Chris Cox (TIFF and LibTiff Mailing List Archive » Transparency ... associated or unassociated alpha is morecompatible?) wrote:
PNG and SGI RGB support associated alpha (aka transparency), just not premultiplied.
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.
Associated Alpha == transparency
Unassociated Alpha == using the extra channel(s) for something else
Premulitplication (black matte) / Matting can only apply to transparency, and varies with the file format or sometimes flags within the file format.
on behalf of Bob Friesenhahn
Sent: Mon 8/6/2007 3:27 PM
To: William Gallafent
Subject: Re: [Tiff] Transparency ... associated or unassociated alpha is morecompatible?
On Mon, 6 Aug 2007, William Gallafent wrote:
> I'm experimenting with TIFF export for RGBA images, bit depth
> eight and sixteen. I need to decide which style of alpha
> channel to use in order to maximise interoperability with other
As best I can tell, alpha is only meaningful in TIFF for the associated alpha case. It is the only case where the purpose/use of the alpha is defined. One is only left to assume the intention of the alpha channel for unassociated alpha.
PNG, SGI RGB, Targa, and many other formats supporting an alpha channel use unassociated alpha. In my own experience, unassociated alpha causes less headache (outside of TIFF) since so many other formats and algorithms work better with unassociated alpha. As an example of where associated alpha causes a headache, consider the algorithm necessary to save the image to a palette-based image format.
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/