About noise in shaders

Question

When I use return fract(sin(dot(uv,vec2(18.13,98.16)))*19978.13); to create random.
Why is there such a clear boundary?

Answer 1

Assuming that the uv (vec2) values are continuous, I would guess that this is caused by the fract function. What it does is t-floor(t), but the floor function is not continuous. That in turn makes the fract a not continuous function too.

This kind of function seems to be discussed here.

The Perlin noise is a common source of pseudo randomness.