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CGLayer Drawing

CGLayer objects (CGLayerRef data type), available in Mac OS X v. 10.4 and later, allow your application to use layers for drawing. CGLayer objects are suited for the following:

• High-quality offscreen rendering of drawing that you plan to reuse. For example, you might be building a scene and plan to reuse the same background. Using a CGLayer, you draw the background scene to a layer and then draw the layer whenever you need it. One added benefit is that you don’t need to know color space or device-dependent information to draw to a CGLayer.

• Repeated drawing. For example, you might want to create a pattern that consists of the same item drawn over and over. Using a CGLayer, you draw the item to a layer and then repeatedly draw the layer, as shown in Figure 12-1. Any Quartz object that you draw repeatedly—including CGPath, CGShading, and CGPDFPage objects—benefits from improved performance if you draw it to a CGLayer. Note that a CGLayer object is not just for onscreen drawing; you can use it for graphics contexts that aren’t screen-oriented, such as a PDF graphics context.

• Buffering. Although you can use layers for this purpose, you shouldn’t need to because the Quartz Compositor makes buffering on your part unnecessary. If you must draw to a buffer, use a CGLayer instead of a bitmap graphics context.

CGLayer objects and transparency layers are parallel to CGPath objects and paths created by CGContext functions. In the case of a CGLayer and a CGPath object, you paint to an abstract destination and can then later draw the complete painting to another destination, such as a display or a PDF. When you paint to a transparency layer or use the CGContext functions that draw paths, you draw directly to the destination represented by a graphics context. There is no intermediate, abstract destination for assembling the painting.

This chapter:

• Discusses how CGLayer drawing works.

• Provides an overview of how to set up and draw to a CGLayer and then draw that layer to a destination.

• Gives a specific example that shows how to set up and use two layers for drawing repeated patterns.

Contents:

How CGLayer Drawing Works
Drawing With a CGLayer
Example: Using Multiple CGLayer objects to Draw a Flag

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