"Developers of the open source graphics format PNG and
its derivative, MNG, have called on Apple to clarify whether its
ownership of key PNG component technologies will be used to hinder
At issue is patent number 5379129, filed by Apple in May 1992
and granted to the company in January 1995. The patent describes a
"method for compositing a source and destination image using a mask
image". Essentially, it covers how computer software may blend two
images using a third, the mask, to determine how the two are melded
The process, known as 'alpha blending', is a common one in
graphical manipulation. Games and multimedia applications use the
technique to fade smoothly between one image and another. And Apple
uses the technique to make Mac OS X's menus, dialog boxes and
windows appear translucent."
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