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How Transparency Layers Work

Quartz transparency layers are similar to layers available in many popular graphics applications. Layers are independent entities. Quartz maintains a transparency layer stack for each context and transparency layers can be nested. But because layers are always part of a stack, you can’t manipulate them independently.

You signal the start of a transparency layer by calling the function CGContextBeginTransparencyLayer, which takes as parameters a graphics context and a CFDictionary object. The dictionary lets you provide options to specify additional information about the layer, but because the dictionary is not yet used by the Quartz 2D API, you pass NULL. After this call, graphics state parameters remain unchanged except for alpha (which is set to 1), shadow (which is turned off), blend mode (which is set to normal), and other parameters that affect the final composite.

After you begin a transparency layer, you perform whatever drawing you want to appear in that layer. Drawing operations in the specified context are drawn as a composite into a fully transparent backdrop. This backdrop is treated as a separate destination buffer from the context.

When you are finished drawing, you call the function CGContextEndTransparencyLayer. Quartz composites the result into the context using the global alpha value and shadow state of the context and respecting the clipping area of the context.

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