[Gimp-developer] Enhancement request - Use alpha channel for non-transparent information

[Joao S. O. Bueno]

On Sunday 20 July 2008, Raphaël Quinet wrote:
> On Sun, 20 Jul 2008 21:22:11 +1000, Snake Arsenic 
<snakearsenic at gmail.com> 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. [...]
>

Hi!
Going beyond all of Raphael's excelent remarks, I still see some 
issues here :
1) The ability to  "ciew teh layer sans transparency" and "edit the 
alpha channel as if it where any other channel" is provided in GIMP - 
you copy the alpha channel to the layer's mask, and edit it (the 
layer's mask). 

2) The request indeed points a thing: GIMP _has_ the ability to set 
each channel "visible" or not - in the layers channel. If the <image> 
channel is disabled in the channels dialog, channel cvaleus are 
considere as zero for all display operations.  However, setting  the 
alpha channel to zero in this way - which is what gimp does -is 
useless - you just can't see anything in the image, as it is rendered 
with alpha = 0 for all layers.
I'd suggest that when the alpha channekll si disabled in the channel 
dialog, image is rendered with alpha =1.0  (255)  insetead.  That 
would:
   a) enable the feature thought when the ability to run channel'son 
and off was included;
   b) Make gimp attend the requesterś (Snake) needs.

Maybe the bug request should be chanegd accordingly? 

Moreover -  I really think it is a more usefull (even if seldom used) 
behavior for disabling the alpha. What do you say of doing it?

> 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 map.
> * 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.
> -Raphaël
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