Extracting the h264 part of a video file (demuxing)

Question

I am trying to demux a video file into the video part (h264, mpeg4, h265, vp8, etc) and the audio part (mp3, aac, ac3, etc) and the subtitle part <

Answer 1

You need to convert an H.264 bitstream from length prefixed mode to start code prefixed mode.This is required by some streaming formats, typically the MPEG-2 transport stream format ("mpegts").

Take a look at https://www.ffmpeg.org/ffmpeg-bitstream-filters.html#h264_005fmp4toannexb

Look at lines from 402 to 424 and from 842 to 843. https://www.ffmpeg.org/doxygen/0.7/crystalhd_8c-source.html

I used it, to extract h264 from mp4.

//Use this filter on your first h264 input AVPacket
AVFormatContext *ifmt_ctx = NULL;
//...
//...   //init input
//...
AVPacket *firstPacket;
//...
//...   //get packet from stream
//...
uint8_t *dummy_p;
int dummy_int;
AVBitStreamFilterContext *filter = v_bitstream_filter_init("h264_mp4toannexb");
if (!filter)
{
    printf("Can't open filter\n");
    exit(1);
}
ret = av_bitstream_filter_filter(filter, ifmt_ctx->streams[videoindex]->codec, NULL,
                                           &dummy_p, &dummy_int,
                                           firstPacket->data, firstPacket->size, 0);
if( ret < 0 )
{
     printf("Can't filter\n");
     exit(1);
}
//  use dummy_p to write to file, as first packet