According to Slashdot, Apple owns U.S. Patent No. 5,379,129, which patents composting a source image to a destination using a mask image. This is an obvious and well-known technique, but Apple seems to think they invented it in 1992. Here's an example. This is the masked bitmap I use in Black Box Chess that shows up in the main game browser.You can clearly see that I'm violating all kinds of patents here.
Funny, I don't recall stealing it from Apple. IIRC, it was covered in one of Microsoft's old programming manuals on how to make a transparent blit.
Interestingly, the PNG folks are looking for examples of pre-existing mask/bitmap code to kill the patent. I honestly wish I had some of my old source code, because I had games from around 1991 that did this.