[Libav-user] Problem with slice based multithread decoding of h264 video

zgs zhengguanshi at 126.com
Fri Nov 6 08:38:49 CET 2015

Hi, all,
I am trying to decode h264 video using FFmpeg h264 decoder and got some problem:
Although the number of threads was the same as I set, the decoder seems to fail to utilize these threads.
    I traced the program by GDB and found that when executing ff_h264_execute_decode_slices()
    in libavcodec/h264_slice.c, the context_count is always 1, so it failed to execute the branch that
    invokes avctx->execute() (which seems to be the correct multithread version).

    the program I used was the example program demuxing_decoding.c under ffmpeg/doc/examples/,
    and I inserted dec_ctx->thread_count = thread_count; dec_ctx->thread_type = thread_type; before
    line 179 which executing avcodec_open2()
    the version of FFmpeg I used was: ffmpeg version git-2015-09-24-7179add

    the video I used was big_buck_bunny_720p_H264_AAC_25fps_3400K.MP4 under 

    One more thing, some videos trigger warning like:
    [h264 @ 0x17ac3e0] Cannot parallelize slice decoding with deblocking filter type 1, decoding such frames in sequential order
    but this video is fine.
    How can I make use of slice based multithread decoding?
    Thanks for your help.


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