I want to realize an application that firstly decode a multi-media file(such as test.mp4 file, video codec id is H264), get a video stream and an audio stream, then make some different in the audio stream, at last encode the video stream (use libx264) and audio stream into a result file (result.mp4). To promote the efficiency, I omitted the decode and encode of video stream, i get the video packet via function av_read_frame, then output it directly into the result file via function av_write_frame. But there is no picture in the output file, and the size of output file is fairly small.

I tracked the ffmpeg code and found that in the function av_write_frame->mov_write_packet->ff_mov_write_packet, it will call function ff_avc_parse_nal_units to obtain the size of nal unit, but the return value is very small (such as 208 bytes).

I find that the H264 stream in the MP4 file is not stored in Annex-B format, so it can't find start code (0x000001).

Now my problem is how can I change the H264 stream to Annex-B format, and make it work?

I added start code at the beginning of every frame manually, but it still not work.

Following is the codes similar with my:

// write the stream header, if any
av_write_header(pFormatCtxEnc);

.........
/**
* Init of Encoder and Decoder
*/

bool KeyFlag = false;
bool KeyFlagEx = false;
// Read frames and save frames to disk
int iPts = 1;
av_init_packet(&packet);
while(av_read_frame(pFormatCtxDec, &packet)>=0)
{
        if (packet.flags == 1)
                KeyFlag = true;

        if (!KeyFlag)
                continue;

        if (m_bStop)
        {
                break;
        }

        // Is this a packet from the video stream?
        if(packet.stream_index == videoStream)
        {
                currentframeNum ++;
                if (progressCB != NULL && currentframeNum%20 == 0)
                {
                        float fpercent = (float)currentframeNum/frameNum;
                        progressCB(fpercent,m_pUser);
                }

                if (currentframeNum >= beginFrame && currentframeNum <= endFrane)
                {
                        if (packet.flags == 1)
                                KeyFlagEx = true;

                        if (!KeyFlagEx)
                                continue;
                        packet.dts = iPts ++;
                        av_write_frame(pFormatCtxEnc, &packet);

                }
        }
        // Free the packet that was allocated by av_read_frame
}

// write the trailer, if any
av_write_trailer(pFormatCtxEnc);

/**
* Release of encoder and decoder
*/

return true;