[x264-devel] Reconfiguring x264 aspect ratio in the middle of stream

Jarmo Torvinen jarmo.torvinen at jutel.fi
Mon Jan 12 10:12:31 CET 2009

> Jarmo Torvinen a écrit :
> > Is it possible to change the aspect ratio 
(x264_param_t.vui.i_sar_width & 
> > height) after the encoder
> > has been initialized with the x264_encoder_open()-call?
> > 
> > There is the x264_encoder_reconfig()-call but while inspecting the 
> > it seems that the vui*-parameters are
> > not copied in that function call. 
> [...]
> > I'm ready to contribute patches.
> That would be a handy features, as there are many ATSC/DVB cases where 
> this would be usefull. Would you like to try implementing this?

Sure, actually I have already made a proof-of-concept style modification 
which seems to work. I'm using
to to change dynamically the aspect ratio of the video while transcoding 
from a DVB ts stream to a ts containing 
h264 video content. 

The POC contains modifications to 
-x264 encoder/encoder.c, 

The modification to the x264 encoder part is really small, actually only 
couple of lines:

// add these lines to x264_encoder_reconfig()

+    h->sps->vui.i_sar_width = h->param.vui.i_sar_width;
+    h->sps->vui.i_sar_height = h->param.vui.i_sar_height;

I see some problems with this approach:

1) it is too time consuming to call the x264_encoder_reconfig() just for 
the purpose 
of changing the aspect ratio, I'd rather have separate API function which 
just would do that. 
But on the other hand, is there enough justification to add another api 
call just for that purpose?

2) is it correct way to directly to modify the h->sps.* parameters, maybe 
it would be better to call
x264_sps_init() instead?

3) according to initial tests, the aspect ratio changes, but the timing is 
not always correct. This is because the 
aspect ratio info is sent in the SPS, so the timing depends on the timing 
of the scene cut. To have correct timing, 
there should be some way to force a scenecut or sending a SPS through the 
API when the aspect ratio changes.

Any thoughts?


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