#version 300 es
precision highp float;

uniform float iTime;
uniform vec2  iResolution;

uniform float HistogramPreserve; // value=1, min=0, max=1, step=1 (No, Yes)

out vec4 fragColor;

float hash12(vec2 p) {
	vec3 p3  = fract(vec3(p.xyx) * .1031);
    p3 += dot(p3, p3.yzx + 33.33);
    return fract((p3.x + p3.y) * p3.z);
}

void main() {
    float n0 = hash12(gl_FragCoord.xy);
    float n1 = hash12(gl_FragCoord.yx + 0.5);
    float f = .5 + .5 * sin(gl_FragCoord.x / iResolution.x * 20. + 2. * iTime);
    
    float h = mix(n0, n1, f);
    float g = min(f, 1. - f); 
    float x = min(h, 1. - h);
    float v = (x < g) 
        ? x * x / (2. * g) 
        : x - 0.5 * g;
    float h1 = v / (1. - g);
    
    h1 = (h < 0.5) ? h1 : 1. - h1;
    
    vec3 col = vec3(mix(h, h1, HistogramPreserve));

    fragColor = vec4(col, 1.);
}