conway.frag

#ifdef GL_FRAGMENT_PRECISION_HIGH
precision highp float;
#else
precision mediump float;
#endif

uniform vec2 resolution;
uniform int pointerCount;
uniform vec3 pointers[10];
uniform sampler2D backbuffer;

float get(float x, float y) {
  return 2.0 * texture2D(
    backbuffer,
    (gl_FragCoord.xy + vec2(x, y)) / resolution).r;
}

vec3 evaluate(float sum) {
  float a = 1.0 - abs(clamp(sum - 3.0, -1.0, 1.0));
  float b = 1.0 - abs(clamp(sum - 2.0, -1.0, 1.0));
  return vec3(a + b * get(0.0, 0.0));
}

void main() {
  float sum =
    get(-1.0, -1.0) +
    get(-1.0, 0.0) +
    get(-1.0, 1.0) +
    get(0.0, -1.0) +
    get(0.0, 1.0) +
    get(1.0, -1.0) +
    get(1.0, 0.0) +
    get(1.0, 1.0);

  float tap = min(resolution.x, resolution.y) * 0.05;
  for (int n = 0; n < pointerCount; ++n) {
    if (distance(pointers[n].xy, gl_FragCoord.xy) < tap * 2.0) {
      sum = 3.0;
      break;
    }
  }

  gl_FragColor = vec4(0.5 * evaluate(sum), 1.0);
}