So look at this thing. I would call this red, and if I was trying to describe it to you over the phone, I might say it's red like an Apple, and you might say a Macintosh Apple or a Fuji? To which I would respond, no, it's a really waxed red delicious apple that's got that really deep color, and then you might say, oh, I only shop at the organic produce market and they don't wax the fruits there. So I don't know what you're talking about. This is a problem with color: how do you create a language for it? When it comes time to build a device that can send a color image to another device, you have to have a way to describe color.
In other words to work with color, we need a way to model it, a set of rules that describe precisely what a particular color is. Now there are a lot of different ways to do this. There are lots of different color models. We've already discussed the RGB or red, green, blue model wherein every color is described as a mix of red, green, and blue values. We've also discussed the CMYK model where colors are described as a mix of cyan, magenta, yellow, and black. There's also the HSL model which also uses three numbers, one for the hue of the color, another for saturation, and the third for lightness.
The Apple color picker that you all have on your Macintosh is an HSL model. This is a standard color picker that comes up in a lot of different applications. I am in text edit right now. What I have got is this color wheel here that's showing me hue around the wheel, saturation as I go out from the center to the edge, and lightness as I move the slider up and down. So you can actually think of this circle here as a slice in a cylinder, and if the cylinder is black at the bottom and white at the top, then moving this up and down is just showing me a different slice.
So I can darken my colors by going down toward a lower slice. Right now, I've selected white. If I wanted yellow obviously I would follow this line here, and as I got further out, you can see the saturation of my color increasing. So this is still another way of modeling color. This model is also sometimes referred to as HSB or hue, saturation, and brightness. All that a color model does is give us a way to represent colors as numbers. Now the reason we have different models is that some models are better suited to some tasks than others. Because a printer's primary colors are cyan, magenta, and yellow, it's sometimes easier to make adjustments in a CMYK space rather than an RGB space.
If I have a particular shade of blue, and I want to find a lighter shade, then working with an HSL model might be easier than working with an RGB model. A color model, then, gives me a way to specify a recipe or formula for a particular color. If I'm using an RGB model, I would describe this particular red using one set of numbers. If I'm using a CMYK model, I would use a different set of numbers to describe the same red. So now I have a way of notating color, but unfortunately, describing color has some additional complications.
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