Section: New Results

Complex scenes

Appearance pre-filtering

Participants : Heitz Eric, Neyret Fabrice.

Last year work and HPG'12 paper "Representing Appearance and Pre-filtering Subpixel Data in Sparse Voxel Octrees" was dealing with the light and view dependant aspect of complex surfaces due to sub-pixels details. This was done by replacing sub-voxel height fields by gaussian slope distribution and height-correlated colors by its gradient, feeding a Cook-Torrance-like microfacet brdf.

In continuation of this and in the same spirit of replacing sub-pixel values by gaussian distributions to be shaded using the frame of microfacets brdf, this year we addressed the filtering of color maps (on surfaces and per se), displacement map, and reflectance maps, thus obtaining a complete model of the local rendering integral (see Figure 11 ).

Note that Eric did his work partly during his 6-monthes stay of University of Montreal in the scope of Exploradoc regional founding. He also colaborated with nVIDIA for an on-going work related to animation of GigaVoxels, and we were invited for a stay of several weeks at Weta Digital, NZ to help them applying our techniques.

Filtering Color Mapped Textures and Surfaces

Participants : Heitz Eric, Neyret Fabrice, Nowrouzezahrai Derek, Poulin Pierre.

Indeed, several ubiquitus CG operations like filtering non-linear functions of the data are still mostly unsolved despite their important flaws. Typically, density, noise data, normals or height are filtered before feeding a color look-up texture, despite the strong non-linearity of the transform forbids factoring it out of the integral. This result on very visible flaws such as thin blue bones+air foams appearing as red muscle at distance in volume visualisation, silhouettes and horizon getting the middle tint instead of the integral of tints, procedural noise bump maps and height fields appearing as smooth instead of rough.

Assuming Gaussian distribution of colors within a pixel or voxel, the filtered colors values can be represented as color lobes (i.e. histogramms) instead of scalars. In all the cases where the subpixel/voxel raw data can also be represented as gaussian distribution (e.g. Perlin noise), the filtering is just the inner product of the two lobes. It can easily be tabuled as a 1D LUT MIP-map which LOD corresponds to the standard deviation and thus the scale. Since microfacet brdf models allow to estimate the visible slope statistics accounting for light and view visibility, this allows for emerging light-view dependant color effect both acurately and very efficiently. Note that the same scheme applies for colors corelated to orientation rather than heights (see Figure 12 ). This provides a multiscale representation where subpixel/subvoxel data is represented through lobes which can be precalculated or calculated on demand from the thiner level.

This work was published at ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games (I3D) 2013 [17] . An extended version "Filtering Non-Linear Transfer Functions on Surfaces" was published at IEEE Transactions on Visualization and Computer Graphics 2013 [11] .

Linear Efficient Antialiased Displacement and Reflectance Mapping

Participants : Dupuy Jonathan, Heitz Eric, Neyret Fabrice.

Here, the last term of the local rendering integration is addressed: the filtering of subpixel/subvoxel geometry and brdf as an appearant brdf applied on a macro-geometry. By re-derivating accurately the brdf of a displacement map assumed to have sub-pixel gaussian distribution (with an exact masking term, more accurate cross-correlated light-view, and offseted appearant lobe), and by noting that the reflectance of the environment can be pre-filtered like the textures of the previous paper, we finally obtain a complete model of pre-fitltered appearance of surfaces (see Figure 13 ). This work, co-first-authored with Jonathan Dupuy, was published at ACM Transactions on Graphics and presented at Siggraph Asia [8] .