In a 2D image some areas often need to be invisible. Example is when ball in a breakout game then the image of the ball will have a background that should not be drawn to the screen. One way to solve this is to set a color as the chroma key and when drawing the screen the color is not drawn. These day transparent drawing is often used with a alpha layer in the image. So chroma keys can be used to create the mask. When setting the chroma key on a image the alpha layer is created and setting the alpha value based on the color value.