In the previous video, we saw that specifying a color with the RGB color space is not easy. So what are the other options? Another useful option is a different color space that is called HSV or a very similar one that is called HSL, which stands for hue, saturation and value, or hue, saturation and lightness. And the interesting property of these color spaces is that it's easier to specify colors. Because they use channels or axes that are more closer to the way we want to specify colors. It's much more natural. It's much more natural to think about color in terms of hue, saturation and lightness. So what are these three channels? Hue is basically the name of the color. Green, red, blue, yellow and so on. Saturation is how vivid the color is, the colorfulness of the color. And lightness is the amount of light, the brightness of the color. Here is a representation of the color space. And as you can see, HSV can be described as a cylindrical color space. And what happens here is that saturation corresponds to the radius of this piece. The value corresponds to the height and the hue corresponds to the angle. And as you change these parameters, you position yourself into a different position of this color space and every single position represents one color. Now to make this even clearer, let me show you a demo using a color picker. Okay, so now let's take a look at an HSL Color Picker to get a sense of how this color space works. Okay, let me describe first what are these elements that you see on the screen. There are a few sliders, as you can see, similar to what we have seen before with the RBG color picker. But now each slider corresponds to one of the new channels that we have in this new color space. So the first one is the h channel, the hue. Then we have s, which stands for saturation. And the last one is lightness. So as I move the slider of the hue channel, as you can see, the color on the left is the color that is currently peaked, changes the hue, okay? So we go from red to purple, blue, green, yellow and red, okay? So now if we keep one of these colors, if we keep hue constant, now we can change the saturation value. So remember that saturation means how vivid the color is. So let me show you how vividness of this color changes as I move the slider here. So we go from very saturated blue to blue with very low saturation. As you can see, as the saturation gets lower and lower, we create colors that are very similar to gray. Okay, and another parameter is lightness, which can be thought of as the amount of light that is emitted by this color. So now we keep hue and saturation fixed, and we change the amount of lightness and see what happens. So we go from dark, very dark, to very bright. This is the lightness channel. Okay, so we have hue, saturation and lightness. As we saw in the demo, choosing colors with HSV or HSL color space is so much easier than with the RGB space. So now you may think, okay, the problem is solved. We can just use this color space all the time and in particular when we want to use color for visualization. Well, there is another problem. The problem is that these color spaces are not perceptually uniform. What does it mean to be perceptually uniform? Well, it means that distances that are calculated in the color space mathematically do not correspond to perceptual distances. So the difference between colors that we actually perceive with our eyes. Let me give you an example to show you how this works. So what you see here, the set of colors that you see in the top row, these are all colors that exactly the same V value selected using the HSV color space. And the colors that you see below them are exactly the same with the chromatic component removed. So all these grays represent the lightness value of the colors that you see on top. And as you can see, we have different levels of gray, which basically means that these colors have different color intensities. We perceive these colors with different intensities, so this is a problem. It's a problem because, ideally, we want to specify colors that have exactly the same lightness, exactly the same V here. But the color space is not perceptually uniform, so we perceive these colors differently.
