© 2003-2006 Mike Russell, All Rights Reserved

Reach for the Sky, and Pick Lab or RGB

Contrast and color are the name of the game - will RGB or Lab save the day for this image?

Although you may simply read this tutorial, your time will be better spent if you copy the original image to your hard drive, and follow the example in Photoshop. This goes for all of the examples.


The sky in this image is whiter than the Cliffs of Dover. The camera's auto-exposure software has sacrificed the detail in the stones, and the color saturation of the grass in a vain attempt to add detail to the sky. It's up to us to recognize that the sky is gone, and focus, therefore, on the important parts of this image.



Above is the RGB correction of this image discussed in the text. Not bad - note the relatively lackluster grass color.



This is the Lab correction discussed below. Note that the stones retain a more neutral color, and the grass is much greener.

There's nothing like doing something to learn it more quickly.
You are encouraged to load the original image and follow the tutorial yourself.

1. Get ready, this is an interesting, but challenging example.

After loading the image and starting Curvemeister, set up your color space and options as shown.


Start by setting Lab mode, which allows separate
control of luminance and saturation
 

Your curves should look like this for this for the purposes of this example. If they don't, set your curve options as described in the illustration on the right.


The only important settings are Color Frames and Black on Left. Both of these options must have check marks.


2. Most automatic programs do nothing to this image because it has no obvious color cast, and there is plenty of pure white in the sky and black in the shadows. The key to fixing this image is the recognition one simple fact: the sky is pure white. It has gone to highlight heaven, and we should busy ourselves with the rest of the image. Since the sky contains no detail at all, we may turn our attention to the stones and grass. These areas of the image are hampered by both luminance and saturation issues.

To begin to address these problems, as usual we will start with the most important steps of all: setting the highlight and shadow points. This image, in common with most images, happens to have both. The darkest area of this image is the shadow at the base of the vertical stone near the left edge of the image.

Use Curvemeister's threshold feature to quickly locate the brightest and darkest important areas of your image.

Click and drag the right or left edge of the Lightness curve.


Curvemeister will show an image with exaggerated pure black and white where the darkest and lightest areas are located.

Note: if you do not see the speckles, verify that there is a check mark next to the Options>Sample>Continuos Update option.

Right click in the shadow under the vertical stone on the left, and select the Set Shadow menu item. Then fine tune the position of the shadow to break up any solid areas of black.

Do a right click in the middle of the darkest area of the image.
Select the Set Shadow menu item to create the shadow point.

Fine tune the location of the shadow point by dragging
it to minimize any large areas of black. Press the control and space keys to magnify the image, if necessary.

Do the same for the highlight. In this case the brightest areas of the image are specks of white on top of the flat stones on the right.


Right-click on the brightest area of the image and set the highlight

To retain detail in the highlight, fine tune the location as you did with the shadows, to break up the white areas.

Setting the shadow and highlight has the immediate effect of opening up the image with more detail in the stone and grass. We could stop here, happy in the knowledge that we have improved this image substantially, but no, there is more to be done.

Note: if the image did not have a neutral highlight, we would use the threshold control of the curve to set the actual limits of the Lab Lightness curve.

3. Go to RGB mode by clicking on the RGB radio button:

4 Check out the RGB image curves. In particular, notice that the endpoints of the Red, Green, and Blue curves have been shifted to accommodate the white point settings you did in Lab mode.

5. Next step: neutrals. The big players in this image are the stones, which I happen to know should be neutral, and they are indeed already very close to neutral already, so I expect there will be no dramatic changes in the image.

First, click on the radio button to get back into Lab mode.

We move the cursor over a likely neutral spot in the image, avoiding the greenish lichen spots, and use the right-click menu to make that point neutral. It turns out this is not much of a change, since things were very close to neutral. We are rewarded with a slightly warmer tone for the whole image.

  




Start by right-clicking on an area of gray rock. Then click on the Set Neutral command. Then fine tune your neutral by clicking and dragging it to the area you want to be pure gray.


In this case, we fine tune by selecting some of the medium gray areas of rock. If you place the neutral over a greenish lichen spot the stones will change to magenta.

 

So far, Lab and RGB are doing about equally well. Verify this by clicking the RGB radio button, and take a look at how your neutral point looks in RGB mode.

 

6. The only important colored item in this image is the grass. Being the English countryside in early summer, the grass was a rich and verdant green. The grass as it looks right now it would make any self-respecting bovine turn up its nose. Our job is to make it more appetizing.

In Lab mode, we can control color completely without compromising our previous brightness and neutral work Here are the curves that get nice green grass.

Warning; these curves are very twitchy, and you will want to resize the curve window much larger than the illustration to get enough control over the positions of the curve's control points.

Before delving into these curves, here are a couple of observations about Lab mode.

    • Lab can represent a gigantic number of colors. In this image, and most photographs, all the data points are confined to the middle three or four squares. In practical terms is these curves are very sensitive. So make the curve window large, and click on the tab for just the a or b channel. Be ready for some momentary wild shifts in overall color if the center of the curve deviates much from the very center of the graph.
    • Each curve of Lab actually controls two independent and complementary colors. The a curve controls green and magenta, and the b curve controls blue and yellow.

On your own, try steepening the a and b curves, as we did with the kite image in the previous example. If you sharpen steeply enough, you will start to see magenta and other colors crawl out of the stones. Perhaps the most important requirement of color correction is to avoid impossible colors. So we will use a different tactic, manipulating the two halves of the a and b curves independently.

The a curve's central cluster of three control points is designed to stop the bend in the green portion of the curve from affecting the the magenta corner. There is a similar group of control points near the center of the b curve that prevent the yellow bend from carrying over to the blue. Voila - grass so green it has to be on the other side of a fence (which this grass indeed happened to be).

Keep in mind that most of the color action is happening within one grid line or less of the center of this image, so it is particularly important to pin that area down accurately. For this operation you may want to switch to the largest window size available, and show just the curve you are working on.


This is a view of the center of the curve, which you may
obtain by making the curve window larger
showing the manually added pin points. These points must be carefully placed to keep the upper right portion of the curve from going crazy.


Instead of adding the "lock-down" points manually, you
may right click the curve, and click Curvemeister's
new Pin Grid>Upper Right menu item. For brevity, the points that will be added by the command are already shown.



The two points to the upper right are there simply to prevent the rather radical "green" bend from carrying over to the upper right, magenta corner of the curve. If these points were not there, the green adjustment would have the effect of greatly increasing the overall amount of magenta in the image. Notice also that the selected point nearest the center is slightly off center. This point was in fact added when we set the neutral in step 5, and in fact that point will in fact be marked with vertical tick marks to signify this.

The grass looks a bit poisonous after just the a curve adjustment above. The cure for this is to add more yellow to the mix, and in the spirit of increasing saturation, we accomplish this by steepening the yellow corner of the b curve, as shown, bending it in what happens to be a mirror image of the adjustment I made to the a curve.

Look at the before and after image at the top of this page to see how the green and yellow been increased, giving more color range, and a more saturated appearance, to the grass.

In RGB mode, there is very little that can be done without altering neutrals and/or the shadow and highlight.

7. Here is the final version of all three curves in Lab mode:

Nothing new here, except the bumps in the Lightness curve. The shadow and highlight are left in the same place as before. I added the midrange point, indicated by the cursor, to increase the bite of the medium gray stones. I added the point below giving even more precious contrast range to the all-important stones, rather than waste it on the shadows.

Adding the midrange point took contrast away from the lighter colored top surfaces of several of the stones, so I added the point just to the right and above the midrange point. This opened up those surfaces. The final point, closest to the highlight, was added to give more contrast range to the very bright horizontal stone on the far right of the image.

8. The final RGB curves look like this:

 

The logic behind the RGB curve is similar to that of the Lab curve. Notice however that not much is happening in the color channels. There is only one point on each curve for a neutral point. In particular, the magenta shimmer in the stones is here to stay in RGB. Nor can we, in RGB, bump the saturation of the grass because increasing green in RGB mode will add green to the entire image, not just the green areas.

Check out once again the comparison image at the top of this page. The grass is much greener on the Lab side of the color space divide.

Experiment - how good can you make this image in HSB and wg-CMYK mode?
 

Table of Contents