[FFmpeg-user] Converting from h264(yuvj420p) to h264(yuv420p) without transcoding

[James Darnley]

On 2013-02-11 23:10, Wu-bin Zhen wrote:
> Hello ffmpeg,
> 
> I have a weird device that only generates h264 video stream with yuv"J"420p
> pixel format. Unfortunately the chrome browser does not support the pixel
> format, so I have no choice but transcoding every frame from yuvj420p to
> yuv420p. It costs a lot of time, and should be lose some quality because
> each frame is going to be encoded again.
> 
> I googled that there is an user modified the gpac MP4Box to manipulate VUI
> property. I remember it is "fullrange" flag.
> https://github.com/golgol7777/gpac/commit/913c0081d6882a713d66764c43827b6f3d05f925
> 
> And it seems the trick is working for some users:
> https://blendervse.wordpress.com/2012/04/02/waiving-the-fullrange-flag/
> 
> My machine is not able to compile his gpac, and the old gpac has some
> limitations. So I wonder if there is a "ffmpeg" way to change the pixel
> format without transcoding every frame. I tried "-vcodec copy
> -pix_fmt=yuv420p" options but it is not working, and the output file still
> uses the yuv"J"420p pixel format.

TO alter the fullrange flag (without encoding) someone would need to
write a bitstream filter for ffmpeg, somewhat similar to the mp4toannexb
filter.

Regardless, if that is true, that is a major issue with chrome.  It
should play the video even if it doesn't know about the fullrange flag
and wouldn't convert to RGB properly.  The only issue would be
exaggerated contrast.

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