Using parallel decoder in libavcodec/ffmpeg

0
votes

The problem

I am writing a simple program in C language which makes use of libavcodec (https://github.com/Dr-Noob/framepos/blob/master/framepos.c)

I have been working with h264 videos. While the decoding works, I can see that it's very slow, since it only uses 1 CPU core (I checked it with top). On the other hand, I know that ffmpeg, which uses the same libavocdec installed in my system, makes use of the parallel h264 decoder. I can test it with:

ffmpeg -c:v h264 -i test.mkv -f null -

With top I can see that it's running in parallel, and the speed is noticeably faster. I would like a solution that always gives me the possibility of decoding the video using all the CPU cores, not only in the case of the h264 codec.

My research so far

Looking at the ffmpeg code, one can see that, to obtain the AVCodec, it uses the function find_codec_or_die. This will end up using avcodec_find_decoder_by_name. Actually, if in my program I use this function, asking for the h264 decoder, I still get the sequential version. Moreover, using gdb in ffmpeg I have seen that the AVCodec in ffmpeg is called ff_h264_decoder, while in my code, gdb does not know which specific type is the codec. The ff suffix makes me think that this one is the parallel decoder (because it looks like ff has something to do with parallel in the ffmpeg context (https://ffmpeg.org/doxygen/2.7/pthread__frame_8c.html)). However, it seems that I am unable to get this codec.

What can I do to decode video in parallel using libavcodec in C?

1
cffmpeglibavcodec
AVCodec doesn't define multi-threading - it is AVCodecContext::thread_count property (which should be set before avcodec_open2()) ffmpeg.org/doxygen/2.7/structAVCodecContext.htm - gkv311

1 Answers

0
votes

Posting gkv311 comment as an answer for future reference.

AVCodec does not have multi-threading functionality. It is stored inside AVCodecContex. So, a posible scheme to run the codec in parallel:

AVCodec *codec = avcodec_find_decoder
AVCodecContext *ctx = avcodec_alloc_context3

ctx->thread_count = n_threads;
ctx->thread_type = FF_THREAD_FRAME;

avcodec_open2(ctx, fmt_ctx->video_codec, NULL)