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Since I don't actually own a "real" chroma key background, I
used my house as a substitute. A real chroma key background
should work even better than this.
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Might as well get the bad news out up front. I know of no
perfectly simple, repeatable technique for one-click selection
of complex subjects (like a person) in a photograph. There
may be products (like Knock-out for Photoshop) that make easier
work of things, but even under the most controlled contitions
this is usually a multi-step affair. That said, the GIMP
provides some decent tools for getting the job done, and with a
little practice you should be able to get this part down to a
few minute exercise. I've done precious little of this kind of
selection and it only took me a few minutes, so take heart.
The key to doing this with a near constant color background that
is different from the subject is to make good use of the
fuzzy selection (aka "magic wand") tool, followed by judicious
use of quick mask to touch up the selection.
Double-click on the magic wand ( ) to bring up the Tool Options
dialog. You'll have to experiement here at first to find the
right setting for the Threshold value, which is the amount of
"fuzziness" to apply to the color you click on in defining a
contiguous region. Remember, the background will probably have
some creases, wrinkles, lines, reflections, shadows, etc. that
all decrease the uniformity of the color. You'll have to find a
Threshold value that's about right for your background, subject
and lighting conditions. Once you've set the Threshold, click
in the background somewhere to set up the initial selection.
You probably will get something similar to that shown at right.
There will probably be regions of the background that aren't
selected and regions of the subject that are. That's OK. Just
as long as you get a decently large chunk of the background and
not too much of the subject, all will be fine. If you need to,
Undo (Ctrl+Z), adjust the Threshold in the Tool Options and try
again.
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Now begin an iterative process of adding to the selection. What
you want to do is hold down the SHIFT key and click again on an
area of the background that didn't get selected, but
should have been. The SHIFT tells the GIMP to add the new
selection to the previous one. In the previous step you can see
that there are a couple of areas under each arm and by the neck
where the boy's shadow caused the color to deviate enough that the
first fuzzy select didn't pick it up. Clicking once on the neck
shadowed area while holding down the SHIFT key results in the
selection is shown at right. It's pretty darn good. There are
areas of the boy selected that shouldn't be, but I'll clean
those up next. In this case, one additional selection was
enough; you may need to repeat for additional areas.
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Now to manually touch up the selection. For this the Quick Mask
tool is the ticket. Convert the selection into a quick mask by
clicking on the little red square in the bottom left corner of
the image window. You should see something similar to that
shown at right.
Think of a quick mask as just another way to view/manipulate a
selection. The parts that are not selected are shown
in a red tint. To change the selection, just paint onto the
image with black or white: black for no selection, white for
selection. You will see that instead of painting actual black
or white, you get red-tinted or not. (Side note for the
curious: selections, channels and masks are all really just
8-bit grayscale images! The difference is that normally you
don't see them that way. Knowing that, however, lets you
understand how to manipulate them using the ordinary GIMP
toolset.)
Looking at this quick mask I can see that I've got a pretty
great selection and just need to remove some parts of the boy
that were selected. I pick a largish, fuzzy brush from the
Brushes dialog (Shift+Ctrl+B), click on the Paint tool
( ), zoom in a
bit (++), make sure Black is selected as the foreground paint
color, and begin painting over spots that should not be
selected. Pan around the image, fixing up all the spots that
need it. If something is not selected that should be, switch to
white (x) and paint it. If you make a mistake and paint into
something that should not be, switch (x) and paint it. Switch
to a smaller brush size near the edges.
With just a little practice you can fix up a selection like this
in a few minutes. When you are done it should look something
like that shown at lower right.
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Now convert the quick mask back to a selection by clicking on
the little selection button just to the left of the quick mask
button in the lower left corner of the image frame. You should
see the image switch back to the "marching ants" selection
outline.
Now invert the selection (Ctrl+Ior Selection/Invert). This
makes the selection the subject rather than the background.
At this point you can feather the selection if you wish. I did
not do so here.
Finally, copy the selection (Ctrl+C).
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If you don't need to resize or do any other work on the image
being pasted, proceed to the next step.
Otherwise, do a New (Ctrl+N) and go with the defaults, except
choose "Transparent" as the fill type. In the new image, Paste
(Ctrl+V) and Anchor the image in the Layers dialog.
Now manipulate the image as necessary (e.g. Image/Scale), then
Select All (Ctrl+A) and Copy (Ctrl+C).
In this case I didn't need to do anything special to the pasted
image.
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Open the new background image (Ctrl+O).
Open the Layers dialog (Ctrl+L) and make sure that the new
(background) image is listed in the drop-down box. Right-click
on the Background layer and select New Layer (there is also a
button for this in the bottom button bar of the Layers dialog
( ).
Here I named the new layer "Subject". Choose "Transparent" for
the fill.
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Go to the new image window containing your desired background
and Paste (Ctrl+V).
Choose the Move tool
( ) in the GIMP
toolbox. Move the pasted image to the desired location in the
image. In the Layers dialog, anchor the pasted image by
clicking on the anchor button.
Voila! Finished image.
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).
Text and/or images are available for personal or commercial use
under certain conditions.
For this kind of fine paint work, a pen tablet like this Wacom
one is very useful. For photo retouching you don't need
any bigger than a 4x5 model.