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ffmpeglibx265

x265: Will I get smaller file size with frame-threads=1?

According to the x265 Command Line Options Documentation about the -F / --frame-threads option:

Using a single frame thread gives a slight improvement in compression, since the entire reference frames are always available for motion compensation, but it has severe performance implications.

Will this affect file size significantly?

Solution

  • Not worth it

    For significantly more encoding time you can have an insignificant quality improvement and almost no file size difference.

    You can always perform a test

    Encode with x265. Command #1 is using default --frame-threads. The value is auto-determined by core count. With my geriatric hardware it is using --frame-threads 3.

    time x265 input.y4m -o default.hevc
real    0m58.430s
user    6m34.437s
sys     0m2.409s

time x265 --frame-threads 1 input.y4m -o frame-threads1.hevc
real    1m29.684s
user    5m38.404s
sys     0m2.992s

    Size is basically the same:

    24858360 (24M) default.hevc
24859280 (24M) frame-threads1.hevc

    Using --frame-threads 1 is significantly slower. In my case 3x slower, but my CPU is ancient so it may be a much bigger difference for you.

    Compare visually and via VMAF, PSNR, SSIM, or whatever prefer to determine quality.

    ffmpeg -v error -i input.y4m -i default.hevc -lavfi "[0:v]settb=AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=AVTB,setpts=PTS-STARTPTS[ref];[main][ref]libvmaf" -f null -
VMAF score: 82.979207

ffmpeg -v error -i input.y4m -i frame-threads1.hevc -lavfi "[0:v]settb=AVTB,setpts=PTS-STARTPTS[main];[1:v]settb=AVTB,setpts=PTS-STARTPTS[ref];[main][ref]libvmaf" -f null -
VMAF score: 82.986954

    A higher VMAF score is better, but it's only a difference of 0.007747 and I can't tell by looking at it.