uniform sampler2D uTexture;
varying vec2 vUv;
#define STEP 0.03125

bool shouldSpawn( vec2 p ) {
  return ( p.x < vUv.x && vUv.x < p.x + STEP && 
           p.y < vUv.y && vUv.y < p.y + STEP ); 
}

void main() {
  vec4 color = texture2D( uTexture, vUv ); // current value

  float x = 0.0;   // Create new pixels on top left
  if ( shouldSpawn( vec2( x, 1.0 - 2.0 * STEP ) ) ) { 
    color.r = 1.0;
  }

  gl_FragColor = color;
}