void setup()
{
  size( 300, 300 );
  background( 0 );
}

void draw() {}

void mouseMoved()
{
  int mx = abs( ( 150 - mouseX ) >> 2 );
  for ( int i = 0; i < 151; i++ )
  {
    for ( int j = 0; j < 151; j++ )
    {
      int g = i * i + j * j;
      g *= g;
      g >>= mx;
      g &= 0xFF;
      if ( g > 0x7F ) g = 0x7F ^ g;
      stroke( g << 2, g, g << 2 );
      point( i, j );
      point( width - i, j );
      point( i, height - j );
      point( width - i, j );
      point( width - i, height - j );
    }
  }
}