Blue Skies

Fig.. 1: JPEG straight from camera (click for larger version if you want to follow along with this tutorial)

If there's one thing that bugs me about the JPEGs that come out of digital cameras, it's what they too often do to a simple blue sky.

Blue sky is blue. It is not green.
Don't give me skies that make me scream.

Blue sky is blue. It is not white.
Don't give me skies that are too bright.

Blue sky is blue. It is not jam.
Don't give me aqua or cyan.

Fig. 1 shows a very typical example of the sort of colour savagery wreaked upon Mother Nature by digital cameras shooting JPEG. The trees and other vegetation are recognizably green; the asphalt is recognizably grey; the wooden portions of the bridge are recognizably that rust-brown colour of a familiar outdoor paint. The water is the sickly yellow green that comes from silt and algae in equal measure. But the sky is not blue, it's blue-green – and so nearly white it's in mortal peril of being washed out altogether.

When you see a green sky in a picture that was taken at 11:08 AM, like Fig. 1, you know something is wrong. To fix it you need a tool that can selectively reduce the amount of green. Technically, this is called eliminating a green cast. Your image editing software may have special tools for this, but just as there is no software that you can trust to drive your car while you sit back to read the morning paper, there is no imaging software that can reliably do automatic colour correction. Many applications make wonderful claims and work wonders on certain images; but I'm pretty sure no application has nailed universal automatic colour correction.

The editing program I happen to have is a non-recent version of full Photoshop. It is not intended for casual users and has relatively few hand-holding tools; but it's all I have to hand. The hand-holding tools it does have are called Auto Levels, Auto Contrast, and Auto Color; they're at least fairly typical of tools available in more accessible programs, like Photoshop Elements and Paintshop Pro.

Looking at Fig. 1, the first thing that needs doing is to darken the sky enough so we can judge its colour clearly. Perhaps Photoshop's Auto Levels and/or Auto Contrast should correct this, but they don't. But by using the almost universal Brightness/Contrast tool to reduce brightness (of the entire image) by around -24 we have a usable compromise. The sky has more colour and the rest of the scene is not yet too dark:

Fig. 2. Brightness/Contrast

Now that we can see the sky colour more clearly, we really need to do something about that green cast. Photoshop's Auto Color correctly finds and removes a different problem – a blue cast – but does nothing for the green in the sky. And that pretty much exhausts Photoshop's collection of auto-wonders.

Moving on, the Levels tool has been touted as one of the more beginner-accessible tools; let's see what we can do with it. When it first opens it's configured to adjust brightness across the entire image, but we've already taken care of that with the simpler Brightness/Contrast tool. Exploring we find there are Red, Green, and Blue channels available. The most obvious thing to try is to drag the Input sliders under the histogram this way and that to see if we can get the sky to turn blue. Problem is, this approach turns the rest of the image to purple by the time you've changed the sky to anything resembling blue:

Fig. 3. Levels, Green channel input

I was ready to give up on the Levels tool, when I chanced upon the solution: moving the Output slider on the right:

Fig. 4. Levels, Green channel output

Fig. 5. Green channel result

Applying this correction we get the results shown in Fig. 5. We now have a version of the image that is at least not offensive to the eye. This is probably where you'll have to stop without more sophisticated tools and skills, and it's quite acceptable for normal amateur use, such as e-mails and photo lab prints.

But for the sake of completeness, the final and rather subtle change is to get rid of the blue cast in all the picture except the sky. The sun is white and is the primary illuminant for this scene, but the blue sky of midday is itself a source of light and casts a blue tinge to the scene. If we try to use Auto Color or Levels or any other tool that works on the entire image, any change we make will also act upon the sky, which is already correct from our previous edits. To work on just the non-sky portions of the image we can either select the sky and invert, or we can use a tool that allows us to work on only the non-bright parts of the scene. I'll use the later strategy via Photoshop's Curves tool.

Opening Curves, I switch to the Blue channel (since I want to get rid of blue), click on the diagonal line somewhere near the top, to lock down the bright end of the tonal scale, click closer to the dark end of the diagonal, then drag down slightly:

Fig. 6. Blue channel curve

Fig. 7. Blue channel curve result

If your monitor is accurately adjusted, you should be seeing Fig. 7 as subtly more pleasant than Fig. 5.

These are simple adjustments that you can make with all the major image editing apps I know of, including Photoshop, Photoshop Elements, and Paintshop Pro (but as I write this, not Picasa or the accessory applets that come with Windows). By clicking on Fig. 1 you can view and download a larger version of the original image. You may find it worthwhile to do that then open it in your usual editing app, then repeat the steps I took using the facilities available in your app to get something very close to Fig. 5, above. You can also click on Figs. 5 and 7 for larger versions of them for comparison with your own results.