Ffmpeg transcoding to H264 MP4 is not producing smooth video

Question

We have app where we record, depending on the configuration, from a camera/desktop or IP camera. The example is in C running under Ubuntu, but our app is windows based one so instead of x11grab we use gdigrab and instead of v4l2 we use 'dshow'.

The idea is no matter what we record from to be encoded to H264 and mp4 container. The code is based on transcoding.c example from ffmpeg.

Everything works so far, videos are playable everywhere, but the client is saying that the produced videos are not smooth and are choppy, even if the reported framerate is 29.97.

I've edited transcoding.c to be the same as our app code.

#include <libavcodec/avcodec.h>
#include <libavdevice/avdevice.h>
#include <libavformat/avformat.h>
#include <libavfilter/buffersink.h>
#include <libavfilter/buffersrc.h>
#include <libavutil/channel_layout.h>
#include <libavutil/opt.h>
#include <libavutil/pixdesc.h>

static AVFormatContext *ifmt_ctx;
static AVFormatContext *ofmt_ctx;
typedef struct FilteringContext {
    AVFilterContext *buffersink_ctx;
    AVFilterContext *buffersrc_ctx;
    AVFilterGraph *filter_graph;

    AVPacket *enc_pkt;
    AVFrame *filtered_frame;
} FilteringContext;
static FilteringContext *filter_ctx;

typedef struct StreamContext {
    AVCodecContext *dec_ctx;
    AVCodecContext *enc_ctx;

    AVFrame *dec_frame;
} StreamContext;
static StreamContext *stream_ctx;

static int open_input_file()
{
    int ret;
    unsigned int i;

    const AVInputFormat *format = av_find_input_format("x11grab");

    ifmt_ctx = NULL;
    if ((ret = avformat_open_input(&ifmt_ctx, ":0.0+0,0", format, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open input file\n");
        return ret;
    }

    if ((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find stream information\n");
        return ret;
    }

    stream_ctx = av_calloc(ifmt_ctx->nb_streams, sizeof(*stream_ctx));
    if (!stream_ctx)
        return AVERROR(ENOMEM);

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        AVStream *stream = ifmt_ctx->streams[i];
        const AVCodec *dec = avcodec_find_decoder(stream->codecpar->codec_id);
        AVCodecContext *codec_ctx;
        if (!dec) {
            av_log(NULL, AV_LOG_ERROR, "Failed to find decoder for stream #%u\n", i);
            return AVERROR_DECODER_NOT_FOUND;
        }
        codec_ctx = avcodec_alloc_context3(dec);
        if (!codec_ctx) {
            av_log(NULL, AV_LOG_ERROR, "Failed to allocate the decoder context for stream #%u\n", i);
            return AVERROR(ENOMEM);
        }
        ret = avcodec_parameters_to_context(codec_ctx, stream->codecpar);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Failed to copy decoder parameters to input decoder context "
                   "for stream #%u\n", i);
            return ret;
        }
        /* Reencode video & audio and remux subtitles etc. */
        if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO
                || codec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO)
                codec_ctx->framerate = av_guess_frame_rate(ifmt_ctx, stream, NULL);
            /* Open decoder */
            ret = avcodec_open2(codec_ctx, dec, NULL);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to open decoder for stream #%u\n", i);
                return ret;
            }
        }
        stream_ctx[i].dec_ctx = codec_ctx;

        stream_ctx[i].dec_frame = av_frame_alloc();
        if (!stream_ctx[i].dec_frame)
            return AVERROR(ENOMEM);
    }

    //av_dump_format(ifmt_ctx, 0, "/dev/video0", 0);
    return 0;
}

static int open_output_file(const char *filename)
{
    AVStream *out_stream;
    AVStream *in_stream;
    AVCodecContext *dec_ctx, *enc_ctx;
    const AVCodec *encoder;
    int ret;
    unsigned int i;

    ofmt_ctx = NULL;
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);
    if (!ofmt_ctx) {
        av_log(NULL, AV_LOG_ERROR, "Could not create output context\n");
        return AVERROR_UNKNOWN;
    }


    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
            return AVERROR_UNKNOWN;
        }

        in_stream = ifmt_ctx->streams[i];
        dec_ctx = stream_ctx[i].dec_ctx;

        if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO || dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {

            encoder = avcodec_find_encoder_by_name("libx264");
            if (!encoder) {
                av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
                return AVERROR_INVALIDDATA;
            }
            enc_ctx = avcodec_alloc_context3(encoder);
            if (!enc_ctx) {
                av_log(NULL, AV_LOG_FATAL, "Failed to allocate the encoder context\n");
                return AVERROR(ENOMEM);
            }

            if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
                enc_ctx->height = dec_ctx->height;
                enc_ctx->width = dec_ctx->width;
                enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;
                if (encoder->pix_fmts)
                    enc_ctx->pix_fmt = encoder->pix_fmts[0];
                else
                    enc_ctx->pix_fmt = dec_ctx->pix_fmt;

                enc_ctx->time_base = dec_ctx->time_base; 
                enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

                av_opt_set(enc_ctx->priv_data, "crf", "18", 0);
                av_opt_set(enc_ctx->priv_data, "preset", "slow", 0);
            } else {
                enc_ctx->sample_rate = dec_ctx->sample_rate;
                ret = av_channel_layout_copy(&enc_ctx->ch_layout, &dec_ctx->ch_layout);
                if (ret < 0)
                    return ret;

                enc_ctx->sample_fmt = encoder->sample_fmts[0];
                enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};
            }

            if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
                enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

            /* Third parameter can be used to pass settings to encoder */
            ret = avcodec_open2(enc_ctx, encoder, NULL);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);
                return ret;
            }
            ret = avcodec_parameters_from_context(out_stream->codecpar, enc_ctx);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to copy encoder parameters to output stream #%u\n", i);
                return ret;
            }

            out_stream->time_base = enc_ctx->time_base;
            stream_ctx[i].enc_ctx = enc_ctx;
        } else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
            av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);
            return AVERROR_INVALIDDATA;
        } else {
            /* if this stream must be remuxed */
            ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Copying parameters for stream #%u failed\n", i);
                return ret;
            }
            out_stream->time_base = in_stream->time_base;
        }

    }
    av_dump_format(ofmt_ctx, 0, filename, 1);

    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Could not open output file '%s'", filename);
            return ret;
        }
    }

    /* init muxer, write output file header */
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error occurred when opening output file\n");
        return ret;
    }

    return 0;
}

static int init_filter(FilteringContext* fctx, AVCodecContext *dec_ctx,
        AVCodecContext *enc_ctx, const char *filter_spec)
{
    char args[512];
    int ret = 0;
    const AVFilter *buffersrc = NULL;
    const AVFilter *buffersink = NULL;
    AVFilterContext *buffersrc_ctx = NULL;
    AVFilterContext *buffersink_ctx = NULL;
    AVFilterInOut *outputs = avfilter_inout_alloc();
    AVFilterInOut *inputs  = avfilter_inout_alloc();
    AVFilterGraph *filter_graph = avfilter_graph_alloc();

    if (!outputs || !inputs || !filter_graph) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
        buffersrc = avfilter_get_by_name("buffer");
        buffersink = avfilter_get_by_name("buffersink");
        if (!buffersrc || !buffersink) {
            av_log(NULL, AV_LOG_ERROR, "filtering source or sink element not found\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        snprintf(args, sizeof(args),
                "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
                dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
                dec_ctx->time_base.num, dec_ctx->time_base.den,
                dec_ctx->sample_aspect_ratio.num,
                dec_ctx->sample_aspect_ratio.den);

        ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                args, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source\n");
            goto end;
        }

        ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
                NULL, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "pix_fmts",
                (uint8_t*)&enc_ctx->pix_fmt, sizeof(enc_ctx->pix_fmt),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format\n");
            goto end;
        }
    } else if (dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
        char buf[64];
        buffersrc = avfilter_get_by_name("abuffer");
        buffersink = avfilter_get_by_name("abuffersink");
        if (!buffersrc || !buffersink) {
            av_log(NULL, AV_LOG_ERROR, "filtering source or sink element not found\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        if (dec_ctx->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC)
            av_channel_layout_default(&dec_ctx->ch_layout, dec_ctx->ch_layout.nb_channels);
        av_channel_layout_describe(&dec_ctx->ch_layout, buf, sizeof(buf));
        snprintf(args, sizeof(args),
                "time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=%s",
                dec_ctx->time_base.num, dec_ctx->time_base.den, dec_ctx->sample_rate,
                av_get_sample_fmt_name(dec_ctx->sample_fmt),
                buf);
        ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                args, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create audio buffer source\n");
            goto end;
        }

        ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
                NULL, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create audio buffer sink\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "sample_fmts",
                (uint8_t*)&enc_ctx->sample_fmt, sizeof(enc_ctx->sample_fmt),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output sample format\n");
            goto end;
        }

        av_channel_layout_describe(&enc_ctx->ch_layout, buf, sizeof(buf));
        ret = av_opt_set(buffersink_ctx, "ch_layouts",
                         buf, AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output channel layout\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "sample_rates",
                (uint8_t*)&enc_ctx->sample_rate, sizeof(enc_ctx->sample_rate),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output sample rate\n");
            goto end;
        }
    } else {
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    /* Endpoints for the filter graph. */
    outputs->name       = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx    = 0;
    outputs->next       = NULL;

    inputs->name       = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx    = 0;
    inputs->next       = NULL;

    if (!outputs->name || !inputs->name) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    if ((ret = avfilter_graph_parse_ptr(filter_graph, filter_spec,
                    &inputs, &outputs, NULL)) < 0)
        goto end;

    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
        goto end;

    /* Fill FilteringContext */
    fctx->buffersrc_ctx = buffersrc_ctx;
    fctx->buffersink_ctx = buffersink_ctx;
    fctx->filter_graph = filter_graph;

end:
    avfilter_inout_free(&inputs);
    avfilter_inout_free(&outputs);

    return ret;
}

static int init_filters(void)
{
    const char *filter_spec;
    unsigned int i;
    int ret;
    filter_ctx = av_malloc_array(ifmt_ctx->nb_streams, sizeof(*filter_ctx));
    if (!filter_ctx)
        return AVERROR(ENOMEM);

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        filter_ctx[i].buffersrc_ctx  = NULL;
        filter_ctx[i].buffersink_ctx = NULL;
        filter_ctx[i].filter_graph   = NULL;
        if (!(ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO
                || ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO))
            continue;


        if (ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
            filter_spec = "fps=fps=29.97,setpts=PTS-STARTPTS,format=yuv420p"; 
        else
            filter_spec = "anull"; /* passthrough (dummy) filter for audio */
        ret = init_filter(&filter_ctx[i], stream_ctx[i].dec_ctx,
                stream_ctx[i].enc_ctx, filter_spec);
        if (ret)
            return ret;

        filter_ctx[i].enc_pkt = av_packet_alloc();
        if (!filter_ctx[i].enc_pkt)
            return AVERROR(ENOMEM);

        filter_ctx[i].filtered_frame = av_frame_alloc();
        if (!filter_ctx[i].filtered_frame)
            return AVERROR(ENOMEM);
    }
    return 0;
}

static int encode_write_frame(unsigned int stream_index, int flush)
{
    StreamContext *stream = &stream_ctx[stream_index];
    FilteringContext *filter = &filter_ctx[stream_index];
    AVFrame *filt_frame = flush ? NULL : filter->filtered_frame;
    AVPacket *enc_pkt = filter->enc_pkt;
    int ret;

    av_log(NULL, AV_LOG_INFO, "Encoding frame\n");
    /* encode filtered frame */
    av_packet_unref(enc_pkt);

    ret = avcodec_send_frame(stream->enc_ctx, filt_frame);

    if (ret < 0)
        return ret;

    while (ret >= 0) {
        ret = avcodec_receive_packet(stream->enc_ctx, enc_pkt);

        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return 0;

        /* prepare packet for muxing */
        enc_pkt->stream_index = stream_index;
        av_packet_rescale_ts(enc_pkt,
                             stream->enc_ctx->time_base,
                             ofmt_ctx->streams[stream_index]->time_base);

        av_log(NULL, AV_LOG_DEBUG, "Muxing frame\n");
        /* mux encoded frame */
        ret = av_interleaved_write_frame(ofmt_ctx, enc_pkt);
    }

    return ret;
}

static int filter_encode_write_frame(AVFrame *frame, unsigned int stream_index)
{
    FilteringContext *filter = &filter_ctx[stream_index];
    int ret;

    av_log(NULL, AV_LOG_INFO, "Pushing decoded frame to filters\n");
    /* push the decoded frame into the filtergraph */
    ret = av_buffersrc_add_frame_flags(filter->buffersrc_ctx,
            frame, 0);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error while feeding the filtergraph\n");
        return ret;
    }

    /* pull filtered frames from the filtergraph */
    while (1) {
        av_log(NULL, AV_LOG_INFO, "Pulling filtered frame from filters\n");
        ret = av_buffersink_get_frame(filter->buffersink_ctx,
                                      filter->filtered_frame);
        if (ret < 0) {
            /* if no more frames for output - returns AVERROR(EAGAIN)
             * if flushed and no more frames for output - returns AVERROR_EOF
             * rewrite retcode to 0 to show it as normal procedure completion
             */
            if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                ret = 0;
            break;
        }

        filter->filtered_frame->pict_type = AV_PICTURE_TYPE_NONE;
        ret = encode_write_frame(stream_index, 0);
        av_frame_unref(filter->filtered_frame);
        if (ret < 0)
            break;
    }

    return ret;
}

static int flush_encoder(unsigned int stream_index)
{
    if (!(stream_ctx[stream_index].enc_ctx->codec->capabilities &
                AV_CODEC_CAP_DELAY))
        return 0;

    av_log(NULL, AV_LOG_INFO, "Flushing stream #%u encoder\n", stream_index);
    return encode_write_frame(stream_index, 1);
}

int main(int argc, char **argv)
{
    int ret;
    AVPacket *packet = NULL;
    unsigned int stream_index;
    unsigned int i;
    avdevice_register_all();

    if ((ret = open_input_file()) < 0)
        goto end;
    if ((ret = open_output_file("test.mp4")) < 0)
        goto end;
    if ((ret = init_filters()) < 0)
        goto end;
    if (!(packet = av_packet_alloc()))
        goto end;

    int frameCount = 0;

    /* read all packets */
    while (frameCount < 300) {
        if ((ret = av_read_frame(ifmt_ctx, packet)) < 0)
            break;

        frameCount++;
        stream_index = packet->stream_index;
        av_log(NULL, AV_LOG_DEBUG, "Demuxer gave frame of stream_index %u\n",
                stream_index);

        if (filter_ctx[stream_index].filter_graph) {
            StreamContext *stream = &stream_ctx[stream_index];

            av_log(NULL, AV_LOG_DEBUG, "Going to reencode&filter the frame\n");

            av_packet_rescale_ts(packet,
                                 ifmt_ctx->streams[stream_index]->time_base,
                                 stream->dec_ctx->time_base);
            ret = avcodec_send_packet(stream->dec_ctx, packet);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Decoding failed\n");
                break;
            }

            while (ret >= 0) {
                ret = avcodec_receive_frame(stream->dec_ctx, stream->dec_frame);
                if (ret == AVERROR_EOF || ret == AVERROR(EAGAIN))
                    break;
                else if (ret < 0)
                    goto end;

                stream->dec_frame->pts = stream->dec_frame->best_effort_timestamp;
                ret = filter_encode_write_frame(stream->dec_frame, stream_index);
                if (ret < 0)
                    goto end;
            }
        } else {
            /* remux this frame without reencoding */
            av_packet_rescale_ts(packet,
                                 ifmt_ctx->streams[stream_index]->time_base,
                                 ofmt_ctx->streams[stream_index]->time_base);

            ret = av_interleaved_write_frame(ofmt_ctx, packet);
            if (ret < 0)
                goto end;
        }
        av_packet_unref(packet);
    }

    /* flush filters and encoders */
    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        /* flush filter */
        if (!filter_ctx[i].filter_graph)
            continue;
        ret = filter_encode_write_frame(NULL, i);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Flushing filter failed\n");
            goto end;
        }

        /* flush encoder */
        ret = flush_encoder(i);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Flushing encoder failed\n");
            goto end;
        }
    }

    av_write_trailer(ofmt_ctx);
end:
    av_packet_free(&packet);
    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        avcodec_free_context(&stream_ctx[i].dec_ctx);
        if (ofmt_ctx && ofmt_ctx->nb_streams > i && ofmt_ctx->streams[i] && stream_ctx[i].enc_ctx)
            avcodec_free_context(&stream_ctx[i].enc_ctx);
        if (filter_ctx && filter_ctx[i].filter_graph) {
            avfilter_graph_free(&filter_ctx[i].filter_graph);
            av_packet_free(&filter_ctx[i].enc_pkt);
            av_frame_free(&filter_ctx[i].filtered_frame);
        }

        av_frame_free(&stream_ctx[i].dec_frame);
    }
    av_free(filter_ctx);
    av_free(stream_ctx);
    avformat_close_input(&ifmt_ctx);
    if (ofmt_ctx && !(ofmt_ctx->oformat->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    if (ret < 0)
        av_log(NULL, AV_LOG_ERROR, "Error occurred: %s\n", av_err2str(ret));

    return ret ? 1 : 0;
}

I'm using the fps filter to keep the framerate/duration of the video if there is variable frame rate. setpts filter is used to generate the pts without making the encoder complain. Also I'm not sure how I can calculate the pts correctly with audio.

What is wrong with my code? Why the videos are not really smooth and choppy?

Answer 1

stream->dec_frame->pts = stream->dec_frame->best_effort_timestamp is definitely wrong. dts and pts should already be correct after the frame has come out of the filter chain - assuming timestamps had been correct prior to filtering.

Second issue: The fps filter is not doing what you think it does. It's merely duplicating/removing entire frames, and then faking (rounding) timestamps on the surviving buffers. That's introducing the problem you are seeing! This filter is only producing appealing results when down-sampling the framerate by a whole number ratio.

You had actually gotten a different behavior when you removed the fps filter, as even though the correct pts was then comming out of the encoder, it was the video container format which doesn't support explicit PTS / varying duration either. So you still need to normalize the framerate prior to encoding.

The framerate filter is for producing a smooth, fixed-framerate stream without introducing stuttering.

You still need setpts=PTS-STARTPTS in order to be able to mux your streams - that's expected. And it should actually make no difference if you place it before or after the framerate filter.