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About Bitmap Images and Image Masks

A bitmap image (or sampled image) is an array of pixels (or samples). Each pixel represents a single point in the image. JPEG, TIFF, and PNG graphics files are examples of bitmap images. In Mac OS X, icons are bitmap images. Bitmap images are restricted to rectangular shapes. But with the use of alpha, they can appear to take on a variety of shapes and can be rotated and clipped, as shown in Figure 11-1.

Each sample in a bitmap contains one or more color components in a specified color space, plus one additional component that specifies the alpha value to indicate transparency. Each component can be from 1 to as many as 32 bits. Mac OS X v10.4 adds support for 128-bit floating-point components. ColorSync provides color space support for bitmap images.

Quartz also supports image masks. An image mask is a bitmap that specifies an area to paint, but not the color. In effect, an image mask acts as a stencil to specify where to place color on the page. Quartz uses the current fill color to paint an image mask. An image mask can have a depth of 1 to 8 bits.

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