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));

}

​

vec3 get_cell(vec2 pos) {

  return texture2D(filter_background, pos).rgb;

}

​

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 = get_cell(pos);

  vec3 left = get_cell(pos - normalRes.xz);

  vec3 right = get_cell(pos + normalRes.xz);

  vec3 up = get_cell(pos - normalRes.zy);

  vec3 up_left = get_cell(pos - normalRes.xz + normalRes.zy);

  vec3 up_right = get_cell(pos + normalRes.xy);

  vec3 down = get_cell(pos + normalRes.zy);

  vec3 down_left = get_cell(pos - normalRes.xy);

  vec3 down_right = get_cell(pos + normalRes.xz - normalRes.zy);

  float bottom_of_screen = step(0., pos.y);

  float curr_empty = is_empty(curr);

  float left_empty = is_empty(left);

  float right_empty = is_empty(right);

  float up_empty = is_empty(up);

  float up_left_empty = is_empty(up_left);

  float up_right_empty = is_empty(up_right);

  float down_empty = is_empty(down);

  float down_left_empty = is_empty(down_left);

  float down_right_empty = is_empty(down_right);

  float all_solid_below = 1. - step(1., down_empty + down_left_empty + down_right_empty);

  vec3 col = (curr * all_solid_below) + // current colour if the cells below are solid

    (1. - all_solid_below) * (

      up * (1.-up_empty)

    );

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

  gl_FragColor = vec4(col, 1.);

}