C++ – Decode h264 rtsp with ffmpeg and separated AVCodecContext

Try using this. It is the fastest and best ffmpeg-way I have figure it out:

 ffmpeg -ss 00:01:00 -i input.mp4 -to 00:02:00 -c copy output.mp4

This command trims your video in seconds!

Explanation of the command:

-i: This specifies the input file. In that case, it is (input.mp4).
-ss: Used with -i, this seeks in the input file (input.mp4) to position.
00:01:00: This is the time your trimmed video will start with.
-to: This specifies duration from start (00:01:40) to end (00:02:12).
00:02:00: This is the time your trimmed video will end with.
-c copy: This is an option to trim via stream copy. (NB: Very fast)

The timing format is: hh:mm:ss

Please note that the current highly upvoted answer is outdated and the trim would be extremely slow. For more information, look at this official ffmpeg article.

First, what does the av_dump_format shows? Are you sure your video stream codec is h264, because you try to open the codec as if it were H264.

In order to open any codec, change your avcodec_find_decoder to pass it the source codec id:

codec = avcodec_find_decoder(formatContext->streams[videoStreamIndex]->codec->codec_id);

By the way, (forget this one if you do not use the c++ code but stick with c#): you do not need to make a copy of the initial AVCodecContext when you are looking for the video stream. You can do: (note that you may want to keep a pointer to the inital codec context, see below).

AVCodecContext* c = s->codec;
if (c->codec_type == AVMEDIA_TYPE_VIDEO) {
    videoStreamIndex = i;
    initialVideoCodecCtx = c;
}

Next point, not really relevant in this case: instead of looping through all the steams, FFmpeg has a helper function for it:

int videoStreamIndex = av_find_best_stream(formatContext, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);

Last point: I think only the first point should do the trick to make avcodec_open2 work, but you might not be able to decode your stream. You opened the codec for the new codec context, but no codec is opened for the inital context. Why did you make a copy of the initial codec context? It is usefull if you want to record your stream in another file (i.e. transcode), but if you only want to decode your stream, it is much easier to use the initial context, and use it instead of the new one as a parameter for avcodec_decode_video2.

To sum it up, replace your code after avformat_find_stream_info by (warning: no error check):

int videoStreamIndex = av_find_best_stream(formatContext, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
AVCodecContext* codecCtx = formatContext->streams[videoStreamIndex]->codec;
AVCodec* codec = avcodec_find_decoder(codecCtx->codec_id);
// tune codecCtx if you want special decoding options. See FFmpeg docs for a list of members
if (avcodec_open2(codecCtx, codec, null) < 0) {
    return;
}

// use av_read_frame(formatContext, ...) to read packets
// use avcodec_decode_video2(codecCtx, ...) to decode packets