[x264-devel] Lossless with 10bit pixel depth

[BugMaster]

On Wed, 10 Jul 2019 10:13:44 +0200, Simon Dumke wrote:
> Hi all,

Hi.

> I was today experimenting with lossless encoding in x264 (using     ffmpeg).
>   
>   
> My experiment was coming in with a raw yuv420 stream, encoding     
> that with h264 high444 witch crf=0. Than decoding it again to raw   
> yuv420 and making a binary diff to the original.
>   
> When using 8bit yuv, this works as expected:
>   
>   
> ffmpeg -f rawvideo -video_size 1024x1280 -pixel_format yuv420      
> -i yuv420-8.yuv -c:v libx264 -pix_fmt yuv420 -profile:v high444     
> -crf 0 -preset:v slow h264-8-420.m4v
>          ffmpeg -i h264-8-420.m4v -c:v rawvideo -f rawvideo
> -pix_fmt         yuv420p10le decompressed-8-420.yuv
>          diff -s yuv420-8.yuv decompressed-8-420.yuv
>   
>   
> The diff, as expected, resulted in identical files.
>   
> Doing the same with a yuv420p10le however, does not:
>   
>   
> ffmpeg -f rawvideo -video_size 1024x1280 -pixel_format        
> yuv420p10le -i yuv420-10.yuv -c:v libx264 -pix_fmt yuv420p10        
> -profile:v high444 -crf 0 -preset:v slow h264-10-420.m4v
>          ffmpeg -i h264-10-420.m4v -c:v rawvideo -f rawvideo
> -pix_fmt         yuv420p10le decompressed-10-420.yuv
>          diff -s yuv420-10.yuv decompressed-10-420.yuv
>   
>   
> Here, while the result looks ok optically, the diff (or a cmp)      show a lot of differences.
>   
> My question: Do i have to do more to have lossless 10bit      
> encoding? Is it even possible? Or is there another error in my       thinking?

For lossless you should set qp=0 (not crf=0). crf=0 works in 8-bit
mode only because crf and qp ranges are aligned (both start from 0).
But in 10-bit mode crf and qp ranges are not aligned (crf starts
from -12, but qp still starts from 0).

> Thanks & regards,
>   
> Simon
>   
> P.S.: BTW - will support for 12 and 14 bit come at any time in      the future?

Probably not. At least there is no plan for this so far.