Inside Macintosh: Imaging with QuickDraw
In addition to the QuickDraw routines for creating and drawing pictures, system software provides a group of routines called the Picture Utilities for examining the contents of pictures. You typically use the Picture Utilities before displaying a picture.
The Picture Utilities allow you to gather color, comment, font, resolution, and additional information about pictures. You might use the Picture Utilities, for example, to determine the 256 most-used colors in a picture, and then use the Palette Manager to make these colors available for the window in which your application needs to draw the picture.
You can also use the Picture Utilities to collect colors from pixel maps. You typically use this information in conjunction with the Palette Manager and the ColorSync Utilities to provide advanced color imaging features for your users. These features are described in Inside Macintosh: Advanced Color Imaging .
The Picture Utilities also collect information from black-and-white pictures and bitmaps. The Picture Utilities are supported in System 7 even by computers running only basic QuickDraw. However, when collecting color information on a computer running only basic QuickDraw, the Picture Utilities return NIL instead of handles to Palette and ColorTable records.