Controversy
The details of chroma subsampling implementation cause considerable confusion. Is the upper leftmost chroma value stored, or the rightmost, or is it the average of all the chroma values? This must be exactly specified in standards and followed by all implementors. Incorrect implementations cause the chroma of an image to be offset from the luma. Repeated compression/decompression can cause the chroma to "travel" in one direction. Different standards may use different versions for example of "4:2:0" with respect to how the chroma value is determined, making one version of "4:2:0" incompatible with another version of "4:2:0".
Proper upsampling of chroma can require knowing whether the source is progressive or interlaced, information which is often not available to the upsampler.
Chroma subsampling causes problems for film makers trying to do keying with blue or green screening. The chroma interpolation along edges produces noticeable haloing artifacts.
Read more about this topic: Chroma Subsampling
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