code: function initMasks() {
  	var i, mask,
  	    result = new Uint8Array( 0x10000 );
  
  	for ( i = 0; i < 0x10000; i++ ) {
  		mask = 3;
  		if ( i & 0x2 ) {
  			// right
  			mask <<= 2;
  		}
  		if ( i & 0x40 ) {
  			// bottom
  			mask <<= 4;
  		}
  
  		result[ i ] = mask;
  	}
  
  	return result;
  }
docs: Inititialze mask lookup table.
  Maps loopy_v values to bitmasks for attribute bytes to get the correct palette value.