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LAB and the Greens of Nature

Photos dominated by greenery scream out for LAB. Cameras lack the sense of simultaneous contrast common to all human beings. When we see lots of similar colors in close proximity, we break them apart. Cameras don't, so we have to rely on a steeper A curve to avoid the flat look.

The thought process for any image starts with an overall gross assessment, without any numbers. Figure 3.9 is too dark, and there's not enough variation in the greens.


Figure 3.9 Images of greenery often need the A channel sloped more sharply than the B, as otherwise trees may become too yellow.

Next, a strategy meeting to decide how to attack whatever problems exist. The battle against Figure 3.9 requires no great generalship: we'll be fighting in LAB, because of its enormous tactical ability to drive a wedge between colors.

Finally, a look at the existing numbers, to see if there's something nonobvious wrong with the existing picture. We look for things that we know or can surmise the colors of, and see whether the current values make sense. Most frequently, we're looking at the AB values only, but sometimes we're lucky enough to know the L as well. For example, the top of the waterfall is the lightest significant object. If we assume that it's also white, we'd like to find a value of 97L0A0B. We put the cursor above at least three different points in the area, and mentally average the results to get an idea of how close we are. Here, remembering that parentheses denote a negative number, I find typical values of 89L(1)A(1)B. That's too dark, but the color seems fine—being off by only a point or two is inconsequential, and anyway these numbers are slightly green-blue, which surely might be right in this context.

The darkest significant area of the image, a deep shadow just above the waterfall, measures 9L0A0B, so close to the target of 6L0A0B that we can leave it alone.

There are no known colors here, other than the trees themselves, which have to be some species of green. A patch to the right of and about a quarter of the way down the waterfall seems to be the yellowest part of the forest. It measures 60L(20)A35B; the darker, bluer leaves above it average 45L(14)A7B. Both are reasonable numbers, in keeping with the definition of natural greens developed in Chapter 2: strongly negative in the A, strongly positive in the B. They solidify our conclusion that there's nothing horribly wrong with the original color in this picture—it just needs to be pepped up.

The recipe calls for putting the steepest part of the L curve where the action is— namely, the forest. Such a curve suppresses detail somewhat in the waterfall, which falls in the very lightest part of the curve. That area has gotten flatter in the L curve, but the price is probably worth paying. The question is, what to do with the AB channels?

Figure 3.10B uses the same AB curves shown earlier in Figure 1.9, the shot of Anza-Borrego. However, I perceive it as too yellow in certain areas, and prefer Figure 3.10A, which has curves much steeper in the A than in the B, as shown in Figure 3.9.


Figure 3.10 Top, a version using the curves shown in Figure 3.9, which treat the A and B channels differently. Bottom, when the A and B curves are identical.

You'll remember that I did almost the same thing with Figure 3.1. Indeed, emphasizing the green more and the yellow less is usually a desirable move with pictures of greenery—and one that can't readily be executed in RGB or CMYK.

If you're curious about the numbers, the extreme points of the forest that used to be 60L(20)A35B and 45L(14)A7B have become 73L(37)A44B and 55L(26)A9B. There used to be 15 points of difference between the two areas in the L channel and now there are 18. There used to be 6 in the A and now there are 11; 28 in the B and now 35. These big changes create the separation between the two points that we were looking for, variation that RGB and CMYK can't find.

You can use different slopes for the A and B curves with any image, but greenery images are one of two major categories that really suggest a different approach. Greenery usually calls for more A than B. The other major category usually wants more B than A.

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