skin rendering

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Ke-Sen Huang has recently added three papers relating to human face and skin rendering to his excellent list of SIGGRAPH Asia 2008 papers. Human faces are among the hardest objects to render realistically, since people are used to examining faces very closely.

The first two papers focus on modeling the effect of human skin layers on reflectance. The authors of the first paper, “Practical Modeling and Acquisition of Layered Facial Reflectance” work in Paul Debevec’s group at the USC Institue for Creative Technologies, which has done a lot of influential work on acquisition of reflectance from human faces (the results of which are now being offered as a commercial product). Previous work focused on polarization to separate reflectance into specular and diffuse. Here diffuse is further separated into single scattering, shallow multiple scattering, and deep multiple scattering (using structured light). Specular and diffuse albedo are captured per-pixel. Unfortunately specular roughness (lobe width) is only captured for each of several regions and not per-pixel, but since normals are captured at very high resolution they could presumably be used to generate per-pixel roughness values which could be useful in rendering at lower resolutions, as we discuss in Section 7.8.1 of Real-Time Rendering. The scattering model is based on the dipole approximation of subsurface scattering introduced by Henrik Wann Jensen and others. NVIDIA have shown real-time rendering of such models using multiple texture-space diffusion passes.

The authors of the second paper, “A Layered, Heterogeneous Reflectance Model for Acquiring and Rendering Human Skin” have also written several important papers on skin reflectance, focused more on simulating physical processes from first principles. They model human skin as a collection of heterogeneous scattering layers separated by infinitesimally thin heterogeneous absorbing layers. They design their model for efficient GPU evaluation, similar to NVIDIA’s approach mentioned above (one of this paper’s authors also worked on the NVIDIA skin demo). “Efficient” here is a relative term, since their model is too complex to be real-time on current hardware, and as presented is probably too complicated for game use. However, ideas gleaned from this paper are likely to be useful for skin rendering in games. The authors also present a protocol for measuring the parameters of their model

The third paper, “Facial Performance Synthesis Using Deformation-Driven Polynomial Displacement Maps” is also from Debevec’s USC group and focuses on animation rather than reflectance. They use the same facial capturing setup, but with different software to capture animated facial deformations instead of reflectance (this too has been turned into a commercial product). This paper is interesting because it extends previous coarse / fine deformation approaches to multiple scales, and uses a novel method to relate the different scales to each other. They use a polynomial displacement map, which uses the same form as Polynomial Texture Mapping (an interesting technique in its own right) but for deformation rather than shading. This method also bears some resemblance to the wrinkle map approach used by AMD for their Ruby Whiteout demo, which they presented at SIGGRAPH 2007.

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