Chroma subsampling
Chroma subsampling is a technique used in video codecs to reduce the amount of data required to store or transmit color information in a video signal. It achieves this by sampling the color information at a lower resolution than the brightness information.
In a video signal, each pixel has a brightness value (luma) and two color values (chroma), typically represented as red-green and blue-yellow color differences. Chroma subsampling reduces the resolution of the chroma information by sampling it at a lower frequency than the luma information. For example, in 4:2:0 chroma subsampling, the chroma information is sampled at half the horizontal and vertical resolution of the luma information. This means that for every four pixels of luma information, there are only two pixels of chroma information in the horizontal direction and two pixels of chroma information in the vertical direction.
Chroma subsampling is commonly used in video codecs because it reduces the amount of data required to represent the video signal, which can make it easier to store or transmit. However, it can also result in visible artifacts such as color bleeding or loss of detail in areas with fine color variations, especially in high-contrast areas. In post-production it makes keyng much worse to operate.
The human eye is less sensitive to changes in chroma information than it is to changes in luma information because of the way the eye is designed. The retina, which is the part of the eye that detects light and sends signals to the brain, contains two types of cells called rods and cones. Rods are responsible for detecting changes in brightness, or luma information, while cones are responsible for detecting changes in color, or chroma information.
However, there are more rods than cones in the retina, and the rods are more sensitive to changes in brightness than the cones are to changes in color. This is because the rods are more numerous and more sensitive to light than the cones, and they are also more evenly distributed across the retina. In contrast, the cones are concentrated in the fovea, which is the central part of the retina that is responsible for high-acuity vision.
Therefore, because there are more rods than cones and the rods are more sensitive to changes in brightness, the human eye is less sensitive to changes in chroma information than it is to changes in luma information. This is why chroma subsampling can be used in video compression without significantly affecting the visual quality of the image.
Different video codecs use different chroma subsampling ratios depending on the intended use of the video. For example, 4:2:0 is commonly used in consumer video formats such as MPEG-4 and H.264 (even this codec have more advanced profiles, while professional video formats such as ProRes and DNxHR, often use 4:2:2 or 4:4:4 chroma subsampling for higher color accuracy and fidelity.
Overall, chroma subsampling is an important technique used in video codecs to reduce the amount of data required to store or transmit color information in a video signal, but it can also have an impact on image quality and should be carefully considered when selecting a codec for a particular application. In particular, for the work of a colorist, it is important not to lose unnecessary detail of colors until the final master files are released. When exchanging with the VFX department, of course, subsampling should be kept to the maximum, using the industry-accepted methods of exchanging material in the form of EXR sequences, with EXR compression methods suitable for the VFX department.