2002.08.23 21:29 "Re: BMP files with alpha/transparency channel?", by Chris Cox
Michael reminded me that many of the readers may not understand the differences between alpha and transparency.
Originally, the term alpha was coined to mean transparency - and there was only a single extra channel associated with the image. Rather quickly, the term alpha grew to mean any extra channels in the image, or a single extra channel used for purposes other than transparency. The historical change of meaning is what confuses most people.
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).
So, for Michael's question:
BMP specifies that the fourth channel is not necessarily related to the image data - thus it cannot be assumed to be transparency (even if it sometimes is). In many cases the fourth channel is used for other attributes (especially in game development!).
And now, back to our regular TIFF (and OT memory management) discussions....