[WorBry]

Originally Posted by lansing

Good comparison to show that x265 really has no advantage over x264 on 1080p materials if we're going for transparent encoding.

Now we'll just have to wait for people with high end computer to do the 4K comparison.

Done. So I ran a parallel test series using the original 2160/50p Crowd Run clip (8bit 420 y4m) as source and reference for the metric tests. For VMAF tests I used the VapourSynth (v3) plugin with the vmaf_4k_v0.6.1.pk model (model=1) which aims to "predict the subjective quality of video displayed on a 4KTV and viewed from the distance of 1.5 times the height of the display device (1.5H)"

The x264 results:





Interesting that the shape of ffmpeg-SSIM vs bitrate plot is quite different to that in the 1080/50p series and the differential between the VMAF and ffmpeg-SSIM scores is larger. The x265 encodes show the same behaviour:



Again the VMAF scores deem that x265 has higher perceptual quality over the lower bitrate range.

As to whether there is an advantage over x264 for 'transparent' encoding; well, I looked more closely at what point at which the VMAF plots hit the maximum score of 100.



For x264 it was at CRF=8 (1296 Mbps) and for x265 at CRF=10 (976 Mbps). So on that basis it could be concluded that x265 is significantly more efficient. That said, if you look at the per-frame VMAF scores, it is clear that the first frame skews the outcome somewhat.

Taking the x264 series first; going from CRF=9 to 16, all frames bar the first frame in each test scored VMAF=100. And in the x265 series also, going from CRF 11 to 17 only the first frame scored less than VMAF=100:



So, if the aggregate VMAF scores are calculated with the first frame excluded (simple average across the remaining 499 frames), CRF=16 (392 Mbps) becomes the point at which VMAF=100 is reached in the x264 series, and CRF=17 (306 Mbps) in the x265 series:



Makes quite a difference. x265 still has the edge on bit savings, but not by as much. I don't have time to calculate 'adjusted' aggregate VMAF scores for the other (lower bitrate) CRF data points. In the 1080/50p series an aggregate VMAF=100 score was never attained for precisely the same reason - the VMAF score of the first frame skewed the aggregate score.

Here are the ffmpeg-SSIM scores obtained in the 2160/50p series at these 'significant' CRF points though:



Edit:

Originally Posted by WorBry

Interesting that the shape of ffmpeg-SSIM vs bitrate plot is quite different to that in the 1080/50p series and the differential between the VMAF and ffmpeg-SSIM scores is larger...

I'll maybe see how the 2160/50p and 1080/50p series compare when plotted against bits/pixel.