When you specify rectangles in the srcRect and dstRect parameters, use the local coordinate systems of, respectively, the source and destination graphics ports. The rectangle you pass in the maskRect parameter selects the portion of the bitmap or pixel map that you specify in the maskBits parameter to use as the mask. If you don't want to clip to the mask region, specify NIL in the maskRgn parameter. If you specify pixel maps to CopyDeepMask , they may range from 1 to 32 pixels in depth. The pixel depth of the mask that you specify in the maskBits parameter is applied as a filter between the source and destination pixel maps that you specify in the srcBits and dstBits parameters.
Intermediate values specify a weighted average, which is calculated on a color component basis.
Thus high mask values for a pixel's color component reduce that component's contribution from the source PixMap record. This procedure is available to basic QuickDraw only in System 7. As with the CopyMask procedure, calls to CopyDeepMask are not recorded in pictures and do not print. The CopyDeepMask procedure may move or purge memory blocks in the application heap. Your application should not call this procedure at interrupt time.