Principles
In developing our new physically-based reflectance model, we were cautioned by artists that we need
our shading model to be art-directable and not necessarily physically correct. Because of this, our
philosophy has been to develop a “principled” model rather than a strictly physical one.
These were the principles that we decided to follow when implementing our model:
1. Intuitive rather than physical parameters should be used.
2. There should be as few parameters as possible.
3. Parameters should be zero to one over their plausible range.
4. Parameters should be allowed to be pushed beyond their plausible range where it makes sense.
5. All combinations of parameters should be as robust and plausible as possible.
We thoroughly debated the addition of each parameter. In the end we ended up with 1 color
parameter and 10 scalar parameters described in the following section.
5.2
Parameters
• baseColor - the surface color, usually supplied by texture maps.
• subsurface - controls diffuse shape using a subsurface approximation.
• metallic - the metallic-ness (0 = dielectric, 1 = metallic). This is a linear blend between two
different models. The metallic model has no diffuse component and also has a tinted incident
specular, equal to the base color.
• specular - incident specular amount. This is in lieu of an explicit index-of-refraction.
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• specularTint - a concession for artistic control that tints incident specular towards the base color.
Grazing specular is still achromatic.
• roughness - surface roughness, controls both diffuse and specular response.
• anisotropic - degree of anisotropy. This controls the aspect ratio of the specular highlight. (0 =
isotropic, 1 = maximally anisotropic).
• sheen - an additional grazing component, primarily intended for cloth.
• sheenTint - amount to tint sheen towards base color.
• clearcoat - a second, special-purpose specular lobe.
• clearcoatGloss - controls clearcoat glossiness (0 = a “satin” appearance, 1 = a “gloss” appearance).
ref: Physically-Based Shading at Disney