[FFmpeg-devel] BUG? interlaced encoding using libx264, nal-hrd

Erik Slagter erik at slagter.name
Mon Jul 18 19:50:34 CEST 2011

> I will try to resolve the complete issue though.

Okay, I seem to have find the origin of the issue.

FFmpeg apparently sets interlacing mode to bottom field first by default. The libx264 
encoder is configured using tff=0 because of this. Then, when you're going to encode tff=1 
material and you specify "tff=1" in the x264opts, the encoder is "reconfigured" 
(x264_encoder_reconfig(x4->enc, &x4->params), line 120) now with value tff=1. If nal-hrd 
is in use (this appears to be essential for interlaced encodings), libx264 complains with 
the noted warning/error.

There seems to be a very simple workaround: add -top 1 to the ffmpeg command line (besides 
x264opts "tff=1", which is also required).

So this is mostly a cosmetic issue, I don't know how this should be solved other than 
changing the default interlacing mode of ffmpeg, but I guess that's unacceptable.

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