Apple patents perturb PNG programmers

Demand to know Mac maker's intentions

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 their work.

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 together.

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.

Alpha blending is typically performed using a greyscale image as the mask - the darkness of each pixel governs how much of the colour of each source and destination pixel in the same location appear in the final image - but Apple's patent allows the mask to be a full-colour image in its own right.

Ownership of patent 5379129 emerged through the Worldwide Web Consortium's move to establish the Scalable Vector Graphic format as a standard. Earlier this year it asked interested parties to submit details of patents they own that might impinge upon the development of SVG.

Apple highlighted 5379129 as part of that process. Unlike Adobe, Sun, Canon and others, Apple has not provided its intellectual property on a royalty-free basis, as have IBM, Quark and Eastman Kodak.

Says the W3C: "Apple informed the SVG 1.0 Working Group very early in the SVG 1.0 process of the patent they listed in their license statement. The SVG Working Group made a concerted effort to produce a specification that does not require implementors to infringe the patent."

Since PNG (Portable Network Graphics) and MNG (Multiple-image Network Graphics) both support alpha blending, its developers are concerned that Apple, as per its filing with the W3C, may only allow anyone to use its patent "on payment of reasonable, non-discriminatory royalties or fees".

That's something a free, open source project like PNG is unlikely to be able to support - particularly since a motivation behind the development of the PNG format is to provide a royalty-free alternative to the well-known GIF (Graphics Interchange Format) file. The CompuServe-developed GIF contains Unisys' LZW compression algorithm for which Unisys levies a fee from software developers who choose to support the format in their code.

The PNG people essentially want Apple to disclaim its patent - unlikely, we reckon, given its response to the W3C - or at least grant the PNG effort a royalty-free licence.

Alternatively, the PNG team could ask for patent 5379129 to be disallowed on the grounds of prior art - if the method existed long before Apple patented it.

Certainly, the group's Web site suggests that the technique may have already been expressed in academic papers dating back to 1984 and SGI's early work on what would become the OpenGL graphics API.

To that list we'd add Adobe's Photoshop application, which has been allowing users to add alpha channels to images since late 1980s. Not that Apple is likely to get shirty with Adobe, even if there was a clash of intellectual property - Photoshop is too important to Mac users.

We have asked Apple to comment on its ownership of 5379129 and how that is likely to affect the PNG team. By close of business, the company had yet to respond. If it does, we'll let you know what it has to say. ®

Related Links

PNG team: PNG and MNG tools Alpha composition patent information
W3C: SVG 1.0 Patent Statements

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