Inside Macintosh: Imaging with QuickDraw
When the user resizes or moves a window, changes the pixel depth of a screen that a window intersects, or modifies a color table, you can use the UpdateGWorld function to reflect the user's choices in the offscreen graphics world. The UpdateGWorld function, described on UpdateGWorld , allows you to change the pixel depth, boundary rectangle, or color table for an existing offscreen graphics world without recreating it and redrawing its contents. You should also call UpdateGWorld after every update event.
Calling UpdateGWorld and then CopyBits when the user makes these changes helps your application get the maximum refresh speed when updating the window. See the chapters "Event Manager" and "Window Manager" in Inside Macintosh: Macintosh Toolbox Essentials for more information about handling update events in windows and about resizing windows.