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  1. I am thinking about the quality improvement when using a 2-pass AVC H.264 encoding vs 1-pass AVC H.264

    In the past I read many recommendations for 2-pass encoding over only 1-pass.
    So I use this 2-pass encoding during the last years.

    When I compare now again the results I found hardly any quality differences.

    Is this statement still true?

    Is there a quality approximation similar to:

    2-pass encoding at bitrate X = 1-pass encoding at bitrate (X + 5%)

    with same other encoding parameters

    ?
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  2. Member azmoth's Avatar
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    I asked a similar question and received a similar answer about 2 pass gaining 5% quality increases. If you notice most of these video converters like Wondershare etc only use 1 pass which is by all accounts the worst encoding method. The recommended is CRF these days. I once received a response from Wonderfox another commercial seller of these plethora of products on the market today, when I asked why is 1 pass used? The answer was something to with smart bitrate allocation method which yields same result as a 2 pass, but they wouldn't elaborate on which parameters were tweaked.

    And for the record I think 2 pass is still superior to alll methods if you like the wait, but I only use 1 pass abr myself. CRF for at 18-22 still lost fine details compared to similar bitrate sizes at 2 pass which gave more balanced bitrate allocation, in my extensive tests, yet some swear by CRF! I've tried a few converters and CRF never cuts it for me, but 2 pass is tortuous, so I settled for ABR when the need arose. If quality is important don't re-encode as even a CRF 15 or a similar 2 pass will show a quality loss to the original.
    Last edited by azmoth; 25th Jan 2022 at 05:47.
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    suppose you are encoding a video at 200kbps
    2 pass will give you better quality then 1 pass


    because 2 pass analys video first and spent bits accordingly
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  4. Member azmoth's Avatar
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    Not so sure on the quality, but theoretically yes. Then at that lower bitrate wouldn't hevc excel better?
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    2 pass encoding is use to save space by lowering bitrate and keeping the quality same but its takes arround twice more time then 1 pass

    if you care about encoding time and dont care about file size then 1 pass mode with high bitrate is used


    personaly i use 2 pass because i want good quality at low bitrate (but yes it takes much time that i dont like)
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  6. Member azmoth's Avatar
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    My 'sweet pass' is usually 2500kbps + for 720p, and 4000kbps + for 1080p at abr, but it depends on the source. Anything lower in bitrate would warrant a 2 pass, but I've no need these days to encode anything as storage is so cheap to preserve the original in a mkv container.
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  7. If you're using the x264 encoder, it encodes video in a virtually identical manner for both 2 pass and 1 pass (CRF) encoding. So if you want a particular file size or bitrate, use 2 pass, but specifying a bitrate effectively specifies the quality without knowing what that quality will be. For 1 pass CRF encoding you specify a quality without knowing what the bitrate will be.

    Don't confuse 1 pass CRF with ABR though, as the latter will produce a less consistent quality than 2 pass, especially for lower bitrates where quality differences become noticeable. It's because like 2 pass, ABR has to achieve a particular bitrate, but without any advance knowledge of how best to distribute the bits, so even though it still encodes the same way (for x264) all it can do is adjust the quality up and down as the encode progresses. I've seen that in a few ABR encodes where a scene is unusually complex (especially at the beginning) and the quality noticeably deteriorates. There's an example of it here, and even though it's a bit extreme, I've seen much the same thing in real-world average bitrate encodes.
    https://forum.videohelp.com/threads/371187-Testing-NVENC-with-the-GTX-960#post2384552
    Scroll down a couple of posts for Bitrate Viewer screenshots showing how each encoding method distributed the bits.

    So the main question becomes does "1-pass AVC H.264" refer to using the x264 encoder, and if so, is it 1 pass CRF or ABR? If you have to select a bitrate for 1 pass rather than a quality, then it'd have to be ABR, in which case 2 pass will be better.

    There's slight differences in the way the encoder makes some decisions for CRF compared to 2 pass, but it's nothing to worry about as the same algorithm is still used for encoding. Plus for a given amount of encoding time, CRF has an advantage because it doesn't have to spend time on a first pass, so for CRF you could theoretically use slightly higher quality settings than for 2 pass, but even though it's slower, still take roughly the same amount of time to encode as 2 pass.

    There's a summery of x264's rate control methods here. It's not heavy reading as I wrote it myself, based on the full article, back when I was pondering this sort of thing. x264's rate control methods.
    Last edited by hello_hello; 25th Jan 2022 at 08:35.
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  8. Member azmoth's Avatar
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    For high motion scenes in an action film, 2 pass or CRF will be more suitable than abr to get the bitrate distribution to places which need it, but I always wonder what the cut off point would be if a really high bitrate is thrown at it when neither 2 pass or 1 pass will make a difference to the perceived quality by the naked eye?

    Plus the psychological aspect of knowing that a CRF encode of your choice eg 22 has given a specified quality would be preferential to most and the less time involved to reach that quality.
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