In this tutorial I'll show you how to simulate a shallow depth of field
using image editing techniques. It is painstaking enough that it is no
substitute for having camera equipment that is capable of the job.
Nevertheless, it can be useful in rescuing the occasional picture.
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Load your image into the GIMP.
I thought this portrait would look better if the background
wasn't quite so distracting.
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The most difficult thing we will have to do is creating a mask
for the subject. We will try to find the "natural mask" that is
already in the image and so use this to make our work much less
tedious.
The best contrast between items can often be found in the green
channel. It's a good place to start looking, in any case. So
the first thing we will do is decompose to individual RGB
channels.
Right-click in the image and choose (Image/Mode/Decompose). In
the Decompose dialog box, choose RGB and click OK.
Examine each channel to find which has the best contrast for
isolating the subject. In this case the green channel image was
the best.
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Close the other two images that you aren't going to use. With a
little work this green channel image will become our mask.
First we'll minimize our work by drawing a selection outline
around the subject with the lasso tool
( ).
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Now invert the selection (Ctrl+I), choose the fill tool
( ), make sure Black is selected as
the foreground color and click in the image.
Clear the selection (Shift+Ctrl+A). You should see something as
shown on the right.
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Now we want to try to force the outline of the subject to
white. Right-click in the image and choose
(Image/Colors/Threshold).
In the Threshold dialog click in the histogram and drag to the
right, as if you were selecting the right half. You will see
the image go to black and white, with everything to the left of
the selection going to black, and everything in the selection
going to white. Experiment with different selections until you
have one that gives the best isolation of the subject in white.
When you are satisfied, click OK.
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We're going to manually touch up the mask to fix any last
imperfections. Here's where the work can get a bit tedious. If
you've found a good contrast between subject and background and
exploited the lasso, fill and threshold steps well you shouldn't
have too much drudge work at this step.
To help us out we're going to composite the mask and the image.
Open the Layers dialog (Ctrl+L) and click on the New Layer
button ( ). Name the new layer "Mask
Guide".
Now go to the original image window (you didn't close it did
you?). Select All and Copy (Ctrl+A then Ctrl+C). Now go back
to the mask image and paste into the new layer (Ctrl+V). It
should paste as a grayscale image. Finally, in the Layers
dialog, anchor the pasted image and reduce the opacity of the
Mask Guide (upper) layer to around 70%, as shown. The image
should look something like what is shown in the lower right.
What this does is allow you to view the image and the mask at
the same time, overlayed, while you work on the mask.
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Before you leave the Layers dialog, select the Background layer
in the mask image (that's the layer we want to work on), as
shown at right. The Mask Guide is just to guide us.
Select a reasonable brush (usually small and fuzzy) from the
Brushes Dialog (Ctrl+B), select the Paint tool
( ) and begin painting the mask.
You want the subject in White, anything that is not the subject
should be Black.
You'll probably need to use a few different brushes. Zoom and pan
around the image. Switch back and forth between Black and White
(x) and take lots of small steps so that you can make use of the
excellent Undo capabilities of the GIMP.
Periodically you can turn off visibility of the Mask Guide (by
clicking on the "eye" next to that layer in the Layers dialog)
to view your mask's progress.
When you are all done, I suggest saving the mask (as an XCF
file) so that all that painstaking work does not have to be
replicated later if you decide to re-edit the image.
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Do a small Gaussian Blur (Filters/Blur/Gaussian Blur (IIR)) of
about 5 pixels or so on the mask.
Then invert the mask (Image/Colors/Invert).
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In the Layers dialog, select the original image in the drop down
box. Then click the Duplicate Layer button
( ).
Double-click on the name of the top layer. In the Edit Layer
Attributes dialog, rename the new layer "Blurred".
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In the original image window, right-click and select
(Filters/Blur/Gaussian Blur (IIR)). Experiment to find a value
that works well for your desired "depth of field". Generally
you won't want to go too crazy with this or the look will be all
wrong. Here I used a value of 40.
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Go back to the Layers dialog and right-click on the "Blurred"
layer; select "Add Layer Mask". In the Add Mask Options dialog
make sure White (Full Opacity) is selected. Your Layers dialog
should now look as shown at right.
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Go to the mask image window and Select All and Copy (Ctrl+A then
Ctrl+C). In the original image window, Paste (Ctrl+V). This
should paste the inverted mask into the layer mask.
In the Layers dialog, click on the anchor button to anchor the
pasted image.
Voila! Your subject should be clear and sharp, while
the background appears soft and out-of-focus.
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