What do you mean by threshold? Works for me like it always did...

The filter does what it is supposed to do.

Strange.  I had a layered image that did that.

Got it quirking again!  You need to have the layer set to preserve all
transparency to see it screw up and perform the threshold.  If not set, it works
properly.  Where's the warning?

Sometimes I think that there should be a big red light that keeps flashing
whenever "keep transparency" is active, because it does so many things that seem
mysterious if you're not aware of it.  But this is not in fact a bug, just a
confusing feature.  It would, however, be good for Color to Alpha to give you a
warning when "keep transparency" is switched on; I am changing the subject of
this bug report to that effect.

I don't think it makes sense to start adding such warnings. We would have to add
it for inactive color channels then as well. If people turn on "Keep Alpha",
then they ask for the alpha channel to be kept. This is completely transparent
to the plug-in.

If there is a plausible use case for running Color to Alpha with "keep
transparency", then I retract my suggestion.  Otherwise, I maintain it.

The main problem with comment #6 is that (1) many non-expert users don't really
understand what "keep transparency" means, and (2) the little check-box for it
does not at all command attention.  I have been burned by this myself, see bug
#145054.

I would be happy to add such a warning myself, if I get a more positive response
-- it would probably take about 30 seconds.  I suggest, "You are running Color
to Alpha on a layer with 'keep transparency' set.  You can do this, but the
result may not be what you expect."

Bill:  That warning dialog makes a whole lot of sense.

In relation to #6:

The more elaborate situation is like this.  You're working on a sketch,
something you want to preview (say on Deviantart.com) but it's in peices (like
two or more pages with overlap, so it'll match right, and various parts of the
image off to one side because you're cheating).  So, to create the mockup to
show, you have to paste a few images in, and for bits on the page you have to
copy/transform, you float the selection and make it a layer.

The newly created (from floating selection) layer has "Keep Transparency" turned
on by default.

Now, hit "Color to Alpha" and guess what you get?  This bug.

I think having something annoy me that "Keep Transparency" is set (and maybe
having Gimp unset it, but that's getting rediculous) when I evoke filters that
munge transparency is worth while.

Eeek, why is "Keep Transparency" turned on for Floating Selections? That's a bad
bug that needs to be fixed immidiately.

Changing milestone to 2.2 because of comment #10.  Should perhaps get its own
bug report, though.

Yes, it should have it's own bug report.

Okay, reported as bug #161042.  Resetting target milestone for this bug to Future.

Making this bug dependent on that bug.  

Fixed in both branches:

2005-03-22  Sven Neumann  <sven@gimp.org>

        * plug-ins/common/colortoalpha.c: unset "Keep transparency", it is
        not what the user wants if this plug-in is being used.