On Sun, 20 Jul 2008 21:22:11 +1000, Snake Arsenic <[EMAIL PROTECTED]> wrote:
> Okay I made a feature request at 
> http://bugzilla.gnome.org/show_bug.cgi?id=543810 but it's missing a 
> solution that will not remove any functionality. [...]

I think that it would be a big mistake to use the alpha channel for
anything else than transparency.  I assume that you are asking for this
because you have some program (I don't know which one) that is
incorrectly using the alpha channel to store bump map information or
something else that is not related to transparency.  It is likely that
this program doesn't use a file format that supports layers or independent 
channels, so its authors of have decided to hijack the
alpha channel in some existing file format.

The correct way to solve this problem is to use layers instead of
abusing the alpha channel (or maybe additional channels, but I think
that using layers would be more convenient in this case).  With layers,
it is very easy to toggle the visibility of the image or the bump map
layer, specular map layer or whatever else you are working on.

So instead of extending the mistakes done by the authors of some other
software, it would be much better to know what file format has been
subverted, and to perform the conversion in the file plug-in:
* When the plug-in loads a file that uses this strange format, it would
  convert the alpha channel into a layer and mark that layer (using a
  special name like the GIF plug-in does for animations, because that
  can be edited easily by the user if necessary).
* You would then be free to edit the image in GIMP and modify the layer
  containing what should be visible or the layer containing the bump
* When saving the file, the plug-in would detect that some layers have
  a special name and would then combine these layers in a way that can
  be read by whatever other program you are using.

So I suggest that you:
1) Identify what file formats need some special treatment.
2) Check if there is a way to detect what files using that format are
   special, so that we do not have to ask the user every time if a file
   using that file format should be read in the intended way (alpha
   channel = transparency) or in the non-standard way (bump map).
3) Suggest improvements to the corresponding file plug-ins, instead of
   requesting major changes in the GIMP core.

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