I've made a test with --merange 12/26/40/57/74/92 for --ctu 64 --qg-size 64 vs. --ctu 32 --qg-size 32
Command line:
for %m in (12 26 40 57 74 92) do (
x265 -p7 --bitrate 500 -f3333 --psnr --ssim --qg-size 64 --merange %m ../big_buck_bunny_1080p24.y4m w64-%m.hevc
x265 -p7 --bitrate 500 -f3333 --psnr --ssim --ctu 32 --merange %m ../big_buck_bunny_1080p24.y4m w32-%m.hevc
)
Results (in ctu block: speed (fps), PSNR, SSIM (dB)):
Code:
ctu
merange | 64 | 32
12 | 6.93 39.980 12.996 | 6.44 39.565 12.742
26 | 6.69 40.303 13.599 | 6.21 39.903 13.345
40 | 6.46 40.393 13.746 | 6.07 39.999 13.499
57 | 6.13 40.404 13.757 | 5.90 40.019 13.512
74 | 5.85 40.409 13.757 | 5.67 40.024 13.515
92 | 5.58 40.413 13.760 | 5.47 40.026 13.514
When --merange grows, the quality increases regardless of the size of --ctu (it may depend on the source movie).
For CPU with only 12 logical cores (6 physical) --ctu 64 is just better in preset slower for 1080p encoding.