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Some of the editing concepts that we're going to cover later will be easier to understand if you know a few rudiments of color theory. As you may remember from biology class, your eyes have two types of light-sensitive cells in them: rods and cones. Rods comprise your night vision. They are your black and white low-light vision. They are what 98% of your vision is made of. You also have cones, which are cells that are sensitive to color. What you may not remember from biology class is that there are three different types of cones.
Some are sensitive to red, some are sensitive to green, and some are sensitive to blue. These colors are significant because they are the primary colors of light. When you mix them together, you can get any other color. Here is an example. I have three circles: one filled with blue, one filled with green, and one filled with red. Where red and blue mixed together, I get magenta. Where blue and green mixed together, I get cyan. Where green and red mixed together, I get yellow. I can mix these in an infinite variety of combinations to get every other color in between.
If I mix equal amounts of all three, I get white. Now you may be thinking, "Well, in finger -painting class, I remember that when I mixed colors together, everything got darker until it just turned into this kind of brown sludgy thing." That's because pigments work subtractively. As you mix them together, they get darker. Light works additively. These are known as the additive primary colors of light, because they mix together to add up to white. The reason we care about this is because some image-editing problems are easier to diagnose with an understanding of how different color channels come to bear on your image.
Sometimes, we will attack problems by looking its specific color channels. In Photoshop, I can go to the Channels palette, which lets me look at specific color channels in my image. Right now, I'm looking at RGB, but if I click on the Red channel, I'm now looking at only the red channel. What I see here is a white circle on a field of black. Where white pixels occur, it means 100% red; where black pixels occur, it means no red at all. Same thing for green and same thing for blue.
When I look at the final image, I see that, sure enough, there is 100% red in these pixels, 100% green here. They are mixing together to create this yellow. Let's take a look at a more real-world example. Here is an image with a fair amount of color in it. We've got some blue up here, some reds down here, a lot of browns over here. If I go and look at the Red channel, I find this. It appears to be a black-and-white image, just the way our last example was. These red posts are showing up predominately as white, because as you will recall, 100% white in the Red channel equates to red in the image.
So it makes sense that these red things would appear as white in the Red channel. Let's go look at the Green channel, and now these red posts, in my final image, appear much darker, because there is very little green in them. I'll switch back to RGB to view my whole image. So, one of the most important things for you to do before we get started here is to make sure that your histogram is set up in a way that you understand, because the histogram in Photoshop has the ability to show you separate histograms for each color channel.
I'm going to go to the Window menu and choose Histogram. This is the Histogram we were looking at earlier. It is just showing luminance, or brightness. In other words, it's showing the distribution of tones from darkest to lightest. Again, we see that I don't have strong highlight detail here, mostly midtone data, perhaps a little low contrast. If I open up this pop-up menu up here and choose Expanded View, I get some additional controls, including this Channels pop-up. Now, by default, your Channel pop-up may be set to something different than this RGB view.
It's probably set to Colors. Now I see this. I see three different histograms, and where they're overlapping, I see some secondary histograms. So you can plainly see back here the blue histogram. You can see hints of the red histogram and a couple of hints of the green histogram. There is also a little bit of magenta, cyan, and yellow in here. These are showing the separate histograms for the separate color channels, in other words, the distribution of individual color channels in the image. So you can see in the lighter tones, there's more blue than anything else.
If you're wondering what this exclamation mark means, it means that the histogram is not necessarily up-to-date for this image. If I click on it, Photoshop will take some time to calculate a more accurate histogram. So, when you're getting ready to do a critical histogram-driven work, it's a good idea to click on that exclamation mark. We'll be returning a lot to Channels and Histograms throughout this course, so you'll get more practice with them, and they should become easier to deal with as we go along.
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