thebookofshaders/10/2d-random-mosaic.frag

// Author @patriciogv - 2015
// Title: Mosaic

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

float random (vec2 st) {
    return fract(sin(dot(st.xy,
                         vec2(12.9898,78.233)))*
        43758.5453123);
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;

    st *= 10.0; // Scale the coordinate system by 10
    vec2 ipos = floor(st);  // get the integer coords
    vec2 fpos = fract(st);  // get the fractional coords

    // Assign a random value based on the integer coord
    vec3 color = vec3(random( ipos ));

    // Uncomment to see the subdivided grid
    // color = vec3(fpos,0.0);

    gl_FragColor = vec4(color,1.0);
}