[x265] Unexpected banding issues with x265 12bit lossy encoding

[PDR PDR]

This is a repost of 


https://bitbucket.org/multicoreware/x265_git/issues/1/unexpected-banding-issues-with-x265-12bit 





Low CRF or QP values, like 0 or 1 for lossy encoding a simple 12bit ramp produces unexpected gaps and duplicate values 

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Test: 12bit ramp, 4K DCI 4096x2160, Y= 0 to 4095. Every x= position coordinate has the same value in the Y channel. 1 frame. 

Result: Repeats and gaps in the waveform at regular intervals. eg. x=252, x=253 both have a Y value of 252. The value Y=256 is missing, etc... x=4060, 4061 both have a Y value of 4060, and Y value of 4064 missing, etc… 

eg. --crf 0 Y waveform PSNR y:81.274910 
[ https://i.postimg.cc/nhFd7Kvt/x265-yuv444p12-crf0.png | https://i.postimg.cc/nhFd7Kvt/x265-yuv444p12-crf0.png ] 


‌ 

[code] 

vspipe --y4m 4k_greyramp_yuv444p12.vpy - | "x265.exe" --y4m - --input-csp i444 --input-depth 12 --output-depth 12 --qp 1 -o x265_yuv444p12_qp1.hevc 

[/code] 

‌ 

Double checked values with vapoursynth picker (missing values/gaps, and repeated values), and psnr testing. 

--lossless is ok - so that mostly rules out a script or toolchain issue; a decoder side issue also less likely. But also checked ffmpeg yuv4mpegpipe on the ffv1 below, instead of vspipe - same issue 

‌ 

eg. --lossless Y waveform PSNR y: inf 

[ https://i.postimg.cc/ZnPcv0nR/x265-yuv444p12-lossless.png | https://i.postimg.cc/ZnPcv0nR/x265-yuv444p12-lossless.png ] 


‌ 

When using --qp 0, --qp 1, or --crf 0, --crf 1 , I also tested some variations like --tune psnr... , tried adding --tskip --cu-lossles, checked several different x265 builds => always gaps, banding without the --lossless switch 

A simple gradient ramp should be lossless at low qp values - it is with 10bit ramps; e.g. even at crf16 with default settings, x264 is lossless on a simple 10bit ramp (probably at higher qp’s or crf’s too) 

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Source file ffv1 yuv444p12le for testing (lossless, PSNR infinity compared to original script) 

[ http://www.mediafire.com/file/2h4xrmvug0lm127/4k_greyramp_yuv444p12_ffv1.mkv/file | http://www.mediafire.com/file/2h4xrmvug0lm127/4k_greyramp_yuv444p12_ffv1.mkv/file ] 
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