This must be the same size as the rectangle passed in the srcRect parameter. 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 specify pixel maps to CopyMask , 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. The CopyMask procedure may move or purge memory blocks in the application heap. Your application should not call this procedure at interrupt time. Plate 4 at the front of this book provides another illustration of the effects of the source and mask pixel maps on the destination pixel map. Inc. All rights reserved.