precision mediump float;

#endif

​

// Pixel position

varying vec2 pos;

​

// Uniforms set by filterShader

uniform sampler2D filter_background; // contains the image being filtered

uniform vec2 filter_res; // contains the image resolution in pixels

​

float is_empty(vec3 col) {

  // If cell is black, the sum of R, G, B is 0

  // Therefore 1-sum = 1, and step returns 1

  // If any channel has some colour, 1-sum < 1 and step returns 0

  return step(1., 1. - (col.r + col.g + col.b));

}

​

float edge_of_screen(vec2 pos) {

  float top = step(0., 1. - pos.y);

  float bottom = step(1., pos.y);

  float left = step(0., 1. - pos.x);

  float right = step(1., pos.x);

  return step(1., top + bottom + left + right);

}

​

void main() {

  // Figure out how far a pixel is

  // Zero stored in z for swizzle convenience

  vec3 normalRes = vec3(1./filter_res, 0.);

  // Get colour of current pixel

  vec3 curr = texture2D(filter_background, pos).rgb;

  vec3 up = texture2D(filter_background, pos - normalRes.zy).rgb;

  float curr_empty = is_empty(curr);

  float up_empty = is_empty(up);

  vec3 col = (up * curr_empty * (1. - up_empty)) + 

    curr * up_empty;

  // Output the cell (white = alive, black = dead)

  gl_FragColor = vec4(col, 1.);

}