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The Alpha Value

The alpha value is the graphics state parameter that Quartz uses to determine how to composite newly-painted objects to the existing page. At full intensity, newly-painted objects are opaque. At zero intensity, newly-painted objects are invisible. Figure 4-2 shows five large rectangles, drawn using alpha values of 1.0, 0.75, 0.5, 0.1, and 0.0. As the large rectangle becomes transparent, it exposes a smaller, opaque red rectangle drawn underneath.

You can make both the objects on the page and the page itself transparent by setting the alpha value globally to graphics context before painting. Figure 4-3 compares a global alpha setting of 0.5 with the default value of 1.0.

In the normal blend mode (which is the default for the graphics state) Quartz performs alpha blending by combining the components of the source color with the components of the destination color using the formula:

destination = (alpha * source) + (1 - alpha) * destination

where source is one component of the new paint color and destination is one component of the background color. This formula is executed for each newly-painted shape or image.

For object transparency, set the alpha value to 1.0 to specify that objects you draw should be fully opaque; set it to 0.0 to specify that newly drawn objects are fully transparent. An alpha value between 0.0 and 1.0 specifies a partially transparent object. You can supply an alpha value as the last color component to all routines that accept colors. You can also set the global alpha value using the CGContextSetAlpha function. Keep in mind that if you set both, Quartz multiplies the alpha color component by the global alpha value.

To allow the page itself to be fully transparent, you can explicitly clear the alpha channel of the graphics context using the CGContextClearRect function, as long as the graphics context is a window or bitmap graphics context. You might want to do this when creating a transparency mask for an icon, for example, or to make the background of a window transparent.

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