On 4/11/2020 12:06 PM, Anton Khirnov wrote:
> ---
> doc/examples/demuxing_decoding.c | 177 ++++++++++++++++---------------
> 1 file changed, 91 insertions(+), 86 deletions(-)
>
> diff --git a/doc/examples/demuxing_decoding.c
> b/doc/examples/demuxing_decoding.c
> index 9bde927321..803e35d25c 100644
> --- a/doc/examples/demuxing_decoding.c
> +++ b/doc/examples/demuxing_decoding.c
> @@ -55,87 +55,93 @@ static AVPacket pkt;
> static int video_frame_count = 0;
> static int audio_frame_count = 0;
>
> -static int decode_packet(int *got_frame, int cached)
> +static int output_video_frame(AVFrame *frame)
> +{
> + if (frame->width != width || frame->height != height ||
> + frame->format != pix_fmt) {
> + /* To handle this change, one could call av_image_alloc again and
> + * decode the following frames into another rawvideo file. */
> + fprintf(stderr, "Error: Width, height and pixel format have to be "
> + "constant in a rawvideo file, but the width, height or "
> + "pixel format of the input video changed:\n"
> + "old: width = %d, height = %d, format = %s\n"
> + "new: width = %d, height = %d, format = %s\n",
> + width, height, av_get_pix_fmt_name(pix_fmt),
> + frame->width, frame->height,
> + av_get_pix_fmt_name(frame->format));
> + return -1;
> + }
> +
> + printf("video_frame n:%d coded_n:%d\n",
> + video_frame_count++, frame->coded_picture_number);
> +
> + /* copy decoded frame to destination buffer:
> + * this is required since rawvideo expects non aligned data */
> + av_image_copy(video_dst_data, video_dst_linesize,
> + (const uint8_t **)(frame->data), frame->linesize,
> + pix_fmt, width, height);
> +
> + /* write to rawvideo file */
> + fwrite(video_dst_data[0], 1, video_dst_bufsize, video_dst_file);
> + return 0;
> +}
> +
> +static int output_audio_frame(AVFrame *frame)
> +{
> + size_t unpadded_linesize = frame->nb_samples *
> av_get_bytes_per_sample(frame->format);
> + printf("audio_frame n:%d nb_samples:%d pts:%s\n",
> + audio_frame_count++, frame->nb_samples,
> + av_ts2timestr(frame->pts, &audio_dec_ctx->time_base));
> +
> + /* Write the raw audio data samples of the first plane. This works
> + * fine for packed formats (e.g. AV_SAMPLE_FMT_S16). However,
> + * most audio decoders output planar audio, which uses a separate
> + * plane of audio samples for each channel (e.g. AV_SAMPLE_FMT_S16P).
> + * In other words, this code will write only the first audio channel
> + * in these cases.
> + * You should use libswresample or libavfilter to convert the frame
> + * to packed data. */
> + fwrite(frame->extended_data[0], 1, unpadded_linesize, audio_dst_file);
> +
> + return 0;
> +}
> +
> +static int decode_packet(AVCodecContext *dec, const AVPacket *pkt)
> {
> int ret = 0;
> - int decoded = pkt.size;
>
> - *got_frame = 0;
> + // submit the packet to the decoder
> + ret = avcodec_send_packet(dec, pkt);
> + if (ret < 0) {
> + fprintf(stderr, "Error submitting a packet for decoding (%s)\n",
> av_err2str(ret));
> + return ret;
> + }
>
> - if (pkt.stream_index == video_stream_idx) {
> - /* decode video frame */
> - ret = avcodec_decode_video2(video_dec_ctx, frame, got_frame, &pkt);
> + // get all the available frames from the decoder
Instead of this, it might be better to call receive_frame() one time
after each call to send_packet(), regardless of the latter consuming the
packet or not. If it doesn't, then you just keep it around until at some
point a call will consume it, then you can fetch the next one.
I say this because if you do
ret = avcodec_send_packet(pkt);
av_packet_unref(pkt);
if (ret < 0)
return ret:
do {
ret = avcodec_receive_frame(frame);
if (!ret)
output_frame(frame):
} while (!ret);
You'll be draining the decoders of all the frames it may have generated,
which may be detrimental in frame threading scenarios, versus something
like (Untested PoC):
do {
ret = avcodec_send_packet(pkt);
if (ret < 0) {
if (ret != AVERROR(EAGAIN))
return ret;
} else
av_packet_unref(pkt);
ret = avcodec_receive_frame(frame);
if (ret < 0) {
if (ret != AVERROR(EAGAIN))
return ret;
} else
output_frame(frame);
} while (!ret || pkt->size);
Which would constantly keep the decoder fed with packets as you retrieve
generated frames.
This is something I've noticed when writing the libdav1d decoder
wrapper. I don't know if it also applies to our frame threading logic,
or if it depends on the decoder used, but if it does, then the CLI would
also benefit from this.
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