Multi-pass CRF with different results

Discuss about generic usage of MediaCoder.

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KW40
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Multi-pass CRF with different results

Post by KW40 » Sun Dec 16, 2018 7:29 am

If the bitrate setting for "Average Bitrate" is 1000 and I change the video encoding settings to "2-pass" with constant crf=18, the resulting file size is, say, 345 mb.

However, if the above setting is 6000 and I change settings as described above, it is 1525 mb.

What do I get wrong, or is this a bug?

flagpole
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Re: Multi-pass CRF with different results

Post by flagpole » Wed Jan 16, 2019 4:34 am

I obviously don't understand the question.

To me this is expected behaviour. Regarding when you set the bitrate it would make sense that the video is approximately 6 times larger.

CRF constant rate factor. A method of encoding that is akin to constant quality - but takes in to account that you are not viewing a series of stills. So reduces the cq for high motion scenes (though it may increase the bit rate)

Usually CRF of 18 is sufficient. One advantage of this is that you don't have to keep changing it depending on the resolution.
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Kippiis
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Re: Multi-pass CRF with different results

Post by Kippiis » Thu Jan 17, 2019 2:38 am

Can you explain in details your problem?

KW40
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Re: Multi-pass CRF with different results

Post by KW40 » Sat Feb 16, 2019 5:38 am

Well, if I choose CRF 18 I expect the resulting file size to be dependent on the content of the clip as defined by the number 18, not dependent of some setting of other encoding modes. Or do I get something wrong about multi-pass CRF encoding?

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Re: Multi-pass CRF with different results

Post by flagpole » Wed Feb 20, 2019 5:32 am

So multi pass CRF is designed to use the CRF algorithm to aportion bandwidth. But constrain to a specified bit rate.
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KW40
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Re: Multi-pass CRF with different results

Post by KW40 » Thu Feb 21, 2019 11:42 pm

I see. Thank you. Though, I find a "real" CRF multi-pass option promising ...

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