Technical Q&A QD27

Problems Creating a Mask for a Picture

Q: I want to create a mask for a picture, such that the mask is 0 wherever the picture's pixels are pure white, and 1 everywhere else. My first try was to simply use CopyBits to copy the rectangle enclosing the PICT onto a same-sized rect in a 1-bit-deep offscreen world. This didn't work, however, as the yellows get transformed to 0, which is not what I want. I tried various transfer modes with CopyBits (from 0 to 100) to no avail. The SeedCFill and the CalcCMask routines don't seem to be what I want either, because it appears that their masks have to be keyed off a certain point or side(s). I can take the brute force approach and go through the pixels of the PICT one by one, checking to see if they're white and setting the mask accordingly, but this seems insane. Is there a good method for doing this?

A: The way to do this is to install a custom color search procedure, then call CopyBits to copy into the 1-bit GWorld and let the search proc determine the color to use. The search proc would simply look at the color asked for and return white if it's white or black if it's nonwhite.

See "Color Manager" in Inside Macintosh: Advanced Color Imaging (forthcoming in print from Addison-Wesley).