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8/18/2019 LightRaysInGIMP Is16 CtDP 2014
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92 c’t Digital Photography 16 (2014)
Bernhard Stockmann
How to Create
Light Rays with GIMP
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The Sun’s rays shining through a church
window – a beautiful effect created
entirely in GIMP
T
he effects produced by incident light can
be diffuse, highly directional or simply
random. Sometimes, when light is scattered
by clouds or formed into arrow-like rays by
windows, it produces effects with a unique,
mystical beauty all their own.
These effects are the result of normally
invisible light rays being dispersed and made
visible by fog, dust or smoke particles in the
atmosphere. Recreating such effects digitally
is a relatively simple matter and can be
executed in the open source GIMP image
editing suite using the simple steps described
on the following pages. This type of edit can
be used to give all manner of images
additional atmosphere. (keh)
I m a g e : f u t u r e s h a p e o n f l i c k r , A l e x a n d e r B a x e v a n i s
Light rays diffused by the clouds orshining ethereally through church
windows are just some of the effectsyou can create using GIMP’s volumetriclighting tools. This workshop showsyou how to apply these fantastic effectsto your own images.
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GIMP Workshop | Volumetric Lighting
Begin by selecting the places where the ‘light rays’ are to appear. To do
this, you need to make a black-and-white copy of the background layer in
which the ‘light’ areas are white and all other details black. To duplicate
the background layer, use either the ‘Create Duplicate Layer’ button at the
foot of the Layers panel or the Ctrl+Shift+D keystroke.
Step1
The basis for this workshop is a photo of the interior of theGuildhall in London. The steps described below show you howto add digital ‘light rays’ that appear to shine through thewindows on the side of the building.
GIMP Workshop – How to Flood a Room with Volumetric Light
After
Before
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Volumetric Lighting | GIMP Workshop
Step2 Now select the Colors > Components > ChannelMixer command and check the Monochrome option. The preview image will now be displayed as agrayscale image.
To emphasize the bright areas, shift one of the three
color channel sliders all the way to the right and
leave the other two set to zero. This produces a
version of the grayscale image in which the tonal
values are determined by the values captured in a
single channel.
The brighter parts of most images are recorded in
either the blue or the red channel, while highlights
are rarely found in the green channel. In our example,
we shifted the blue slider all the way to the right and
kept the other two at low settings. The result is a
high-contrast image in which the dark gray tones
tend toward black, thus further emphasizing the
bright windows.
Step3 Now use the Colors > Levels dialog to adjust the lastremaining gray tones. The Input Levels histogramreveals that the image still contains quite a few dark
gray tones. Move the left-hand (black point)
triangular slider to the right to shift the dark gray
tones to black. Similarly, moving the right-hand
(white point) slider to the left brightens the
remaining light gray tones so that they become
pure white.
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GIMP Workshop | Volumetric Lighting
Now use the Brush tool with a wide
tip to ‘paint out’ any remaining
highlights that you don’t wish toinclude in your ‘light ray’ effect.
Step4
Duplicate the edited monochrome
layer and hide the duplicate (in our
example, the ‘Lights copy’ layer) by
clicking the eye icon to the left of the
layer thumbnail. Now select the
visible monochrome layer and apply
the Filters > Blur > Gaussian Blur filter
with a Radius value of 30.
Step5
Step6 Optionally, you can also color the‘sunlight’ you have created on theblurred layer to give it a warmer
yellow/orange tone. To do this, select
Color > Colorize and shift the Hue
slider to a value between 30 and 40.
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Volumetric Lighting | GIMP Workshop
Step7 Switch the layer mode of the edited layer toScreen. This blends the bright areas in theedited layer with those on the layer beneathto create a ‘glow’ effect.
This is where we produce the all-important
‘light rays’. Reactivate the topmost copy layer
by clicking the eye icon on the left.
Now navigate to Filters > Blur > Motion Blur
and select appropriate values. We used a
Linear/Length value of 256 and an Angle of
168 degrees. Applying this effect can take
some time, depending on how powerful yourcomputer is and the values you select.
Step8
You can further emphasize the rays using
the Filters > Enhance > Unsharp Mask filter.
This tool makes the image sharper by
increasing the amount of contrast between
the individual object edges. An Amount
value of about 2.30 and a Radius of 5 make
good starting points for your experiments.
You can increase the length of the ‘rays’ by
re-applying the Motion Blur filter to the
edited layer using the same settings as
before. Doing this to our sample layer
produced the result shown on the right.
Step9
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GIMP Workshop | Volumetric Lighting
Altering perspective in the ‘rays’
layer adds vitality to the scene.
Activate the Perspective tool bypressing Shift+P and zoom out a
little (using the scroll wheel with
the Ctrl key pressed) to give
yourself a clearer view. In our
example, we ‘stretched’ our
image toward its top right corner.
Press Enter to confirm your
changes.
Step10
To ensure that only the white
rays remain, delete all the
remaining black pixels using the
Colors > Color to Alpha dialog.
Step11
Use the eraser to delete any stray
rays that appear in the wrong
places.
Step12
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Volumetric Lighting | GIMP Workshop
Adapting the color of the rays to fit the scene
makes them look more realistic. To do this,
duplicate the ‘rays’ layer and switch the layermode of the layer beneath to Value. This uses
the color values from the lower layer to color
the one above.
Step13
Another way to add emphasis to the light rays
is to darken the entire base image (i.e., the
background layer) by dragging the image curve
slightly lower in the Colors > Curves dialog.
Adjust the opacity of all three layers to balance
their individual effects and give the image its
final polish.
Step14
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