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The mhSkin2 Shader:

Introduction

The mhSkin2 shader is the result of my research into creating a skin shader with the benefits of a Sum of Gaussians approach, but with added user controls and greater speed. The result is a shader which can be used just as easily as the mhSOGskin shader, but is up to 70% faster in renders at the cost of full accuracy (although results are nearly identical to the naked eye). More options are exposed to the artist, so setup time can be a bit longer, but the results speak for themselves, and the default settings can safely be used for many cases. Recommended uses for the mhSkin2 shader are:

• “Hero” characters AND secondary/tertiary characters
• Cases where maximum physical accuracy can be sacrificed for greater render speed
• Cases where artists desire additional shader control 

Theory

The mhSkin2 shader is the results of MANY months of research, headaches, and tweaking to get the best possible balance of quality and speed out of a skin shader. Picking and choosing and testing and combining from almost a dozen popular skin models have resulted in a skin shader that is fast, good looking, easy to use, and easily customized. I forego the 6-layer Gaussian profile of the Sum of Gaussians shader in favor of an averaged 3-layer profile, carefully balanced to give similar (nearly identical) results to the full profile. I also opted to use an IOR In/Out method for calculating Fresnel falloff, which may be new to some artists. I’ve also chosen to expose the most requested parameters from those who tested the shader in its early stages. These include individual weighting for Diffuse, SSS, and Glossy layers, in addition to individual weights for the three layers of SSS. Access to all three SSS radi are available as well. My biggest goal was to retain a system where individual maps don’t have to be created for each layer of skin, and indeed this shader relies only on a single image texture to create the desired layers automatically within the shade

Settings

Base and Diffuse

Base Texture - The map that will be used for the SSS calculations. Diffuse Texture - Allows the user to plug in an additional map for makeup, dirt, or other materials that rest on top of the skin. Diffuse Weight - Controls the amount of diffuse mixed on top of the SSS. Can be controled with a scalar map. Diffuse Roughness - Controls the roughness of the diffuse component of the shader

SSS and Glossy

Overall SSS Weight - Controls the weighting of SSS in the shade Overall SSS Scale - Allows the user to change the scale of the shader as a whole to compensate for models and scenes using different world scales. IOR In/Out - The shader uses IOR in and out values to calculate a Fresnel value to use for reflections. IOR in represents the IOR of the medium outside of the skin (can be left at 1 for most cases) and IOR out is then used to control the Fresnel falloff of reflections. The equation is (IORin – IORout)^2 / (IORin + IORout)^2 = Fresnel Value. Sheen/Specular Color - Allows the user to control the color of glossy reflections. Sheen/Specular Weight - Controls the weighting of glossy reflections within the shader. Sheen/Specular Roughness - Controls the roughness of the glossy reflections in the shader. Normal Map Texture - Allows the user to plug in an RGB map to control surface normals. Normal Strength - Controls the strength of the normal mapping. Can be controled with a scalar map. Shallow/Mid/Deep SSS Weight - Allows fine control over the weighting of each SSS layer. Shallow/Mid/Deep SSS Radii - Allows the user to change the scattering radius of each individual layer. Because this shader uses the averages of measured values for these settings you shouldn’t touch them unless you know what you’re doing. The defaults should work fine in almost all cases