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Digging up the luma palette images reminded me of a useful PNG I made back around 1996 or so, back when this file format was quite new. A peculiarity of the PNG file format is that it stores alpha separately, unmultiplied. For 3D work it is the norm for the color stored to be premultiplied by the alpha. I won’t go into the how and why; this topic is covered in our book on pages 139-140 and is discussed on Wikipedia, among many other places.

One nice feature of premultiplied images is that you can just ignore the alpha channel entirely when displaying them for preview. This is equivalent to compositing the image over a black background. With PNG, you are required to examine the alpha channel and multiply the RGB by it in order to get the right color to display. Unmultiplied alpha has a 2D photo sense to it, the RGB image exists everywhere and the alpha is masking some part of it. The alpha is not an integral part of the pixel, as it is in 3D.

On to the image:


I cobbled this together to be able to quickly check if a particular piece of software was respecting the alpha channel in PNG. Back then, most software didn’t, so what would be displayed is “This viewer does not support transparency”. Today, it’s pretty rare to find such flawed PNG readers commercially (I couldn’t find a current example for you to try). Still, I’ve found this image useful as a quick reality check for whether software is using the alpha in a PNG.

Lest we forget, it was the LZW patent in the GIF format that helped popularize PNG as a patent-free alternative for the web. The Unisys patent finally fully expired back in July 2004, so it’s a moot point now, but for awhile this was a patent enforced for tens of millions of dollars, with over 2,000 licensees. My favorite quote on the whole controversy was from a flak at Unisys giving spin about their positive role enforcing a patent on a technology unknowingly used in a file format that they didn’t invent:

But Unisys credited its exertion of the LZW patent with the creation of the PNG format, and whatever improvements the newer technology brought to bear.

“We haven’t evaluated the new recommendation for PNG, and it remains to be seen whether the new version will have an effect on the use of GIF images,” said Unisys representative Kristine Grow. “If so, the patent situation will have achieved its purpose, which is to advance technological innovation. So we applaud that.”

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