[x264-devel] libx264 encoding parameters that influence SPS and PPS

[declan harrison]

On Sat, Sep 18, 2010 at 9:35 PM, Jason Garrett-Glaser
<darkshikari at gmail.com> wrote:
> On Sat, Sep 18, 2010 at 6:23 AM, declan harrison
> <harrison.declan at gmail.com> wrote:
>> Hi
>>
>> Using ffmpeg command line (see below) with an input flash video file
>> to create an output flash video file whose size is smaller than the
>> input file and bitrate differ but in most other ways the files should
>> be the same (profile, level etc).
>>
>> So I'm trying to create this output file to have basically the same
>> SPS and PPS that the input flash video file has.
>
> This is generally not possible.

Hi Jason

I take your point on this but I have made some progress by doing the following.

So I have worked back from the methods x264_sps_init and x264_pps_init
in file encoder/set.c and mapping this back to the file
libavcodec/libx264.c to see what parameters use which ffmpeg options.
Using this technique I have sorted most of the mismatches. However Im
working on the final 2 parameters "log2_max_frame_num_minu4" and
"log2_max_pic_order_cnt_lib_minus4" and these appear to be a bit more
involved. They are using the parameters vui.i_max_dec_frame_buffering
(controled by refs option) and i_bframe_pyramid as well. I reckon if I
can sort these two out then that should be good.

However I was thinking of a slightly different approach but Im not
sure if it will work, I was wondering what your thoughts on whether
this new approach would work or not.  Basically my ultimate goal is to
send part of the original input stream to the player and then append
the parts that I need to re-encode at the end of this stream.  I
suppose you could say Im attempting to concatenate two streams
together. So rather than try and match exactly the input SPS and PPS I
was wondering if I changed the sps_id and the pps_id of the re-encoded
H264 stream would the player be able to decode these two streams?
Currently ffmpeg doesnt expose the x264 parameter (sps-id) to allow
this to happen but Im currently building a change that sets this value
to a non zero value in ffmpeg. Will be able to test this when its
done.

I would need to add an extra entry into the
AVCDecoderConfigurationRecord for the additonal SPS and PPS.

Do you think this approach in theory could work?

Declan