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Video encoders use a lot of techniques to fit the bitrate restrictions. With high compression level even the simple MJPEG compression may partially or totally discard the chroma information.

I wonder if it is possible to specify that discarding the chroma should be used in the first place. That is, if the encoder cannot compress a frame with a decent quality, it should drop chroma and compress only luma, so the image will have less artifacts than if the encoder tried to fit chroma too.

Maybe with libx264, at least? It has a lot of tunables.

The idea is inspired by analog TVs and VCRs that lose track of the chroma signal when it's weak. With SECAM, it can make image quality even better, as SECAM fire tends to make black&white text unreadable.

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This will depend on the specific encoder.

For libx264, you can use -x264-params chroma-qp-offset=6 to set a higher quantizer relative to luma. This will be applied throughout the encoding.

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  • chroma-qp-offset is great, but instead of discarding chroma or luma, it only roughens their quantization, so with extreme values (-12/12) the image appears even blockier :/
    – bodqhrohro
    Dec 15, 2019 at 13:35

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