Snow 3: sparkle

The last ingredient is the one you notice when you walk in real snow: glints. Tiny ice crystals catch the sun at just the right angle and flash for a moment.

We cannot simulate every crystal, but we can fake their look. Chop the ground into a very fine grid (floor(pos.xz * 110.0) gives every little cell a number), ask the hash21 for a random value of the cell, and let only the luckiest 0.4% of cells sparkle: step(0.996, g).

If the glints stood still they would look like white dust. Give each its own flicker: 0.5 + 0.5 * sin(time * 6.0 + g * 40.0). The random number g works as a phase, so the cells blink out of step.

Finally, ice crystals only flash where the sun reaches, so multiply by how much the point is lit: saturate(diff * 2.0 + 0.2). Sparkle in shadow would look wrong.

float g = hash21(floor(pos.xz * 110.0));
float glint = step(0.996, g) * (0.5 + 0.5 * sin(time * 6.0 + g * 40.0));
col += float3(1.0, 1.0, 1.0) * glint * saturate(diff * 2.0 + 0.2);

Add this just after the snow color, before the fog (so distant sparkles fade away, too).