I've seen similar issues but can't seem to come to a resolution myself. I have a 4K rip that is in hevc, 10bit depth, which my older phone can't play since it lacks GPU acceleration. Therefore, I'm reencoding it on ffmpeg to a YUV420p scheme. I've done this by GPU acceleration and without, at multiple qualities and bitrates. I figured I'd measure each output against the original video and simply use the one with the higher VMAF score as the one I use to reencode some other 4K videos with the same 10bit depth.
However, the VMAF score for every output, whether CPU or GPU encoded, 1M or 10M bitrate, is somewhere around 32.2%. This is despite the fact that the higher bitrate ones generally look pretty decent. After looking around, I saw that some people recommend using a filter for
setpts=PTS-STARTPTS to sync up the frames, since maybe the encoding process desynced some frames? This still doesn't help.
Here's where it gets weird. I looked in the log when running the VMAF filter to pick one frame in particular and test it. I found one frame where the VMAF score reported a score of 0.0000:
<frame frameNum="487" integer_motion2="7.639184" integer_motion="9.133736" integer_adm2="0.516201" integer_adm_scale0="0.902589" integer_adm_scale1="0.619710" integer_adm_scale2="0.445753" integer_adm_scale3="0.428904" integer_vif_scale0="0.443259" integer_vif_scale1="0.513409" integer_vif_scale2="0.532767" integer_vif_scale3="0.533751" vmaf="0.000000" />
Then in ffmpeg I ran a command to extract this frame in both source and encoded with something like:
.\ffmpeg.exe -i "(sourceVid)" -vf "select=eq(n\,487)" -vframes 1 source.png"
to just get a sample of the frame that VMAF has ranked at 0. Looking at the two pngs, they seem identical to the eye. When running the two pngs through VMAF in ffmpeg, they report a score of 98, which is what I actually expect and is obviously not 0.
This is driving me mad. I can't tell why VMAF would keep reporting a much lower rate than expected. It does seem like an issue with frame syncing, but I can't see a way to resolve it. Any help would be appreciated