code: public void clearAlphaMap() {
  		pushTransform();
  		GL.glLoadIdentity();
  		
  		int originalMode = currentDrawingMode;
  		setDrawMode(MODE_ALPHA_MAP);
  		setColor(new Color(0,0,0,0));
  		fillRect(0, 0, screenWidth, screenHeight);
  		setColor(currentColor);
  		setDrawMode(originalMode);
  		
  		popTransform();
  	}
docs: Clear the state of the alpha map across the entire screen. This sets
  alpha to 0 everywhere, meaning in {@link Graphics#MODE_ALPHA_BLEND}
  nothing will be drawn.