Understanding skin roughness
Thanks to Digital Human League member Koki Nagano for insight into understanding skin roughness:
Skin roughness/topology serves as a reserve of the tissue, providing the skin surface layer with elasticity through a network of very fine grooves and ridges which allow the skin to deform. These fine-scale structures vary in size and patterns throughout the body. The statistical variation of such structures, or roughness, is an important component to skin that affects it at many scales.
When light hits the skin, it is generally split into different components. Part of the light enters the skin, scatters, and comes out (better known as subsurface scattering). A small percentage of light scatters but does not penetrate (better known as diffuse). Finally, some of the light reflects back off the skin surface (better know as specular). This specular is the very first surface layer reflection and is greatly affected by the surface roughness.
When doing a scan of Emily at ICT, the face and textures are captured at 0.1mm of detail, which comes out as 6k maps (including displacement). Therefore, the geometry is able to get detail down to the pores and fine wrinkles. We call this the “meso” structure.
However, there is still a great more detail in skin between pores which breaks up the surface highlights even more. Traditionally one way to mimic this “micro” geometry break up was to use a BRDF such as Cook-Torrance or GGX. Since certain areas of the skin are shinier than others a roughness map is still needed for these to be used correctly.
In the Digital Emily 2.0 data set, ICT also provides a 16k displacement map through its micro- geometry scanning method. We call this the “micro” structure which is added to the “meso” scale map. This “micro” structure is able to capture detail down to 10 μm, yielding a great deal more geometric complexity.
In the “micro” surface reflection theory, the resulting roughness is simulated through raytracing a collection of mirror-like tiny facets called “micro-facets”. Although the acquired skin micro- geometry from our process is still an order of magnitude away to achieve this completely, this new level of detail achieves two things:
- Eliminates the dependency on specific BRDF models such as Cook-Torrance or GGX, which are used to simulate micro-surface reflections. Since the new displacement simulates specular break-up more through geometry instead of the BRDF, it reduces the roughness value even more. Therefore, the BRDF can now be replaced by a simple Blinn-Phong or Phong.
- Eliminates the need for a specular roughness map. Since the new displacement map is based on scans of different areas of the face, it will vary the roughness of the detail accordingly. Therefore, the specular roughness map can now be replaced by a constant value.