[x265] coding efficieny comparision between x265 and x264(also include HM)

[idxa at sina.com]

Hi, I have compared x265 with x264, and find that coding efficieny of the two encoders is in the same level and is less about 20%-30% than HM11.0.
 
When testing,  I adjusted parmeter --qp to get same bitrates of x265 and x264, and then compare the PSNR. 
x264 --preset veryslow --sar 640:360 --fps 25 --tune psnr --no-mbtree --psnr -o out.264 -q xx input.yuv --dump-yuv recon.yuv
x265 --input input.yuv --input-res 640x360 --fps 25 --qp xx --recon recon.yuv -o out.hevc -F 6
 
My question is what factors cause the performance difference between x265 and HM. I thought  of the following 3 reasons.
1. some feagures (like multiple refs, open-gop, pass more motion vector candidates to motion estimation, weighted bidir) are not included in x265
2. x265 port x264 motion eastimation code which make some compression loss
3. the framework of wpp and frame parallelism reduce the coding efficiency
 
thanks,
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