Quick 'n' Dirty Color Correction with LAB Color

(Republished from the SitePoint Design View #30)

If you’re anything like me, Photoshop’s LAB Color option has always headed the list of ‘strange things to ignore in Photoshop’. Like most designers I know, I’d long ago decided that two color spaces — RGB and CMYK — were more than enough for this little black duck.

That was until I came across Dan Margulis’s Photoshop LAB Color a little while back. I must admit that when I ordered this book I wondered to myself how anyone could write 350+ pages about a single color space. After working through the book, I can see that Dan probably could have written a lot more.

Make no mistake — LAB color is a little weird. Familiarity with either RGB or CMYK probably won’t have done a lot to prepare you for working in the alternative universe that is LAB color.

Although that weirdness is no doubt a major reason for your average Photoshop user to ignore LAB color, it’s also at the heart of LAB color’s real power. The color model behind LAB color is so ‘left-field’ that it has the ability to describe colors that don’t even exist (outside the mind at least), let alone have the ability to be described by the domestic RGB and CMYK color spaces. Strange but true.

Now, although an in-depth explanation LAB would be beyond a single post — for that I recommend Dan’s book, we can tackle:

1) a thumbnail sketch of how LAB color works

2) a simple, repeatable LAB color correction method that will improve 95% of your photos at a step

That should give you the grounding to start your own trial and error tests.

LAB Color — The Thumbnail Guide

The easiest way to start is by looking at the LAB channels. If you have Photoshop handy, it might be useful to open a photo and switch to LAB color to follow along (Image > Mode > Lab Color).

Quick 'n' Dirty Color Correction with LAB Color

First let’s take a peek at the Channels palette.

Channel L = Lightness

The Lightness channel shouldn’t be too difficult to grasp. A cursory glance will show that it looks pretty much like a normal grayscale image. Using Photoshop’s Curves or Levels tools on this monochrome channel will have an immediate and predictable effect on your full-color image.

Channel A = Red /Green

This is where things start getting … different. Rather than representing a single color (as RGB and CMYK channels do), Channel A actually displays a “battle” between two contrasting colors — red and green.

In Channel A, the darker an area is, the greener it is. The lighter an area is, the redder. Any area that is neither red nor green will be shown as exactly 50% gray. Pretty loopy, I know.

If you’re having trouble getting your head around this idea, try switching on only the L and A channels (CTRL-click them) and you’ll probably start to get a feel for how the system works.

Channel B = Yellow /Blue

Channel B takes the same approach as Channel A, only the colors are different. Yellow and blue are the colors displayed, with lighter areas corresponding to more yellow and darker areas corresponding to more blue.

Okay, now you’re probably thinking, “That’s all very nice’n’all, but how does it help me?”

Bear with me.

The human eye is highly sensitive to color, but it does have a quirk or two. Specifically, we tend to judge any color as being more vibrant when it’s placed on top of its contrasting (or opponent) color.

For instance, in the diagram below, the green square seems to be almost vibrating when surrounded by red (if you don’t see it that way, it’s ok — you probably just have alien DNA).

Quick 'n' Dirty Color Correction with LAB Color

Greens tend to appear brighter and more vibrant when placed on a red backdrop. Herein lies the true power of LAB color.

If we use Photoshop to intensify our ‘Channel A’, not only do we make our green bits a little greener — we make everything else a little bit redder (in relative terms).

The result is greens that appear even more vibrant.

Of course, this logic doesn’t only apply to the green image areas — the red image areas are similarly boosted, and we can manipulate Channel B to produce a similar effect on our yellows and blues.

If you’re confused, maybe a practical example will help.

No-brainer LAB Color Correction

1). After switching your color mode to LAB color, hit CTRL-M to launch PhotoShop’s Curves dialog. The Lightness channel (Channel L) will be selected by default.

Quick 'n' Dirty Color Correction with LAB Color
2). Select the lowest point on your curve line (0%) and drag it to the right, to the 10% grid line (see example). Next, grab the highest point on the curve (100%) and drag it to the left, to the 90% grid line.

We’ve just compressed the tonal range of our L channel, making the shadows in our image appear richer and our highlights appear brighter.

3). Next, switch the Channel drop-down at the top to Channel A and create the same curve shape — compressed between 10% and 90%.

As you slide the top of your curve left, your image should take on a greener tinge, but sliding the bottom of your curve right will balance things again. If you’re feeling bold, you might venture further than 10%, but keep your curve symmetrical unless you want to introduce a tint or “caste.”

4). Finally, select Channel B and recreate the same curve shape again. Hit OK and your color correction should be complete.

This is the result I achieved using a random stock image from SXC.hu. Your result should be similar.

Quick 'n' Dirty Color Correction with LAB Color

Is this final image perfect?

Obviously not by a long shot. Too much cloud detail has been blasted out and some shadow areas have filled in too much. Some educated tuning wouldn’t go astray.

Nevertheless, there is a richer, clean quality to the new image, despite the fact we used no particular skill or judgement to produce it. It was more or less a push-button result.

In truth it’s hard to truly appreciate LAB’s real magic until you begin to really get a feel for its strengths, and how changes in the LAB color space affect your images.

After a few months of working with Dan’s book, I’m starting to see LAB color as the sports car of color correction. Fast, precise and very responsive.

I think it’s certainly worth tinkering with on your next color correction.


Category: other Time: 2007-02-22 Views: 1

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