2002.08.23 22:09 "Re: BMP files with alpha/transparency channel?", by James Burgess
On Friday, August 23, 2002, at 02:29 PM, Chris Cox wrote:
So, here's the current state of terminology:
Alpha channels are a superset of extra channels and can contain anything (bump maps, saved selections, spot color plates, annotations, overlays, several other maps/layers used by 3D rendering, etc.). There can be many alpha channels associated with a single image.
Transparency is a subset of alpha channels with data closely related to the image data and specifying transparency of the image data. This still doesn't specify the matting or premultiplication of the data with the transparency - that has to be specified somewhere else in the format or format specification. In general there will be only a single transparency channel associated with an image (complex compositing like Photoshop or After Effects may have more for masking of existing data, but that has to be spelled out carefully in the file format specification).
This might be terminology for some people but it will be a long time before I can think of alpha as anything other than the transparency of that pixel for use in compositing in the "Porter Duff" sense of the word. I say this only so that anyone who doesn't already understand what might be meant by Alpha would a least go off and find out what Mr Porter and Mr Duff had to say about the subject.
And now, back to our regular TIFF (and OT memory management) discussions....
We should try and get some more discussions on gamma and monitor calibration and going.