istokeeptheskyimageasthebackgroundorbaseimage,
and to place the foreground image as a layer above that
base image.
So go to the foreground image, select it ([Ctrl] + [A]) and
copy it ([Ctrl] + [C]). You can now close the foreground image,
discarding any changes. You’ll now have just the one image
open – the sky – and you’ll notice in the Layers dock that
there’s only the one layer. Next, select Edit > Paste as > New
Layer and you’ll notice that the foreground image appears
as a new layer called Clipboard. You might want to rename
it as Foreground by right-clicking it and selecting Edit Layer
Attributes. At this point, you could just delete the top part
of the foreground layer to allow the dramatic sky from the
sky later to show through. However, this is destructive, in
the sense that once you’ve exited GIMP and, therefore,
flushed its undo buffer, you can’t undelete anything.
Instead, we’re going to apply a transparency mask to the
foreground layer. So right-click the foreground layer and
select Add Layer Mask, accepting the default of initialising
to white (full opacity), before clicking on Add. You’ll notice
that a white rectangle, representing the mask, appears next
to the foreground layer. Before going any further, and
assuming that your two images are correctly registered,
lock them both to ensure you don’t accidentally shift one
layer with respect to the other. To do this, click the right-
most box to the left of the thumbnail image of both layers
and a chain icon will appear to show the lock.
Now all that remains to be done is to make the mask on
the foreground image transparent in the areas where you
want the sky to show through. Do this by selecting the mask,
as indicated by it having a white border, and painting it black
where you want it to be transparent. Select black as the
foreground colour in the Toolbox dock and select the
paintbrush tool. Now just paint the mask black, as necessary,
starting with a large brush, and reducing the size (the square
bracket keys reduce and increase the brush size). Zoom in as
necessary to define the edge more accurately.
If you accidentally make too much of the mask transparent,
just make it opaque again by painting in white. When you’ve
finished, save the image to preserve the layers and masks, and
also use the Export option to generate a JPEG file in which all
the layers and masks are correctly combined.The automatic method
You could use GIMP to create an HDR image with more than
two photos, but generally speaking if you want to go beyond
the sort of effect we’ve just seen by combining a much larger
number of photos, a dedicated HDR package is the way to go.
Here we’re going to see how to use one called Luminance
HDR (http://qtpfsgui.sourceforge.net/)which, in our
experience, gives good results.
The first example we’ve chosen, while not an especially
common scene, was picked because it has an unusually
high-dynamic range. The scene in question (see top-left) was
taken just inside a cave entrance, looking out. A common way
of photographing this view is with the outside landscape
exposed normally so that the cave is rendered almost totally
black, framing the image. While this would be a perfectly
acceptable composition, we wanted to do something a bit
different, so we shot no fewer than six photos, with different
exposures separated by 1EV.
At one end of the range the outdoor scenery was rendered
well, but the cave interior was totally black, while at the other
end, the scene outside the cave was totally white, but we
could see some detail in the cave. Even in this latter case, the
cave interior was still modestly illuminated and, while we
could have gone further, we took the view that caves are
supposed to look somewhat gloomy.
Our first step in turning these into an HDR image is to start
Luminance HDR. Using the program is pretty intuitive. Choose
‘New HDR Image’ and then, in the HDR Creation Wizard, select
the images you want to combine. The images are loaded and
an estimate of the exposure appears for each. You can correct
these if needed. Click Next to go to the wizard’s second
screen. Unselect the Preview tickbox, which will cause the
Compute button to change to a Finish button, which you
should now click on. After some time, you’ll be taken to the
Tonemap window.
By the time the Tonemap window is displayed, the HDR
image will have already been generated, but there’s a
problem. In much the same way that a camera’s CCD can’t
record an HDR image, ordinary (non-HDR)monitors can’t
display them and printers can’t print them. In that sense,
creating a HDR photo is a pointless exercise, or at least it
would be without tone mapping. Tone mapping aims to
process an HDR image, preserving as much as possible of
the tonal range it contains, while making it suitable for
display on ordinary hardware. Inevitably this causes some
artefacts, which can range from subtle to extreme – although
even the latter can result in a visually attractive result, albeit
not altogether natural.Tonemap options
At the left of the Tonemap window are loads of options that
control the way in which the tone mapping operates. Next is a
preview of the currently selected options and, for those who
want a bit less choice, the small images at the right represent
various presets that you can select. We suggest that you
browse the presets first of all, and then, if none of them really
fit the bill, start playing with the individual controls. Bear inAnymovementbetweenexposuresis potentiallya
problem. Clouds moving in the sky is a common example,
although a city scene with moving people and vehicles is
clearly much more of a challenge. While the two methods
we’ve looked at here don’t produce exactly the same sort
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your choice of which of them to use.
Unlike some other HDR software, Luminance HDR
claims to handle movement. However, there’s a limit to
how well this can work, and the result might not be
entirely satisfactory. Nevertheless, in the case of scenes
that contain large moving objects, there might well be
no alternative.
With the case of movement of clouds in the sky, the
method of combining two images in GIMP may well give
better results. This is because the horizon acts as the
demarcation between the two images so now, of course,
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just one of the pair of exposures. The fact that the clouds
in the other exposure might be in an entirely different
part of the sky is, therefore, irrelevant.HANDLING MOVEMENT
In Luminance HDR’s tone-mapping stage, the HDR image is rendered in such a
way that it can be displayed on screen or printed.
36 |^ |^ April 2020