Forum member andrew-bkk brought this article by Dave Shea to my attention:
I found the first comment by Keith Lang so very interesting. So I’m going to post it here for all those people who never click the article links.
[quote=Alvy Ray Smith]Ed Catmull and I invented the integral alpha channel in late 1977 at the New York Institute of Technology. I remember the moment very clearly. Ed was working on his sub-pixel hidden surface algorithm for SIGGRAPH paper submission*. He was rendering images over different backgrounds using the new technique. I was working with him since I knew where all the interesting background images lay in our file system. We had six of the rare 8-bit framebuffers at NYIT at this time. I would position a background in three of them (equivalently one RGB framebuffer) over which he would render a foreground element. A different background required a new rendering, a very slow process then.
Ed mentioned that life would certainly be easier if, instead of re-rendering the same image over different backgrounds, he rendered the opacity information with the color information once at each pixel into a file. Then the image could be composited over different backgrounds, without re-rendering, as it was read pixel-by-pixel from the file. I immediately said that this would be extremely easy to accomplish, confident because I had written our image file saving and restoring programs. Versions for saving and restoring 8-bit and 24-bit images existed, and I knew exactly the changes to extend the code to 32-bit images. I started right then and by the next morning had the full package, complete with Unix manual pages using “alpha” and “RGBA” terminology, ready for use. All Ed had to do was write the alpha information into a fourth framebuffer. We called it that because composition uses the classic linear interpolation formula aA + (1-a)B [read a as “alpha” (Greek letters are difficult for .htm documents)] where a controls the amount of interpolation between, in this case, two images A and B. I would save the four framebuffers (equivalently, one RGBA framebuffer) into a file with the new code, savpa4. Then Ed or I or anybody could use the newly revised restore routine, getpa, to composite the file image over an arbitrary image already in the framebuffers. getpa could detect a fourth channel in a file image and use it to composite as the image was read from the file. That was it. The integral alpha channel had been born. The “or anybody” is a key phrase: The integral alpha channel severs the image synthesis step from the compositing step.
* Catmull, Edwin, A Hidden-Surface Algorithm with Anti-Aliasing, Computer Graphics, 12:6-11, July 1978.
The first working, practical real-world alpha channel, implemented in code is 33 years old.
#1 From http://alvyray.com/Awards/AwardsAcademy96.htm[/quote]
Wow, someone got an Acadamy Award for creating alpha transparency. Cool.
Keith’s comment at the bottom just makes it better. IE9 seems to be ok with this. We can only pray that IE9 finally drives out the last notable vestiges of IE6… : (