On Sun, Oct 18, 2009 at 4:07 PM, Ray C. <for...@gimpusers.com> wrote:
> I have several photos that are square but had an oval matt over them. Over the
> years the oval has UV damage. I am stumped on how to get the UV damaged part
> of the photo to match the original.

I think the Colors > Levels could one of be the best tool for that,
even if it may take time to get the right correction. Hopefully your
photos will all have the same fading and so you could *save* the level
correction and re-use it on all of them, instead of tweaking the
levels for each photo.

Start by duplicating the base layer (the one with the photo, I'll call
it A), add a mask to that new layer (I'll call it B), so that only the
faded part is visible (no need to make a perfect mask, here it's just
a way to easily see the faded part on B and the not faded part on A)

Then, the Levels tool... well...  I general I find it easier to start
by tweaking the "Value" channel, so that the light/darkness of both
images matches - even if the colors do not match yet. An easy way to
do that is to add a white layer on top of the others and set its mode
to "Color". You now see your image in black and white - with the
central part being too light.

Select layer B and select the Levels tool, then move the left slider
(the black triangle, on the Input Levels) until the lightness matches
the not faded borders (around 32 seems a good value on your example).
Now you can toggle the visibility of the white layer, we won't need it
anymore - and keep the levels dialog open.

Now the colors. The faded part looks like it had too much green, so in
the dialog select the Green channel. Move either the left or the
middle slider (on the Input Values) to the right.
Moving the middle slider to the right will make the image less green,
moving the left slider will do the same, but it will also increase a
bit the contrast. It's up to you to see what seems to fit better. And
repeat for the other channels... :)

Here are some values that seems to work for me, for each channel:
Value: Left input slider at 32
Red: Middle input  slider at 1.05
Green: Middle input slider at 0.82
Blue: Middle input slider at 0.96
The colors are still rather dull, but at least they should match the
colors of the part protected by the mat.

> Then I can restore and colour balance of the whole piece. I would appreciate
> anyone's suggestions on how to proceed with this.

I hope this helped, now I'm afraid I don't have a solution for how to
make a good mask easily. The only method I know is careful painting of
the mask. (Maybe a feathered ellipse could work too)

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