Programming with Quartz

Chapter 10 Image Masking

Broadly speaking, image masking is a process that controls the coverage of paint to apply when painting an image. Image masking is typically performed using an image mask. The terms image mask, masking image, and mask are interchangeable terms to describe an image whose sample values indicate a percentage of paint to apply but not the color of the paint itself. Such an image is sometimes called a stencil mask because the mask does not itself have any intrinsic color; instead, color “pours” through the stencil. An image mask has only one component value, the coverage value.

An image mask is not the only way in Quartz to accomplish image masking. Colors and images can also serve as masking devices. Each masking device—image ...