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Chroma meaning11/30/2022 ![]() This is uncompressed video. Four chroma samples in the first row and four in the second row. The general form is J: a: b, and this describes how many chroma samples are taken per block of Jx2 pixels. In the first row of that block, we sample a pixels for color. In the next row of this block, we sample b pixels for color. As a reminder, luminance is always sampled at every pixel. Now that the Chroma information is available separately, it is possible to compress that information. The method used is sub-sampling, in other words less samples are taken than are actually needed. To what extent there is too little sampling depends on how much you want to compress and how much color information you are willing to lose. For example, the number of chroma samples can be halved horizontally or vertically, or both. The notation that indicates this is, to say the least, not very intuitive Chroma subsampling explained: How do you compress color data? The amount of detail in the luminance channel (black and white image) is sufficient to have a clear impression of the image. The chroma channels are much less clear. ![]() When we visually split a photo according to this model, it immediately becomes clear that the luminance channel is much more important. ![]() The basis of this technique is very simple: our eye is less sensitive to spatial variation in color than to spatial variation in luminance (brightness). To take advantage of this, images are not saved in RGB encoding (where each pixel is assigned a value for red, green and blue), but in YCbCr. So each pixel still has three values. Y is the value for the luminance, Cb and Cr are the Chroma (color) values. We spare you the math of the conversion between RGB and YCbCr. ![]() Chroma subsampling is one of the many compression techniques used in both video and photo. Some of the information is discarded to make the file smaller. This is especially important for video because the files are very large, so the savings are proportional. A second reason why this is especially important for video is that the bandwidth of transmission channels (such as your internet connection) is limited. You couldn’t possibly stream an uncompressed video file. Even for your HDMI cable, an uncompressed video file is just too big to get every frame on your screen in time. Compression is therefore indispensable. ![]()
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