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LoRd_MuldeR · 27th December 2012, 18:46

Your understanding of 2-Pass and CRF seems to be wrong

Actually they use the same rate-control algorithm and thus none of them "compresses more efficiently" than the other one. After all, if we have two files, one encoded with 2-Pass mode and one encoded with CRF mode, and both of these files happen to come out at the same file size (i.e. the same average bitrate), then the quality of those files will be pretty much identical!

The difference between 2-Pass mode and CRF mode is that with 2-Pass mode you can know the final average bitrate (i.e. the final file size) in advance, because the encoder will constantly re-adjust the so-called "rate factor" in order to hit the selected (average) bitrate. At the same time, CRF mode, as the name implies, simply uses a constant "rate factor" value. Therefore with CRF mode the final average bitrate (file size) cannot be known in advance. And that's it.

Finally it is wrong to think about the CRF value as a quality indicator/selector! Indeed, as long as you don't change any other options, the same CRF value will give approximately the same visual quality for different sources. But as soon as you change other encoding options, this is no longer the case! For example, many people believe that if they use "slower" encoder settings and the same CRF value, they will get the exactly same quality as with "faster" settings (and that CRF value) but at a lower bitrate (smaller file). In reality, however, the very same CRF value may result in pretty different quality for "fast" and for "slow" settings, which means the resulting bitrates (file sizes) are not even comparable anymore! Therefore you may very well end up with bigger file (higher average bitrate) when using the "slower" settings (for the same CRF value). Only thing you know for sure is that "slower" settings will improve the compression efficiency, i.e. the "quality per bit" ratio. But you don't know how the overall quality will change, for a given CRF value. So you may need to re-adjust your CRF value after changing other settings...

27th December 2012, 18:46	#4 \| Link
LoRd_MuldeR Software Developer Join Date: Jun 2005 Location: Last House on Slunk Street Posts: 13,248	Your understanding of 2-Pass and CRF seems to be wrong Actually they use the same rate-control algorithm and thus none of them "compresses more efficiently" than the other one. After all, if we have two files, one encoded with 2-Pass mode and one encoded with CRF mode, and both of these files happen to come out at the same file size (i.e. the same average bitrate), then the quality of those files will be pretty much identical! The difference between 2-Pass mode and CRF mode is that with 2-Pass mode you can know the final average bitrate (i.e. the final file size) in advance, because the encoder will constantly re-adjust the so-called "rate factor" in order to hit the selected (average) bitrate. At the same time, CRF mode, as the name implies, simply uses a constant "rate factor" value. Therefore with CRF mode the final average bitrate (file size) cannot be known in advance. And that's it. Finally it is wrong to think about the CRF value as a quality indicator/selector! Indeed, as long as you don't change any other options, the same CRF value will give approximately the same visual quality for different sources. But as soon as you change other encoding options, this is no longer the case! For example, many people believe that if they use "slower" encoder settings and the same CRF value, they will get the exactly same quality as with "faster" settings (and that CRF value) but at a lower bitrate (smaller file). In reality, however, the very same CRF value may result in pretty different quality for "fast" and for "slow" settings, which means the resulting bitrates (file sizes) are not even comparable anymore! Therefore you may very well end up with bigger file (higher average bitrate) when using the "slower" settings (for the same CRF value). Only thing you know for sure is that "slower" settings will improve the compression efficiency, i.e. the "quality per bit" ratio. But you don't know how the overall quality will change, for a given CRF value. So you may need to re-adjust your CRF value after changing other settings... __________________ Go to https://standforukraine.com/ to find legitimate Ukrainian Charities 🇺🇦✊ Last edited by LoRd_MuldeR; 27th December 2012 at 19:06.