groundColor property
Optional lower-hemisphere color for ambient lights.
When set, the ambient term becomes a two-color hemisphere: surfaces facing up receive color, surfaces facing down receive groundColor, and the two blend linearly with the world-space normal's Y component. This models sky light plus ground bounce (a fill light from below) without any extra light passes. Null keeps a flat, direction-independent ambient. Ignored for non-ambient light types.
Implementation
Color? groundColor;