Re: [Libquicktime-devel] More patches

[Burkhard P. <pl...@ip...>]

Hi,

Am 20.11.2012 18:11, schrieb Joseph Artsimovich:

> It doesn't look like it's going to work. When I set the following:
>
> codec->avctx->time_base.num = 1;
> codec->avctx->time_base.den = lqt_video_time_scale(file, track);
> codec->avctx->ticks_per_frame = lqt_frame_duration(file, track, NULL);
>
> avcodec_open2() then fails with "MPEG1/2 does not support 600/1 fps". It
> doesn't even look at ticks_per_frame.

Ahh ok, now I remember it wasn't that easy. And of course framerate isn't
only used for rate control, it's also be written to the MPEG sequence header :)

ticks_per_frame is a field in some MPEG4 header so I'd guess it's used
mainly by CODEC_ID_MPEG4.

> It also lools like even if you remove that check, 1/600 ratio would get
> written to MPEG bitstream.

No it won't, because MPEG-1/2 has no way to way to store arbitrary
framerates.

> In fact that's the whole point of
> ticks_per_frame, to be able to put 1/50 to MPEG bitstream for 50i
> content. So, this behaviour is by design.
>
> The solution I am thinking about is doing what my previous patch did,
> but only for XDCAM. This will make it impossible to generate
> variable-duration XDCAM frames, but I don't expect anyone to ever need that.

I would make it depend on the codec_id because it also applied to IMX.
For CODEC_ID_MPEG2VIDEO (and probably others) set time_base to the framerate
like you did earlier, and increase PTS by one for each frame.

Just found this snippet in my other libavcodec frontend:

static void set_framerate(ffmpeg_video_stream_t * st)
   {
   if(st->format.framerate_mode == GAVL_FRAMERATE_CONSTANT)
     {
     st->stream->codec->time_base.den = st->format.timescale;
     st->stream->codec->time_base.num = st->format.frame_duration;
     }
   else
     {
     st->stream->codec->time_base.den = st->format.timescale;
     st->stream->codec->time_base.num = 1;
     }
   }

Burkhard