I want to talk a little bit more about contrast between the objects that you have in the foreground, and the color that you use for the background. Why? Well, because virtually every visualization, always has at least these two layers; the background and the objects that you draw on the foreground. Right? Think about it, virtually every visualization out there, has some sort of background and objects depicted in front of the background. Right? So, now it turns out that the color that you use for the background, affects the colors that you can perceive in the foreground. So, the color that you use for depicting the objects, okay? And let me show you a couple of images. So, in this image you can see that different shades of gray look different according to what background there is. So in these images, the one on top and the one at the bottom, they differ only for the background that is used. But the progression of colors that you see in these scales, it's exactly the same. There's no difference. But when we try to perceive them, we see that when there is one background, some elements are more visible than others, than when you have a different kind of background, in particular when the background is totally white or very bright, you won't be able to perceive very well the elements of the scale, that are very bright. Closer to the white shades, right? And the same is true when you have the one at the top, when you have colors that are similar to the darker and gray background, it's harder to perceive differences between these colors. So, this is important to keep in mind, because every time you use quantitative color scales in a visualization, you have to keep in mind that the background that you use is going to affect the perception of these elements. Especially, when you have a white background, you may want to be careful with elements from the screen, that are very close to white or very luminant like a bright yellow. You won't be able to perceive very well objects that are bright yellow on a white background. Okay? So, another important and related concept, is the concept of color constancy. What does he mean? Well, it means that exactly the same patches of color they are perceived as different, if the background is different. So let me show you an example. So here we have two rectangles that are actually colored exactly with the same color, but they have different backgrounds. And the effect is so strong that you just can't perceive it right. Okay? There's no way for you to perceive it right, but these two rectangles are actually the same color. So, if I show you the same picture with a uniform background, you would see that these two colors are actually the same. But as I put back the background with different shades, you'll see that the two rectangles are perceived as different. The same is true, so in this case we have a difference only in luminance, but the same is true also when the difference is exclusively chromatic. So here is another example. Here we have two rectangles, that have again exactly the same color, but the background differs, and the only difference here again is chromatic. All the other channels are the same. The luminance is the same, and saturation is the same. Okay? But again, since these two patches of color that are exactly the same have different backgrounds, they are perceived very differently. So in turn, how do you use these knowledge when you are designing new visualizations? Well, you have to be careful because, so if you have exactly the same background everywhere, typically you don't have a problem. Right? But there are visualizations out there, where you may have backgrounds of different colors. And in that case, you have to be aware of the fact that color perception may be different in different regions of your visualization. Or sometimes you have visualizations where there are regions that are one next to the other. And the perception of the objects on top of these layers is going to be affected by the color of the objects that are surrounding this object of interest. So in general, be careful, because color perception depends on the background, and in general on the surrounding elements, the elements that surround different patches of color. And again, this is called color constancy and the idea that colors are perceived differently, when they have different backgrounds. Yeah, so that's all for this part and keep in mind that these are very subtle effects but they may actually affect the way your visualization is perceived in a very fundamental way. So, be careful.
