Fake subsurface scattering brightens silhouettes using the Fresnel term tinted by a flesh color

True subsurface scattering simulates light entering a translucent body and scattering inside. A cheap shader approximation: (1) Compute the Fresnel term — clamp(-dot(rd, nor), 0, 1) — which peaks at 1 at silhouettes, 0 at face-on surfaces. (2) Multiply by how much sun hits the surface (sun diffuse) to light only the sun-side silhouettes. (3) Tint with a reddish-orange flesh color. Add this to the diffuse accumulation. The result is a warm bright rim around sun-facing edges, simulating light leaking through the skin. IQ calls this ‘physically inspired’ — it is an artistic approximation that makes characters look alive.

Examples

float fres = clamp(1.0-dot(rd,nor), 0., 1.); col += fres*sunDiff*sunCol*vec3(0.3,0.1,0.02);

Assessment

Describe three regions of a creature silhouette where fake SSS brightens vs remains dark, and which controls each.