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Photoshop Top 40
Illustration by John Hersey

6. RGB, CMYK, Lab


From:

Photoshop Top 40

with Deke McClelland

Video: 6. RGB, CMYK, Lab

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! #6! Feature #6 is the color models by which I mean RGB, CMYK, and Lab. And Photoshop is absolutely exceptional in its degree of support for these big three. It even supports infinite variations of the two device dependent models, which are RGB and CMYK. There is one and only one Lab, and I'm going to tell you all about it here inside of this video.

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Photoshop Top 40
7h 13m Intermediate Dec 21, 2009

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There's nothing people love more than lists, and Photoshop Top 40 offers a great one, highlighting the best features in Photoshop. Deke McClelland counts down to #1 with a new video each week, detailing one great feature after another in this popular digital imaging application. The videos cover tools, commands, and concepts, emphasizing what's really important in Photoshop.

This is an ongoing course that will be updated monthly.

For the newest updates please go to our blog entry for Deke's Photoshop Top 40.

Topics include:
  • Assembling multiple pieces of artwork with layer comps
  • Creating a black-and-white image from a color photograph
  • Merging multiple channels to create an alpha channel with calculations
  • Selecting images with the Pen tool
  • Masking images using the Brush tool
Subjects:
Design Photography
Software:
Photoshop
Author:
Deke McClelland

6. RGB, CMYK, Lab

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! #6! Feature #6 is the color models by which I mean RGB, CMYK, and Lab. And Photoshop is absolutely exceptional in its degree of support for these big three. It even supports infinite variations of the two device dependent models, which are RGB and CMYK. There is one and only one Lab, and I'm going to tell you all about it here inside of this video.

We're looking at an image from Oleg Kozlov and this image will service very well at the beginning of our discussion. Later on I'm going to switch to a more colorful version of this cat which is this image right here. This is kind of a Na'vi version of the cat, if you're familiar with the movie Avatar. And in addition, to the highly saturated colors inside the feline itself, we also have these color swatches up and down the left and right hand side. So we've got the full throttle versions of Red, Yellow, Green, Cyan, Blue, and Magenta, which are the RGB and CMY primaries.

Then we have half intensity versions of each one of those colors. Along the right-hand side, we have Macbeth styles swatches, which are more representative of colors that you see in the natural world, and we'll come back to this image because I want you to see what you lose when you switch between color spaces. But first let's see what happens when you switch color models in the first place. I'm going to go back to the original cat right here and we are going to bring up the Channels palette, and I want you to see this is an RGB image. Now, RGB is the color space of your computer monitor, of your digital camera, of your scanner, of anything that projects or receives light.

So anything that can capture an image, anything that's going to display an image is going to be an RGB device. CMYK is going to be your printer essentially, and we'll come to that in a moment. And then Lab is device independent. It's designed to capture a wider range of colors that we see in the real world. All right, so an RGB image is made up of three channels Red, Green, and Blue. Now notice that all these channels are showing up in grayscale at the moment. That's Photoshop's default setting. The reason it is, is because it makes it easier to mask an image using feature #15, alpha channels.

However, if you're interested in understanding how these channels work, you might want to see them the way that Photoshop sees them. And to see the channels in color, you go up to the Edit menu or the Photoshop menu on the Mac, then you choose Preferences, and then you choose this command right there, Interface. And that's going to bring up the Interface panel of the Preferences dialog box. Then turn on Show Channels in Color, and just like that they appear in color. Click OK, and that's what the Blue channel looks like, and that's what the Green channel looks like. That's what the Red channel looks like. You can even turn on just a pair of channels to see how they mix together.

If you turn on Red and Green for example, you're going to get all you yellows, you're going to get all your oranges, all the colors found between Red and Green on the big color wheel. And then of course, to get the final whites inside the image you've to go ahead and add Blue as well. All right, so that's RGB. The next color space on the list is CMYK. CMYK, the only reason they ever go to CMYK is because you want to professionally output your image. You're going to send it out to a commercial printer. If you're printing it at home, or at your office, or anywhere locally, if you're printing it to an inkjet printer, then leave it in RGB and let the printer driver do the work.

You only send it to CMYK if you're sending it out to a commercial printer, if it's actually leaving the building. All right, so let's say it is. I'll go up to the Image menu, I'll choose Mode, and I'll choose CMYK Color. Now you're going to get an alert message here that's going to tell you, hey, this is one way to make a CMYK image. If you go this route then you're going to use whatever profile you specified back in feature #10, the Color Settings command, which by default is U.S. Web Coated (SWOP) v2. If you want to convert to any other CMYK profile, then you choose the Convert to Profile command from the Edit menu.

That's entirely up to you. However, I'm going to say Don't show again, so we don't have his message every single time, and I'm going to click OK. And now notice that we have a total of four channels, each of which represents a different primary ink or plate, if you prefer, because the ink is actually laid onto a plate when your commercial printer outputs the image. Cyan, the purpose of the Cyan ink is to absorb Red light and reflect back Green and Blue, which is why Cyan is a combination of Green and Blue. It's a Blue-Green color.

Magenta is absorbing the Green light and reflecting back Red and Blue. And Yellow is absorbing the Blue light and reflecting back Red and Green. Now in a perfect world that's all you would need. You would just need Cyan, Magenta, and Yellow mixed together. You can see though this is not a perfect world, we're missing our ultra-lustrous rich shadows, which is why we need an extra plate of black ink to burn those shadows into place. So that's CMYK. It's a very different color space than RGB incidentally. And then finally, what we would do.

You wouldn't go, by the way, you don't go from RGB to CMYK and then to Lab. You don't just play around willy-nilly in the different color models unless you want to ruin your image over time. So what we would do if we're going to go to Lab next, it would be smart to undo the CMYK color transformation there. So we get back to our RGB image. Then that way we haven't messed up any of the colors inside the image. Then go up to the Image menu, choose Mode, and choose Lab Color. Now, Lab is also a three channel space, but very different channels this time around.

Here is Lightness, which is all luminance levels inside the image. Here is a and b, and by themselves they don't make any sense. So what we need to do is turn on the Lightness channel at the same time. So I'm going to turn on Lightness and b at the same time. And by the way, a and b are two different axes of color. So all the colors are located in the a and b channel. But you're going to need Lightness turned on in order to see what in the world they mean. This is Lightness plus b, and what you're getting inside the b channel is your temperature values. These days we think of b as temperature.

And by temperature I mean the colors ranging from Blue to Yellow. So from cool over to warm. Now, the opposite axis of color, if you're thinking about all the colors mapped on a big color wheel, the opposite axis is going to be your tint axis. I'll go ahead turn off b for a moment, and turn on a, and your temperature in your tint axis is going to be essentially your turquoise to your pink colors. Now you'll hear people say it's your Green to your Magenta range. But Green and Magenta have very specific definitions whereas Turquoise and Pink don't.

That's why I prefer them. And that's basically the way the colors look as well. If you add both your tint, your a channel and your b channel temperature together, you're going to get all the colors inside that image. Now why in the world would you want to switch to Lab? Lab is not a color model that you ever need to switch to. It's not like CMYK where you need to ultimately switch to it for commercial printing purposes. Lab is more of a creative channel. So, for example, let's say that I wanted to enhance the Saturation values inside the image.

Rather than using feature #28, Hue/ Saturation, you could take advantage of Lab combined with feature #9, Levels, and you're going to get better results too. So let me show you what that looks like. I'll go ahead and press Ctrl+L, Command+L on the Mac to bring up the Levels dialog box. I will eave Lightness alone because the luminance values are just fine. What we want to change is the saturation. So we're going to edit the a and b channels. And what I'm going to do is I'm going to increase these black values to 80, really through the roof and notice that enhances the turquoise values like crazy inside this image.

Then I'm going to subtract 80 from 255, which works out to 175. That balance is the tint values. So I've enhanced the pinks inside the image as well. Now, let's switch down to b and we'll do the exact same thing. If I increase the black value to 80, then I'm going to blow out the blues inside the image. And I can temper that by decreasing the white value by that same amount down to 175, and then I balance out the Yellows as well. And now I click OK. And so just to give you a sense of what we've been able to do, this is before the original saturation of the tiger and this is after, much more saturated this time, without harming any of the detail inside of the image.

Whereas something like Hue/Saturation has a tendency to harm detail if you go too far with it, and it increases things like JPEG artifacts, all kinds of stuff, whereas Lab does a really great job. All right, anyway, so let's take a look at what happens when you're working inside of a layered composition. Here's my big layer composition right there, and notice that we have just a ton of layers going on here. So we've got the swatches layer, which is the smart object. You may recall that as feature #18. Then we have this cat layer that's clipping a bunch of adjustment layers.

Now adjustment layers don't tend to survive from one color model to another. So you usually have to get rid of them in one way or other. So let me show you what it looks like when we are converting a layered image. I'll go up to the Image menu, I'll choose Mode, and I'll choose CMYK Color. And the first thing that Photoshop is going to ask is, do we want to Rasterize this smart object. We don't necessarily need to. Typically by the way you can leave a smart object in a different color mode. This smart object right here, swatches, will remain an RGB smart object even if we're going to CMYK, and that's just ducky where Photoshop is concerned.

You can make that conversion on the fly, in other words. So we would say Don't Rasterize. Here is the big one though. Changing modes will discard some adjustment layers. Change mode anyway. Well, actually it's going to discard all of our adjustment layers, as you'll see. If I click OK, which is the wrong choice, by the way. It should not be highlighted like that. But if you click OK, you're just going to basically lose all your adjustment layers and we no longer have Na'vi cat anymore, we just have a standard everyday average earth tiger. That's no good. So press Ctrl+Z, Command+Z on the Mac to undo that modification.

Go back up to the Image menu, choose Mode, and choose CMYK Color. Now this time around, if I say Don't Rasterize, it doesn't matter because then I'm going to turn around, say Flatten which I have to do in order to preserve the color adjustments and that's going to flatten the entire image. So what do you do, how do you keep that Smart Object and yet keep the affect of the color adjustments? You do this. You undo once again, Ctrl+Z, Command+Z on the Mac. You make sure that in this case the cat and all of its adjustment layers are selected because all the adjustment layers are clipped to the cat, only the cat layer needs to be selected in my case.

I'll go up to the Layer menu and choose this command, Merge Clipping Mask, or I press Ctrl+E, Command+E on the Mac. That goes ahead and merges out the cat, now we won't get bugged about that specific piece anymore. If I go to the Image menu, choose Mode, and choose CMYK color, Photoshop starts by asking if I want to Rasterize this smart object. No, I do not. Notice this is a different message this time around. Do you want to flatten because you could lose some layer appearances, thanks to blend modes. So feature #11, if you've got all kinds of blend modes operating inside of your layer composition, then you probably just want to go ahead and flatten your artwork because it might mess things up.

Start by not flattening and see how it goes with some of the more elaborate blend modes will require you to flatten. Anyway, I'll go ahead and say Don't Flatten in my case, because I don't have any blend modes operating. And notice that my layers are preserved. However, the appearance of the image is diminished. This is before, watch these. Okay, this is the RGB version of the image. You are going to see all the colors change but what I really want you to look at is the swatches along the left -hand side of the image here. This is the RGB version of the image.

This is the CMYK version. Did you see those colors drop right there? In other words, they became less saturated because CMYK can't handle the degree of saturation that RGB can. They both have problems displaying some of each other colors, but RGB is a much bigger space. All right, so you lose something when you go to CMYK, which is why it's the last thing you do. You should do all your creative work inside of RGB, or Lab, or a combination of the two if you like. And then, at the very end convert over to CMYK, if you have to and then maybe sharpen the image. That's about it.

All right, I'm going to undo that modification so that we're back to RGB. Watch what happens when I convert over to Lab. If I choose Mode, and then Lab Color, we're going to see the same Rasterize message. I'm going to say Don't Rasterize. We're going to see the same Flatten message. I'm going to say Don't Flatten. And then we should see the change. There is no change. There is actually a change going on you're just not perceiving it is what it comes down to, and you never will. You should never see a difference in your image between RGB and Lab, the way that Photoshop is structured.

All right, just to give you a sense so of how different the colors are in these different color models, I'm going to switch over to this final demonstration file right here. Looks the same, but it's a different file. Notice what we have, this is an RGB composition, very important to note and this is the original RGB image that you're seeing on screen, all flattened, of course this time around. Above it is the CMYK version of the image. So it was converted to CMYK and then back to RGB and then this guy here, it's converted from RGB to Lab and then back to RGB. They all had to come back to RGB because we have one color model per composition inside of Photoshop.

All right, so to give you a sense of how different the image has become, I'm going to turn on CMYK, the CMYK cat that is. You will see a change on screen. You will see the colors diminish. Of course, they become less saturated; sometimes they darken up as well. And then in order to compare these two, the CMYK cat and the RGB cat, I'm going to select the CMYK cat and I'm going to take advantage of this blend mode right here called Difference, and that allows you to explore the differences between the pixels. I showed you how it inverts an image back in feature #11, blend modes video, but this time we'll see another use for it.

You can explore the differences between two similar images. Anything that turns black is absolutely the same, anything that's some other color is different. So all the color swatches along the left-hand side are different. The eyes are different, the nose is different. You can see these different color swatches up here in the upper right. In fact, you're just not seeing everything. Actually much, much more of this image is different than that, which is why I have got this Levels adjustment layer right there. I'll turn it on. Everything that's not black is different.

And if you're saying white, that's extraordinarily different, by the way quite different. I'll go ahead and bring up my histograms so you can see the degree of difference. All these colors inside the image are different. All these pixels, that is just that first bar and nothing more, is the same converting from RGB to CMYK and back to RGB. So in other words, when you convert from RGB to CMYK, big destructive modification, there's no going back. You never want to take it back into RGB, whereas you'll hear people characterize Lab that way sometimes.

They will say that Lab is a destructive modification. You're changing a lot of pixels by the way, when you go back and forth between RGB and Lab. The implication being, if it's destructive you shouldn't do it. That is absolutely the wrong implication, in my humble opinion. I'm going to turn off CMYK and I'm going to turn off the Levels adjustment layer for a moment, and I'll turn on Lab. You don't see any change happening on screen and that's because fundamentally the changes that are occurring aren't worth worrying about, is basically what it comes down to.

There are some changes going on, but everything you do inside of Photoshop changes the colors of pixels to some extent or other and that's generally a good thing. You're trying to change the colors, if you go into a mode like Lab. Anyway, let's see how different the colors are. I'll select the Lab cat layer. I'll go to the Blend Mode menu. I'll choose Difference. And at first it looks totally black. It's as if not a single pixel has changed. Once again, we have to explore that histogram, so I'll go ahead and turn on that Levels adjustment. Now anything that isn't black has changed, which is to say several of the swatches have changed a little bit.

Anything that's black though, some of the swatches haven't changed at all and then I would say something like more than 50% of the pixels inside the tiger have changed, but still nearly 50% will stay the same. That's pretty amazing. So let's take a look at the histogram. Not much of a histogram going on at all here. So the degree of change that happens between RGB and Lab is not much at all. It's not anything like the degree of destruction that happens between RGB and CMYK, which is good because you don't have to take advantage of Lab.

You do have to take advantage of CMYK. In other words, if you got to go to CMYK, why then you have to accept the collateral damage. It's just the way it goes. Lab is a choice that you make, there shouldn't be much damage associated with it. And sure enough there's not. So feel free to go back and forth between RGB and Lab as much as you need to, to get the job done. And that is it my friends, this is your quick and dirty introduction to one of the most powerful features inside of all Photoshop, the big three color models RGB, CMYK, and Lab.

Find answers to the most frequently asked questions about Photoshop Top 40.


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Q: Is there a way to batch convert an entire folder of photos from the RBG color mode to the CMYK color mode without having to open and convert each individual image?
A: In the Actions panel in Photoshop, create an action that converts an image from RGB to CMYK. Then link to that action from File > Automate > Batch inside Photoshop.
Next, in the Bridge, select a folder of images. Choose Tools > Photoshop > Batch. Select the action inside the ensuing dialog box.
Or, in Photoshop, select File > Automate > Batch, and select the action and the folder inside the dialog box.
See also: Photoshop CS2 Actions & Automation, Chapter 2 “Action Essentials.”
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