Snow 1: a field of snow

Snow is a height field too, like the sea, but a frozen one. Rolling dunes from a couple of sines, plus noise at two sizes for the lumpy texture: big lumps (noise2(xz * 1.7)) and tiny grain (noise2(xz * 7.0)).

We march a ray over the surface with the same relaxed-step trick as the ocean, but now we stop as soon as we are within a hair of the snow (d < 0.002), and remember that we hit it.

The light is what makes snow look like snow, and it has two rules:

1. The sun is low, so the bumps cast long soft shading: max(dot(n, sun), 0.0).
2. Shadows are not grey, they are blue. Snow in shadow is lit by the blue sky, so blend from a blue shadow color to a warm white: lerp(shadow, lit, diff). This one choice is what makes it read as snow.

Distant snow melts into the sky with a bit of fog.

float diff = max(dot(n, sun), 0.0);
float3 shadow = float3(0.55, 0.66, 0.88);    // blue, from the sky
float3 lit = float3(1.0, 0.97, 0.92);        // warm, from the sun
col = float3(0.93, 0.96, 1.0) * lerp(shadow, lit, diff);