A layer mask is a grayscale image attached to a specific layer; it is the same size as that layer. The mask’s pixels set the transparency of the corresponding pixels in the layer. A white pixel in the mask specifies that the corresponding pixel in the image is completely opaque, and a black pixel specifies full transparency. A gray pixel specifies partial opacity. The pixel’s color components are unchanged, so a pixel can be strongly colored and completely transparent at the same time. This means that in a layer with a layer mask, every pixel requires 4 bytes.
Figure 14-19. The Mask menu
Figure 14-20. Adding a mask to the layer