subdivision schemes. Thanks to the method any subdivision scheme may be analyzed and it is even possible to construct new ones. Indeed, having programmed a single tool it is possible to implement any subdivision scheme (3D as well as 2D) just by feeding the tool with simple text files representing control graphs.

We currently study the relations between L-systems and subdivision, with Cedric Gérot, Viktor Ostromoukhov and Nicolas Stuart. The objective of this research project is to use L-systems to define the subdivision algorithms. Based on the blossoming theory it is possible to insert control points on a spline curve without changing its shape. Subdivision schemes are a particular case of this insertion, where many new control points are inserted at predefined positions. This enables the precomputation of the knot insertion algorithm to produce a subdivision mask. Irregular subdivision gets rid of the predefined position constraint, but prevents the construction of the subdivision mask, therefore reducing the amount of precomputation. We use L-systems to guide the knot insertions to enable the definition of non-uniform subdivision schemes, while still being able to precompute a subdivision mask.