Re: [Gimp-user] New question on Gimp

[Steve Kinney <admin pilobilus net>]

On 11/11/2015 05:12 PM, Jack Denman wrote:
> Using Gimp, I would like to modify the highlights without modifying the
> middle tomes on the shadows. I would like to increase the contrast of
> just the highlight areas or darkening a little without modifying the
> middle tones or the shadows. I don't care how complex the procedure is,
> just how to do it. I am not asking about "blown out" highlights. Any
> answer would be appreciated.

You might want to try the Curves tool, it lives at Tools > Colors >
Curves.  I always configure my main toolbox (edit > preferences >
toolbox) to include the non-default buttons for the Curves, Levels,
Threshold, Brightness/Contrast, Hue/Saturation and Color Balance,
and remove several default buttons for tools I rarely or never use
(Zoom, Ink, etc.).

With the Curves tool active, right click on your image canvas and
the Curves dialog will appear.  Right click and drag the the
diagnoal line to add nodes; changes made in the upper/right part of
the curve will affect the highlights.  Direct adjustment here will
sometimes prove useful.

Or, and this will likely be a more effective approach, you can
create a copy of your image on a new layer, and use a layer mask to
make everything but the highlights transparent.  Then, adjustments
made to that layer will only affect those regions, while the darker
parts of the image layer below show through unaffected.  Where the
mask it white, the layer's image content is fully visible.  Where
the mask is black, the layer's image content is fully transparent.

First create a copy of your image on a new layer.  Go to the Layers
dock, right click, and select Duplicate Layer or New From Visible.
To add a mask to your new layer, right click the layer icon in the
Layers dialog, select Add Layer Mask.  In this case you will select
the option: Grayscale copy of layer.

Now to configure the mask.  First left click on the thumbnail of the
mask in the Layers dock, to make it active for editing.  Then
holding down the Alt key, left click on the layer mask thumbnail.
This will make the mask visible in your image canvas; you should see
a grayscale copy of your image there.

What you want to do is make everything in the mask except the
highlights black, or at least rather dark, and lighten the
highlights until they are a little blown out.  For this, I would use
the Curves tool:  Activate the tool and left click on the image
canvas.  When the Curves dialog appears, click and drag the line
down to the bottom of the box, about 1/3 of the way in from the left
side of the box.  Play with this and adjust it until your highlights
are looking rather bright, fading to black in the darker areas you
do not want to change.  When it looks like that, click the OK button.

Hold down the Alt key and left click the layer mask again, to turn
its visibility back off.  Right click on the layer thumbnail (not
its mask) to make it active for editing.

Now make your light, color and contrast adjustments with the new
layer current.  You will see the changes only in the areas where the
mask is gray or white; the rest of that layer is transparent.  You
might want to use the Brightness/Contrast tool, or run some filters
against the layer to get the effect you want.  The changes should
blend in smoothly in your finished image, without affecting the
darker parts of it at all.

Another very useful thing that layer masks can do is "erase" parts
of a layer, by painting on a white layer mask with black; when you
later find an error in your erasure, no problem; bring the part you
should not have "erased" back, by painting on it with white.

Learning how to use layer masks effectively is one of the big
differences between "beginner" and "intermediate" level skills and
results.

:o)

Steve