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In this exercise, I'll introduce you to Photoshop's other three channel color mode which is lab. I've updated my composition and saved it as B&W adjustment.psd. It contains that black and white adjustment layer although it's turned off. Now converting between color modes is technically a destructive modification that is Photoshop has to go in and rewrite all the pixels inside the image. Now converting to lab, because it offers a very large color gamut is about as nondestructive as it gets still it's probably a good idea to go ahead and duplicate our image before we start.
So I'll go up to the Image menu and choose a Duplicate command and I'll call this one Lab variation and click OK. All right, now I'll go up to the Image menu choose mode and then choose Lab Color and a note before I go any farther there are some disagreement out there about how you should pronounce lab. There is a group of folks that are of the mind that you should pronounce each and every letter as an L-A-B. I'm with what I understand to be the majority which is folks who just call it Lab. And one of the reasons for this is A; rolls off the tongue a little more easily and B, it's not strictly speaking an acronym.
The L stands for lightness or luminance once again depending on who you're talking to, but the A and B are arbitrary designations given to perpendicular color axes and I'll show you what that means in just a moment. Anyway, I'll go ahead and choose Lab and then Photoshop tells me it's going to go ahead and discard an adjustment layer specifically that black and white layer there we created in the previous exercise. Do I want to allow that to occur and keep the other layers, or do I want to Flatten the image in order to maintain its appearance? I'm going to say OK for now to keep the layers, but you'll see the background just shifted on screen.
So this is the way it looked before, I'll press Ctrl+Z, Command+Z on the Mac see the blues and violets when that background shifted and this is the way the background looks in the lab mode. So you may figure, well, this is an example of how converting from RGB to lab is destructive, because after all we saw some color shift on screen. The truth of the matter is it's based on this grad layer subject to the Hue blend mode and that blend mode math happens a little differently in the Lab mode that it does in RGB. If I'd flatten the file we wouldn't see that color fluctuation and let me demonstrate that to you.
I'll press Ctrl+Z or Command+Z on the Mac to go back to the RGB mode and I'll go out to the Image menu choose mode and choose Lab Color. This time I'll say that I want to Flatten the image and notice that the background doesn't shift at all. Now the pixels are getting rewritten so again it's a destructive modification and of course I just threw away all my layers so that's pretty darn destructive. However, you are never going to notice a perceptual difference on screen assuming that you flatten when you convert either from RGB to lab, or from CMYK to lab.
A right, now I'll switch to the Channels panel, so we can see what's going on. Once again we have the composite version of the image atop. It's called Lab, because we're working in lab mode, and then we have each of the component channels. We've got the Lightness channel which is the L in lab and so far as Photoshop is concerned anyway. And that Lightness channel is a serviceable grayscale version of the image similar to but different than Photoshop's automatic grayscale conversion. The other channels don't look anything like the image.
If you click on a, for example, you're going to see this mishmash of in this case inverted grays. If you click on b you're going to see a weird gray version of the Toucan as well. So what's going on here? Well basically, what Photoshop is doing is its removing lightness from the color equation, and then it's mapping the colors out on a circle and it's bisecting that circle. So drawing lines through the circle at 90 degrees angles and one of those lines is a and it maps out the colors from essentially green at the dark end to magenta at the light end.
It isn't strictly speaking true? I'll show you what's going on there in a moment and then the b channel maps out the colors from blue on the dark end to yellow on the light end. So if you've ever used Camera Raw, you know that there is those Temperature and Tint values right at the beginning of the development experience b is the same as temperature and a is the same as tint. Now if you want to see what these channels look like in color you can click on one and then turn on the other. So in this case, I have both the a and b channels turned on which means I can see all of the colors in the image, but without the luminance it doesn't make any sense.
So what I'm going to do instead is turn off a, leave b turned on and then turn on Lightness, so that we can see the luminance of the image mixed with those yellows and blues that I was telling you about. And we end up getting a very interesting take on this photograph. All right, now I'm going to turn off b and I'll turn on A now you can see what I was talking about they're not really strictly speaking greens and magentas, it's more like turquoise and hot pinks. But once again in the business, folks say that the a channel conveys a greens and the magentas just so as you know.
All right so you may wonder on what practical use is the Lab color mode? Well, there are all kinds of things you can do with it. From a masking perspective however, we've got this Lightness channel right there and that is yet another take on the image that can be useful for masking. I've never managed to make anything of the a and b channels where mask is concerned, but every once in awhile, I'll find that Lightness channel quite useful. In the next exercise, I'll show you a practical benefit to the lab mode which has nothing to do with masking, but you may find it useful in your day-to-day compositing work.
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