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.
Anyone can give me any hint?Thanks very much. 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;
You might try this: libavcodec/h264_mp4toannexb_bsf.c. It converts bitstream without start codes to bitstream with start codes.
Using your source file, does ffmpeg -i src.mp4 -vcodec copy -an dst.mp4
work? Does it work if you add -bsf h264_mp4toannexb
? (all using the same version/build of ffmpeg as you are trying to use programmatically of course)