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

[Wu-bin Zhen]

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.

Any advise will be appreciated.
-w