2005.03.17 23:44 "Re: [Tiff] Re: Tiff file with Transparency mask (former 'Photoshop Masked area TIFF images')", by Bob Friesenhahn
I am at the moment specifying a new raster map product and I intend to use the transparency mask for encoding the padding (void areas).
I recommend testing with Photoshop. Anything you can create using Photoshop is likely to work with it. Some applications are likely to have problems with handling a separate transparency mask. They may complain or even crash. I know that the software I maintain does not handle it right now.
In my testing, I find that associated alpha works well with Photoshop, but of course that consumes more resources than a simple bilevel mask since the alpha channel needs to be the same depth as the other channels, and math is required. Unassociated alpha can be used with Photoshop, but only if an experienced user tells Photoshop how to use it. Unassociated alpha is presented as an extra channel which can be assigned for use (by the user) as a mask.
Do you have a sample file that we can test with?
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/