2007.07.03 18:37 "[Tiff] BigTIFF extension?", by Phil Harvey

2007.08.06 22:48 "RE: [Tiff] Transparency ... associated or unassociated alpha is morecompatible?", by Chris Cox


Yes, I know.

I'd like to have some way to specify un-premultiplied/unmatted transparency, but it would be a major change to TIFF. (adding new alpha channel types helps, but it would require changes to a lot of software before it would work correctly)

"Correct according to the original Targa specifications" and "what most users of the TGA format expect and understand" are completely different things... Photoshop tries to follow the format specs., but sometimes users don't follow the specs, much less understand the specs.


Cc: Tiff List

Subject: RE: [Tiff] Transparency... associated or unassociated alpha is morecompatible?

On Mon, 6 Aug 2007, Chris Cox wrote:

> PNG and SGI RGB support associated alpha (aka transparency), just not premultiplied.

Thanks for correcting my wrong use of terms. It is a pity that TIFF

does not provide a way to specify associated alpha which is not

> TGA is supposed to support associated alpha (transparency), but most

> users of TGA don't seem to understand the difference and get

> transparency and general (unassociated) alpha channels confused.

> When Photoshop 7 shipped with transparency support in TGA format,
> the users got so confused that Adobe had to go back to using
> unassociated alpha. The TGA users still can't agree on what they
> want, much less what they understand.

That is interesting to know. As far as I am aware, GraphicsMagick
handles alpha correctly (according to written specification) for TGA
files. I spent quite a bit of time double checking it and fixing

Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/