![]() For example, using the 4:2:2 ratio, both rows contain chroma information from two out of the four pixels. The third number in the ratio represents how many pixels in the second row of the sample area contain unique chroma information. The second number in the ratio represents how many pixels in the first row of the sample area contain unique chroma information. The first number in the ratio is ‘4’ in most cases. The sample area size is the reference point for how much chroma compression takes place in a given pixel area. For example, the number ‘4’ means that the sample area is 4 pixels in length and 2 pixels high, creating two rows of 4 pixels each (8 pixels in total). The first number in the ratio represents the size of the sample area, in reference to pixels. It’s important to note that all pixels contain luminance information and only the colour (chroma) information is being compressed here. The ratios (4:2:2 etc) refer to how many pixels contain unique chroma information in a given area of pixels. To avoid confusion, let’s look at the most up-to-date method to read a chroma subsampling ratio. ![]() In the past, chroma subsampling ratios represented slightly different elements within a video signal. Therefore, we can compress the chroma data in a video file or signal to save valuable bandwidth when storing and transmitting video. For perspective here, think of a black and white video/TV, which only contains luminance information. The luminance is what carries all the picture detail and the chroma is just added to create a colour image. In fact, the 4:2:2 chroma subsampling ratio (half of the original colour data) has virtually no impact on viewing experience compared to 4:4:4 (uncompressed). Our human vision is much more sensitive to brightness (luminance) and motion than colour (chroma). With that in mind, let’s take a deeper look into compressing colour, why we do so and what these ratios actually mean. This topic was discussed in the previous section and helps us understand chroma subsampling. ![]() This allows us to separate all of the luminance (brightness) information of the video and preserve it, while compressing the chroma (colour) information separately. Chroma subsampling uses a colour difference encoding system, usually ‘YCbCr’.
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