Terragen images
colour correction and other adjustments
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Part 2 | Contrast/brightness
adjustments and converting to Black and White
Jean Claude Grégoire & Esteban Glas
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Now, suppose you have an image of which you love the little
saturated colours, but nevertheless there is something wrong, something
you don't like perfectly: a certain lack of contrast. If it was a black
and white photo, you should say: it's too pale, too faded, too... gray,
it lacks blacks and whites.
But with the preceding methods, the problem is that when you ameliorate
the contrast between the dark and the light tones, you modify the colour
too. Here you absolutely DO NOT WANT to modify the so subtle colours you've
got with Terragen.
In this page we will explain a rather complicated method. There's another easy way to deal with contrast available here.
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| The solution is: playing with the LAB channels. |
"What a strange beast could this be?!?!" You'll probably
say; "That's much too complicated for me, only for professional photographers
and infographists!"
You'll see it's not complicated at all. It's very simple, actually. There
are lots of books written on the subject, but I'll explain it in a few
words. In contrast with the RGB mode, where the three channels are Red,
Green and Blue, the Lab mode, which has three channels too, is made of
an entirely different way. The three channels are:
- The "L" channel, is the "Lightness" channel and defines the contrast between the darkest and the lightest
areas of your image; it's almost like a B&W image which only contains
a grayscale, but no colours.
- The "a" channel, which defines the relative proportion between magenta
and green in the image.
- The "b" channel, which defines the relative proportion between blue
and yellow in the image.
Wow! That's the solution, because you won't modify the colours but only
the black and white proportion of the image. Thus you only have to work
with the "L" channel. Yeah! You're right: the colours won't be affected
too much by this.
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Open the image in your editor. The example we have
chosen has very subtle tones of blues and yellows. If you apply
the method of the curves adjustment layers with the black and white
eyedroppers, you'll get an oversaturated image.
There is an easy way to correct it with the help of the Lab channels,
without modifying your beautiful subtle colours.
On the other hand, this is a kind of image which could get a lot
of style in a black and white version.
This conversion can be achieved very easily with the LAB channels
too, because this last method can give much better images than the
commands "Image > Adjustment > Hue-Saturation" or "Image >
Mode > Grayscale".
Let's see how to proceed.
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Uncorrected image as rendered by Terragen
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Adjustment of the light balance
without modifying colour |
Fig. 1. The LAB channels tab |
1. Choose "Image > Mode > Lab colors".
2. Go the the LAB channels Palette.
3. Select the "L" Channel (fig. 1). |
4. Choose "Image > Adjustment > Curves". Now that you've selected
the "L" Channel, this adjustment will only apply to this "L" Channel.
So it's impossible that you modify with this method the subtle arrangement
of blue and yellow tones of your image.
5. Now left-click on the lower part of the curve, and drag and drop
it a little more to the right and to the bottom. You click on the
upper part of the curve, and drag and drop it a little to the left
and to the summit (fig. 2). |
Fig. 2. Where to click |
Fig. 3. The resulting curve |
You should get an S shaped curve like the one we show here(fig.
3). |
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| By doing this you see that your image has improved it's
contrast level. In this way you can modify the curve until you get,
so to speak, an excellent black and white photograph on your
screen (see fig. 4). In order to get this view you'll need to turn
off the "a" and "b" channels. |
Fig. 4. Adjusted L channel - no colour |
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Important
The S shaped curve usually gives the best results. We strongly advise
you not to try the sort of corrections like the one shown in figure
5, which look quite like the corrections made by the "auto" command.
Actually, a very sloped curve means much contrast, but the horizontal
ends of the curve on this figure indicate that you have lost all
detail and all contrast in the darkest and lightest areas.
Remember: As the curve gets more vertical, it means more
contrast; on the other hand more horizontal means less contrast.
Like in a landscape: vertical slopes give as a result a more contrasted
landscape than horizontal ones. Easy, isn't it? |
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| When you are satisfied with your "black
and white photograph", click OK and it's time to come back to
the colors. Therefore, go in the channels window and click on the
"LAB" composite channel. The colors are back. Then return to the RGB
mode by choosing "Image > Mode > RGB" (see fig. 6). |
Fig. 6.a. The adjusted image |
Fig. 6.b. The sharpened image |
Now you only have to sharpen your image a bit and to
convert it to JPEG format for the web. |
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Transforming images to black and white
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Perhaps you found the B&W image was wonderful, and you had wanted
to keep it as the best one for the Web! It can be easy done.
When you are happy with your "black and white photograph" (fig. 4),
click OK but do not come back to the RGB mode. You just have to "delete"
the colour channels "a" and "b". How? Actually, that's very simple. |
Fig. 7. Dragging and dropping the a channel into the
trash |
Drag and drop the "a" channel into the trash at the
lower right end of the Channels Palette (fig. 7). |
Fig. 8. Dragging and dropping the Alpha 2 channel into
the thrash |
Now the names of channels have changed to "Alpha
1" and "Alpha 2" (fig. 8).
"Alpha 1" is the former "L" channel, and "Alpha 2" the former "b"
channel. You should now drag and drop the "Alpha 2" channel to the
trash.
Choose "Image > Mode". You'll see that the menu indicates that
you are in the "multi-channels" mode. You just have to choose "Image
> Mode > Gray Levels", and you have a very nice B&W image.
Just a little sharpening and it's ready for the Web (fig. 9). |
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Fig. 9. Sharpened B&W image for the Web |
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Appendix 1 | Difficult cases
When the adjustment is only good for a part of the image
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Now we should examine a difficult case: when the
adjustment is good for a part of the image, and bad for the rest
of the same image.
See next image to enhance (left). The sky looks good, but the land
is too pale and lacks contrast. Beside the sky, it has a lot of
light; a "washed-out" look.
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| As we have already described before, you should go to
the LAB mode, select the "L" channel and choose "Image > Adjustment
> Curves". You play with your curve (right). Now you see that this
curve is good for the land, but that the sky looks much too dark. |
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The result in full colours can be seen here at the left.
The sky is unacceptably dark. What can we do? |
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Start with your original Terragen Image, and change
to LAB mode. You don't need to select any channel; what we are now
going to do is select only the terrain. Go to "Selection > Colour
Range". Click on the first eyedropper tool (marked 1), then on the image (1.). Then click on the "+" eyedropper
(marked 2), then at
some places in the image (2s). You see the result of your selection
in white in the Color Range window. If you have selected too much,
click on the "-" eyedropper (3) then in the image, etc. When
you're satisfied with your selection, click OK.
Now you see your selection in the image (marked with "marching
ants" borders). It'll be annoying for further work, so you
type <Ctrl+H> and the selection disappears, but it still works
in the background.
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Go to the channels palette and select the "L" channel.
Then choose "Image > Adjustments > Curves". Play with the curve
until you are satisfied with the result, i.e. your "black and
white photograph" (fig. 14). You see that the sky has remained
unaffected by the alterations on the curve. |
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Now click the composite "LAB" channel for going back
to a colour image and choose "Image > Mode > RGB" (fig. 15).
Your sky is not modified and your land is much better than in the
unadjusted image.
You already know the rest of the work.
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| NOTE: it
is possible to render masks in terragen for land, clouds, water and
sky. Those techniques are described by C-wied in his tutorials
(beware! page with annoying pop-ups). A good way to maximize the use
corrections is the use of masks for different parts of the render
in order to affect just the desired sections. |
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| Appendix 2 | Another very simple method
Separation between the color and the contrast adjustments
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You've rendered a Terragen image and you want to
- modify the contrast;
- modify the colors.
See the image to enhance (left). It looks pretty good, yet you're not satisfied with the colors you find too "monotonous". You'd like to add a bit more "zip" in it. But when playing with the curves, you don't come to a satisfactory result, because either the color is good or the contrast, but never both together.
Your problem has an easy solution: you just have to work on the colors and the contrast separately .
Before beginning, we must say that we have exaggerated a bit the colors and contrast for a better understanding of what we want to demonstrate. |
Begin with duplicating your image as a second layer. Call it "copy for contrast adjustment" and deselect it when you're working on the first one. Select the first layer; call it "original for color adjustment" and choose "New Adjustment Layer > Curves". Choose "group with preceding layer". Play with your curves until you're satisfied with the enhanced-colours image (right). Now you see that these curves are good for the colors, but that the contrast is too weak.
Here you can get violent and unnatural colors because you haven't adjusted the contrast yet. |
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Select the "copy for contrast adjustment" layer and add a curves adjustment layer. Group it with the preceding one. Now you adjust the curve until you find a good contrast. The result can be seen here on the left: excellent contrast but the colors aren't the ones you wanted.
You have to combine your two layers so that you like the result.
Therefore each adjustment layer has to be grouped with the preceding one. The blending option for the contrast layer has to be changed from "normal" to "luminosity". |
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See the layers palette (right):
The blue
grouped layers are:
- The original layer for color
adjustment;
- The curve adjustment layer for color
adjustment.
The yellow grouped layers are:
- The
copied layer for contrast adjustment (which is selected);
- The
curve adjustment layer for contrast adjustment.
The selected
layer has its blending mode set on "luminosity". |
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The result of this blending mode (luminosity) is that the contrast adjustment layer only works on the LIGHTS of the image, without any influence on the colors.
The final image can be seen on the left. |
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