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The chroma bug is a visual distortion present in the output of some models of DVD players.
The bug manifests itself as jagged lines or streaks of horizontal lines running through some portions of a picture, usually on diagonal edges of high contrast colors, making that portion look like it has a texture. The chroma bug effect may be clearly seen on big progressive displays, but could go unnoticed on smaller interlaced displays.
The chroma bug is technically known as the chroma upscaling error (CUE).
The chroma bug is a visual artifact that usually appears in areas of deep red and blue but can appear anywhere, as long as there is high contrast between areas. It was not an issue before large progressive displays came into wide use.
MPEG decoders in DVD players must use a different upsampling algorithm for interlaced and progressive frames. The reality is that most of the decoders in early DVD players only apply one algorithm for both kinds of frames, usually the one for interlaced frames, as most displays before the year 2000 were interlaced displays and only a very small portion of them were above 30 inches in size. When large progressive displays became popular, this inherent bug became obvious.
The distortion in the video is due to the MPEG decoders in the DVD players, which are not converting 4:2:0 chroma information from the DVD into the proper 4:4:4 or 4:2:2 format that video encoders require. In the MPEG specification, it is stated that the MPEG decoder should select two different upsampling algorithms to convert 4:2:0 to 4:2:2 correctly. One algorithm should be used for “interlaced” frames, while the other should be applied to “progressive” frames. What happened is that most decoders in DVD players only use one algorithm for both (for interlaced frames), which causes the chroma bug to appear in progressive frames.