Smoke grenade 2: light and a floor

Right now the smoke is a flat grey blob. Real smoke has a bright, sunlit side and a dark core. The recipe is the same one the clouds used: ask how much smoke lies between each point and the sun. One extra density lookup a little towards the sun is enough.

Here the shadow lookup uses baseD (the plain ball) and not the final density. Later we will carve bullet holes into the smoke, but a hole should not change where the shadows fall, and the cheaper function keeps the shader fast.

The smoke needs something behind it to be seen against. We add a floor with a checkerboard (so you can tell later that holes show the scene behind), a warm horizon haze, and the same sky. The floor is a plane at y = -1: for a ray that points down, t = (-1 - ro.y) / rd.y and the point is ro + rd * t. Checker: fmod(floor(g.x) + floor(g.y) + 100.0, 2.0) (the +100 keeps the number positive, and the same trick works in every shader language).

float sh = baseD(pos + sun * 0.3);
float3 lit = lerp(float3(0.22, 0.24, 0.3), float3(0.97, 0.94, 0.88), saturate(1.0 - sh * 1.3));
float a = 1.0 - exp(-d * dt * 6.0);
L += T * a * lit * 0.9;
T *= 1.0 - a;