Colorizing the shadow penumbra with warm tones rather than grey makes rendered images more cinematic

In physically-naive rendering, shadows fade from key-light color to black. In film and in IQ’s practice, the penumbra zone is tinted warm (orange-red) rather than grey, because the eye/film perceives penumbras as slightly red-orange in reality. Practically: instead of multiplying diffuse by a scalar shadow factor, multiply by a vector that ramps differently in R, G, B channels — red decays more slowly than green/blue through the transition, creating an orange penumbra. Mixing linear and quadratic decay per channel controls the hue.

Examples

vec3 shd = vec3(sha, sha*sha, sha*sha); — red = linear, green/blue = quadratic; transition zone appears orange.

Assessment

A student replaces the vec3 shadow multiply with a scalar and reports the scene looks flat. Explain exactly what the colorized penumbra adds and which specific areas change most.