precision highp float;

#endif

​

varying vec2 pos;

​

uniform float minx;

uniform float maxx;

uniform float miny;

uniform float maxy;

​

const float MAX_ITERATIONS = 100.0;

​

const vec4 startColour = vec4(0.3, 0.0, 0.9, 1.0);

const vec4 endColour = vec4(0.8, 0.0, 0.5, 1.0);

​

// https://en.wikipedia.org/wiki/Mandelbrot_set#Computer_drawings

float iterateMandelbrot(vec2 coord) {

  // initialise x and y as 0

  float x = 0.0;

  float y = 0.0;

  // iterate for maximum iterations

  for(float i = 0.0; i < MAX_ITERATIONS; i ++) {

    // exit condition

    // return how many iterations as % of max

    if(x * x + y * y > 4.0) {

      return i/MAX_ITERATIONS;

    }

    float xtemp = x * x - y * y + coord.x;

    y = 2.0 * x * y + coord.y;

    x = xtemp;

  }

  return 1.0;

}

​

void main() {

  // lerp between the min and max coordinates

  float x = ((maxx - minx) * pos.x) + minx;

  float y = ((maxy - miny) * pos.y) + miny;

  float i = iterateMandelbrot(vec2(x, y));

  gl_FragColor = mix(startColour, endColour, i); // vec4(i, i, i, 1.0);

}