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Old 22nd November 2018, 09:11   #6494  |  Link
LigH
German doom9/Gleitz SuMo
 
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Join Date: Oct 2001
Location: Germany, rural Altmark
Posts: 6,752
--qcomp <float>
Quote:
qComp sets the quantizer curve compression factor. It weights the frame quantizer based on the complexity of residual (measured by lookahead). It’s value must be between 0.5 and 1.0. Default value is 0.6. Increasing it to 1.0 will effectively generate CQP.
The default value 0.6 is a balance between a constant quantizer (regardless of the video content) and the complexity of the video content (degree of details and amount of motion) providing chances to spare bitrate by increasing the quantizer slightly in scenes where it may be sufficient to preserve enough quality with little noticeable loss.

IIRC, if you could decrease it to 0.0, the encoder would try its best to keep a constant bitrate (CBR), which would cause a very varying amount of quality loss (I might be wrong here, for x265, though). Increasing it to 1.0 instead would cause a constant quantization which would not take advantage of the possible ways to spare bitrate in scenes where convenient quality preservation could already be achieved with less bitrate, at a coarser quantization than the target.

You may increase this value a little (e.g. towards 0.8) when you notice that there is too much loss of precision in areas with very little detail, e.g. darkness and smooth ramps, especially in cases when your target bitrate is rather low. On the other hand, there may be other (psycho-visual) options to let the encoder not spare too much bitrate.
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