Randomness without random()

There's no Random.Next() in a shader — remember, the function has to give the same answer for the same input, and pixels can't share a random generator. Instead we use a hash: a function that scrambles its input so thoroughly the output looks random.

float hash(float2 p)
{
    return frac(sin(dot(p, float2(127.1, 311.7))) * 43758.5453);
}

Give it a position, get a pseudo-random number 0..1. The same position always returns the same value, so the noise doesn't flicker. (The magic numbers are just big, unrelated constants; sin of a huge number is chaotic.)

Combine with floor from the tiles lesson: every pixel in the same cell hashes the same cell ID → one random value per cell.