uniform float uRandom; // JavaScript will change each frame

void main() {
  ...

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