2001.02.22 00:39 "Re: PhotoShop alpha channels and LibTiff", by Joris Van Damme

any idea why?

this seems like a terribly cruel thing to do.

I'm not an adobe employee or something, but I can make a guess here, if that's OK.

Photoshop's PSD file format allows for a number of layers, alpha channels, etc. In such a sceme, there is no such thing as a single alpha channel, but only a number of channels, some of which represent colors in layers, or in a global picture, others alpha's that can have just about any local layers or global image meaning and must even considered to be editable and enablable/disablable by users all the time. I guess this makes adobe reluctant to define such a thing as a single alpha channel, including when they write out other formats that allow for something else/more than a single alpha.

Also, it might be a bit of a strategic move. (Mind you, I do say 'it might', I don't actually know anything.) I mean, TIFF certainly has the potential to store just about everything a PSD can store. But TIFF's weak point is the spec doesn't reflect the last generation of image editors and in general if image usage anymore. This TIFF spec and updating it is the responsability of... adobe, and, now now now, the lack of agreement on such topics as multiple alpha's, layers, etc... is becoming a tiff problem...

is there any work-around?

You can't possibly, in my opinion, consider adding a meaning that doesn't even reflect anything definable by the intentional writer. So the only possible thing to do is treat the image the same way. I strongly suspect adobe adds proprietary data that describes the intended meaning of the channels in detail, or at least offers the user the option to have this done. That is, if my assumptions are correct. I'd expect photoshop is probably given the information to read as much back as possible, so if there is any intrest in reading photoshop tiffs, I'd recommend downloading the adobe sdk, which includes pdf's describing lots of adobe format stuff.

Joris