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92 c’t Digital Photography 16 (2014)

Bernhard Stockmann

How to Create

Light Rays with GIMP


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93c’t Digital Photography 16 (2014)

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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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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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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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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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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