QP vs CRF
Posted: Sun Oct 25, 2020 11:45 pm
Hello,
I have been encoding movies with Handbrake for years on a 2011 iMac, using x264 codec with the CRF mode (or the 2 pass ABR mode). Using the "slow" preset it achieves an encoding rate of about 10fps on a 1080p video: quite slow, but acceptable for my usage.
Switching to h265 (which would be nice to reduce file sizes) was not really an option, as the with x265 the encoding speed drops to 4fps.
Then I recently got a mini-PC, with a Celeron/UHD600: definitely less powerful than my old iMac, but at least it has some hardware support (QSV) for h265 encoding. I have played a bit with it and it achieves about 20fps with the "balanced" preset, which is twice faster than x264 on the old Mac: it could be an alternative, but it has no CRF mode, only the QP mode.
From what I understand QP is less smart than CRF, as the latter is actually a kind of "variable QP" (higher QP on high motion frames). So I would need to use a lower QP value to compensate for that, and at the end I wouldn't obtain significantly smaller files...
However, this article ( https://slhck.info/video/2017/02/24/crf-guide.html ) tends to show that the advantage of using CRF instead of QP is the most important for low QP/CRF values and much less obvious for medium values (see the graphs that compare values of 17 and 23). Since I generally use medium values (such as 23!), this means that I wouldn't waste that much space by using QP...
What do you think ?
I have been encoding movies with Handbrake for years on a 2011 iMac, using x264 codec with the CRF mode (or the 2 pass ABR mode). Using the "slow" preset it achieves an encoding rate of about 10fps on a 1080p video: quite slow, but acceptable for my usage.
Switching to h265 (which would be nice to reduce file sizes) was not really an option, as the with x265 the encoding speed drops to 4fps.
Then I recently got a mini-PC, with a Celeron/UHD600: definitely less powerful than my old iMac, but at least it has some hardware support (QSV) for h265 encoding. I have played a bit with it and it achieves about 20fps with the "balanced" preset, which is twice faster than x264 on the old Mac: it could be an alternative, but it has no CRF mode, only the QP mode.
From what I understand QP is less smart than CRF, as the latter is actually a kind of "variable QP" (higher QP on high motion frames). So I would need to use a lower QP value to compensate for that, and at the end I wouldn't obtain significantly smaller files...
However, this article ( https://slhck.info/video/2017/02/24/crf-guide.html ) tends to show that the advantage of using CRF instead of QP is the most important for low QP/CRF values and much less obvious for medium values (see the graphs that compare values of 17 and 23). Since I generally use medium values (such as 23!), this means that I wouldn't waste that much space by using QP...
What do you think ?