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Old 12th December 2016, 11:05   #1118  |  Link
r0lZ
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Join Date: Jul 2003
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Welcome to the Doom9 forums, audiolord !

Why do you want a specific size? Is it not better to target a specific quality? Although somewhat related, size and quality are not identical, and in the case of a movie with very clean images mainly generated with computers (like The BFG), it is possible to compress it very well without much quality loss.

It is totally absurd to pretend that it is necessary to encode at a certain bitrate to obtain a good quality. If you want a good demo, encode a movie made with only black frames at a certain bitrate. Of course, the quality will be perfect. Now, encode a movie made of complex and rapidly changing frames with much noise at the same bitrate. The quality will be terrible! It is therefore impossible to predict the quality of the encoding with a specific bitrate. But if you encode with a reasonable value in CRF mode, the black movie will be perfectly encoded and consume only a few MB, and the complex movie will be equally good but will require much more disc space. Encoding both movies with the same bitrate simply doesn't make sense.

Anyway, it is possible to change the quality of the encoding in the last tab. By default, BD3D2MK3D encodes in CRF mode, and by default I have left the CRF value to the default of the x264 encoder (CRF 23), because IMO that value is an excellent compromise between quality and size. (After all, the programmers of x264 know what they are doing!) Of course, you may prefer to use a lower value if you really think that the quality is not good enough at CRF 23. But do you think that, or do you really see that the quality is bad? IMO, you are victim of a prejudice due to bad information on the internet. Serious tests have proven that the human eye is usually not able to see the difference between a good encoding at CRF 23 and the same encoding at CRF 20 or even lower. Personally, I use values between CRF 18 and CRF 23 depending of the quality of the source image, but IMO it is never useful to encode a movie with lower CRF values. CRF 18 should be reserved to very special cases where the image quality is extremely important. (So far, I have used that value only to encode Sin City 2.)

I've read somewhere that lowering the CRF value by 3 is approximately equivalent to doubling the file size (the bitrate) of the video track. So, if you use CRF 20 instead of 23, the final file size should be around 11 GB, with the audio, but the final file size is difficult to predict because in CRF mode it depends mainly of the difficulty to compress the images. If you really want a specific size, you should use the 2-pass mode, but please DO IT ONLY if you want to put the movie on a media limited in size (like on a DVD). Using 2-pass instead of CRF produces ALWAYS a slightly less good quality, requires much more encoding time and has NO ADVANTAGE at all, except the possibility to define a precise final file size. It is a common mistake to encode in 2-pass, and if it's so frequent, it's probably because peoples recommend absurd encoding settings on the internet.

So to summarise, here are my recommendations: Encode in 2-pass if and only if you really need to control precisely the final file size. Otherwise, encode in CRF and use a CRF value between 18 and 23 depending of your wishes and the quality of the original images. Do not be surprised if your final file size is less than expected, because that means ONLY that the movie was easy to encode, and certainly not that it has been badly encoded. And finally, trust your eyes, and do not trust the bad information on the internet.

[EDIT] Nico has replied before me, but he is right. Your question doesn't make much sense if we don't know what encoding mode you are using.
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Last edited by r0lZ; 12th December 2016 at 11:11.
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