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Update README.md (more details on the CRF value used by each label)

See merge request framasoft/peertube/official-plugins!21
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Chocobozzz 2023-10-11 07:05:52 +00:00
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peertube-plugin-transcoding-custom-quality/README.md
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# PeerTube transcoding custom quality
This plugin creates a transcoding profile in which admins can decide the quality of the transcoding process.
Here are the CRF value used by ffmpeg for each label :
- 'Low' -> CRF = 36
- 'Medium' -> CRF = 33
- 'Good (Peertube default)' -> CRF = 30
- 'Very good' -> CRF = 27
- 'Excellent' -> CRF = 24
- 'Perfect' -> CRF = 21
- 'Unreasonnable' -> CRF = 18
- 'Insane' -> CRF = 15
**Increasing quality will result in bigger video sizes**.
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Once installed and your value set in the plugin parameters, you have to choose "**custom-quality**" option in Administration/Configuration/VOD Transcoding/Transcoding profile.
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Explanations (source : https://trac.ffmpeg.org/wiki/Encode/H.264)
CRF means _Constant Rate Factor_.
This method allows the encoder to attempt to achieve a certain output quality for the whole file when output file size is of less importance. This provides maximum compression efficiency with a single pass. By adjusting the so-called quantizer for each frame, it gets the bitrate it needs to keep the requested quality level. The downside is that you can't tell it to get a specific filesize or not go over a specific size or bitrate, which means that this method is not recommended for encoding videos for streaming.
The range of the CRF scale is 0–51, where 0 is lossless, 23 is the default (for ffmpeg), and 51 is worst quality possible. A lower value generally leads to higher quality, and a subjectively sane range is 17–28. Consider 17 or 18 to be visually lossless or nearly so; it should look the same or nearly the same as the input but it isn't technically lossless.
The range is exponential, so increasing the CRF value +6 results in roughly half the bitrate / file size, while -6 leads to roughly twice the bitrate.
Choose the highest CRF value that still provides an acceptable quality. If the output looks good, then try a higher value. If it looks bad, choose a lower value.
Note: The 0–51 CRF quantizer scale mentioned on this page only applies to 8-bit x264. For 10-bit support, refer to the documentation on ffmpeg.org