2007.08.06 22:27 "Re: [Tiff] Transparency ... associated or unassociated alpha is more compatible?", by Bob Friesenhahn

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.

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