Re: [Gimp-user] Subtracting an image from another image.

[Steve Kinney <admin pilobilus net>]

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On 10/08/2014 03:26 PM, GreenFeather wrote:
> Hi, I'm fairly new to GIMP. I need help.
>
> Let's say, I have a base image. Then another image which is
> comprised of the base image and a transparent layer of blue,
> but is merged into one layer. How can I subtract the base image
> from the other image, resulting in a layer of blue?
>
> Here, I'll show what is needed in my case.
>
> I have my base image, here 
> <http://gimp.1065349.n5.nabble.com/file/n43846/Arcanist_Costume_-_F.jpg>
>
>
> and here is the other image, which is the base image plus a
> rainbow layer 
> <http://gimp.1065349.n5.nabble.com/file/n43846/Rainbow_ark_female.jpg>
>
>
> As I said before, the second image is one layer, and I can't
> click undo because that image isn't originally mine.
>
> How would I subtract the base image from the second image that
> leaves me with a rainbow gradient layer, that looks something
> like this 
> <http://gimp.1065349.n5.nabble.com/file/n43846/Rainbow.jpg>
> (image not to scale) (note, this is just an example and I can't
> use this one because it does not match the exact shades of the
> seconds image, which is what I need)

Hey GreenFeather

If I had to do the trick described above, I think I would manually
reconstruct the gradient used to colorize the sample image.

First I would load up the colorized sample and the original sample
in one GIMP image, original above the colorized version, and set
the layer mode of the original (top layer, blue) to Divide.  This
gives a crude rendering of the gradient that was used to make the
colorized layer.  Do "New from visible" in the Layers dialog
(right click on the top layer to select the option) to get a layer
with the visible effect of the "divide" layer as its own native
colors, so the eyedropper can pick them up:

http://pilobilus.net/xfer/gimp1/snap0.png

Then I would add a new transparent layer to create an
eyeball-calculated gradient on, with guide lines (dragged in from
the top & side rulers) as shown:  Horizontal guides centered on
the middle of the color bands, and a vertical guide as shown to
keep things neat.  The guides make it easy to select a series of
precisely placed rectangles, to limit the effect of drawing
gradients on the transparent layer to one region at a time:

http://pilobilus.net/xfer/gimp1/snap1.png

Next I would zoom in on the upper right rectangle made by the grid
lines, and use the eyedropper to set the foreground and background
colors to match the unmixed colors in the middle of the crude
gradient in the "new from visible" layer.  Then I would click on
the transparent layer to make it "current", select the Gradient
tool (in default configuration as shown here), and click and drag
straight up along the vertical dividing line.  Keep re-doing the
gradient until it looks right, example:

http://pilobilus.net/xfer/gimp1/snap2.png

Next, I would make the crude new-from-visible layer current again,
switch the color selector's current foreground and background
colors (tiny arrow at lower left of color swatches in toolbox),
and use the eyedropper to pick up the next color for my new gradient.

Make the transparent layer current, select a rectangle between the
top and 2nd horizontal guide lines, and match the sample gradient
as well as possible - just as you did for the first region.
Repeat for every color change and we end up with a "rainbow"
gradient that closely matches the original:

http://pilobilus.net/xfer/gimp1/snap3.png

In this shot, we see a comparison between the effect created by
using the new gradient in Color mode (left side) to modify the
blue original (middle), and the sample image made with the
"unavailable" gradient (right side).  The match is quite good,
except that the yellow/red transition at the bottom of the new
gradient needs fixed:

http://pilobilus.net/xfer/gimp1/snap4.png

Here's the resulting .xcf file:

http://pilobilus.net/xfer/gimp1/Rainbow_ark_mixed-gradient.xcf

If I wanted to make a real GIMP gradient out of this, I think I
would set the manually drawn gradient to normal mode, rotate the
image 90 degrees, and line up the Gradient Editor tool below the
one I just made.  Make 'em match and save the result as a freely
re-useable gradient.  I rarely have a use for the gradient editor,
but it's the right tool for this job:

http://docs.gimp.org/en/gimp-gradient-dialog.html#gimp-gradient-editor-dialog

:o)

Steve


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