Clipping is the computer graphics term for choosing which parts of a graphical object to display or hide. In 2D graphics, it's very common to have one image to define parts of a screen to either only show other images, or to never show other images. Such an image is known as an image mask. In this recipe, we'll use two related images. One is an image of the inside of a room, showing a window out to a night skyscape. The second image is transparent, except for the rectangle where the window is. We'll use the second image to only allow other sprites to be seen when they pass by the window rectangle. The following figure shows how we can only see the parts of the moving blue bird sprite when it is overlapping with the window rectangle:
Figure 6.61 – Image mask being used to only show the bird when it flies through the window rectangle area of the screen
