Gradient Noise by Inigo Quilez - iq/2013 https://www.shadertoy.com/view/XdXGW8

noise.glsl

float noise(vec2 st) {
    vec2 i = floor(st);
    vec2 f = fract(st);

    vec2 u = f * f * (3.0 - 2.0 * f);

    return mix( mix( dot( random2(i + vec2(0.0, 0.0) ), f - vec2(0.0, 0.0) ),
                     dot( random2(i + vec2(1.0, 0.0) ), f - vec2(1.0, 0.0) ), u.x),
                mix( dot( random2(i + vec2(0.0, 1.0) ), f - vec2(0.0, 1.0) ),
                     dot( random2(i + vec2(1.0, 1.0) ), f - vec2(1.0, 1.0) ), u.x), u.y);
}