Quartz capabilities such as alpha compositing and shadows allow for interesting results without a lot of effort. So far you've seen these effects applied to the drawing of individual graphic objects. But what if you want to apply a shadow or perform alpha compositing to a group of objects as if it were a single graphic primitive? Quartz makes this possible through the use of transparency layers. Transparency layers group a set of drawing operations and then operate on that group as a whole. Sometimes transparency layers are referred to as transparency groups.

You can see one effect possible with transparency layers by comparing Figure 15.11 with Figure 15.12. Note that in Figure 15.11, each individual graphic primitive has its own shadow. But the graphic in Figure 15.12 is shadowed as a single object. The rectangles don't cast a shadow over those underneath, but instead the entire graphic has a single shadow. The shadow cast is a complex shape and is only drawn at the border of the graphic as a whole.

Figure 15.12: Shadows on objects as if they are a group