Hi,


RaphaŽl Quinet wrote:
One thing that has not been mentioned in this discussion so far is
that there are two kinds of transparent pixels: those that have been
made transparent during the current GIMP editing session, and those
that were already transparent when the file was loaded from disk or
obtained from a plug-in.


Like nomis and Sven, I don't see there being any inherent difference here.


1) For the pixels that have been made transparent during the current
   editing session, one point of view could be to consider the alpha
   channel as a "hiding mechanism" that does not modify the RGB data.
   However, if the goal is to be able to un-erase some pixels that
   were accidentally made transparent, then an "undo brush" would be a
   more appropriate solution to that problem.  So the concept of
   "hiding without destroying" is not really necessary if the same
   data can be retrieved from the undo history with the same
   convenience.

I don't follow your logic here. It is undoubtedly useful to consider an alpha channel as a way to hide data, therefore it is not necessary to modify data in the RGB channel when we modify the A channel.


In any case, masks and alpha channels are also used to hide data across sessions for several file formats (eg. png), and we have ven had a bug report against the GIMP (http://bugzilla.gnome.org/show_bug.cgi?id=82882) because someone expected this behaviour from the tiff plug-in (apparrently this is provided by other programs).

2) For the pixels that have been obtained from an external source,
   then the "hiding" concept does not fit because the RGB data is
   undefined.

This is where we start to voilently diverge :) I disagree that this is the case. It may be considered to be the case in some specific circumstances, but in general, we should not destroy data, IMHO. Not destroying it means, inevitably, allowing it to be manipulated. Your suggestion amounts to pre-multiplying RGB by alpha regardless of the file format, and I consider that just wrong.


Beyond the next release,
this is also why we should consider removing the "anti-erase" mode of
the eraser tool in a future release and replace it by an undo brush.

As a matter of interest, how would an undo brush work? I'm having trouble getting my head around the idea.


Cheers,
Dave.

--
Dave Neary
[EMAIL PROTECTED]


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