Blt（）创建图层效果。不工作。我使用错误的逻辑功能还是什么？

I have one window that is drawn to by various objects to create a layered effect (think of a heads up display where one object draws a compass, the other draws the grid lines, another the altimeter reading etc). So, each object has a black memory bitmap that it draws to. When I call that objects Draw function, the memory bitmap is blitted to the application window. The memory bitmaps are all black to begin with and the object draws on it. Black is the transparent color so it is masked off. The result is an overlay effect. I have one window that is drawn to by various o