libav producing MP4 file with extremely high frame rate

Question

I am trying to write a program to generate frames to be encoded via ffmpeg/libav into an mp4 file with a single h264 stream. I found these two examples and am sort of trying to merge them together to make what I want: [video transcoder] [raw MPEG1 encoder]

I have been able to get video output (green circle changing size), but no matter how I set the PTS values of the frames or what time_base I specify in the AVCodecContext or AVStream, I'm getting frame rates of about 7000-15000 instead of 60, resulting in a video file that lasts 70ms instead of 1000 frames / 60 fps = 166 seconds. Every time I change some of my code, the frame rate changes a little bit, almost as if it's reading from uninitialized memory. Other references to an issue like this on StackOverflow seem to be related to incorrectly set PTS values; however, I've tried printing out all the PTS, DTS, and time base values I can find and they all seem normal. Here's my proof-of-concept code (with the error catching stuff around the libav calls removed for clarity):

#include <iostream>
#include <opencv2/opencv.hpp>
#include <math.h>

extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/opt.h>
#include <libavutil/timestamp.h>
}

using namespace cv;

int main(int argc, char *argv[]) {
    const char *filename = "testvideo.mp4";
    
    AVFormatContext *avfc;
    avformat_alloc_output_context2(&avfc, NULL, NULL, filename);
    
    AVStream *stream = avformat_new_stream(avfc, NULL);
    AVCodec *h264 = avcodec_find_encoder(AV_CODEC_ID_H264);
    AVCodecContext *avcc = avcodec_alloc_context3(h264);
    
    av_opt_set(avcc->priv_data, "preset", "fast", 0);
    av_opt_set(avcc->priv_data, "crf", "20", 0);
    avcc->thread_count = 1;
    avcc->width = 1920;
    avcc->height = 1080;
    avcc->pix_fmt = AV_PIX_FMT_YUV420P;
    avcc->time_base = av_make_q(1, 60);
    stream->time_base = avcc->time_base;
    
    if(avfc->oformat->flags & AVFMT_GLOBALHEADER)
        avcc->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    
    avcodec_open2(avcc, h264, NULL);
    avcodec_parameters_from_context(stream->codecpar, avcc);
    
    avio_open(&avfc->pb, filename, AVIO_FLAG_WRITE);
    
    avformat_write_header(avfc, NULL);
    
    Mat frame, nothing = Mat::zeros(1080, 1920, CV_8UC1);
    AVFrame *avf = av_frame_alloc();
    AVPacket *avp = av_packet_alloc();
    int ret;
    
    avf->format = AV_PIX_FMT_YUV420P;
    avf->width = 1920;
    avf->height = 1080;
    avf->linesize[0] = 1920;
    avf->linesize[1] = 1920;
    avf->linesize[2] = 1920;
    
    for(int x=0; x<1000; x++) {
        frame = Mat::zeros(1080, 1920, CV_8UC1);
        circle(frame, Point(1920/2, 1080/2), 250*(sin(2*M_PI*x/1000*3)+1.01), Scalar(255), 10);
        
        avf->data[0] = frame.data;
        avf->data[1] = nothing.data;
        avf->data[2] = nothing.data;
        avf->pts = x;
        
        ret = 0;
        do {
            if(ret == AVERROR(EAGAIN)) {
                av_packet_unref(avp);
                ret = avcodec_receive_packet(avcc, avp);
                if(ret) break; // deal with error
                av_write_frame(avfc, avp);
            } //else if(ret) deal with error
            ret = avcodec_send_frame(avcc, avf);
        } while(ret);
    }
    
    // flush the rest of the packets
    avcodec_send_frame(avcc, NULL);
    do {
        av_packet_unref(avp);
        ret = avcodec_receive_packet(avcc, avp);
        if(!ret)
            av_write_frame(avfc, avp);
    } while(!ret);
    
    av_frame_free(&avf);
    av_packet_free(&avp);
    
    av_write_trailer(avfc);
    avformat_close_input(&avfc);
    avformat_free_context(avfc);
    avcodec_free_context(&avcc);
    return 0;
}

This is the output of ffprobe run on the output video file

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'testvideo.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.76.100
  Duration: 00:00:00.07, start: 0.000000, bitrate: 115192 kb/s
  Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 115389 kb/s, 15375.38 fps, 15360 tbr, 15360 tbn, 120 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]

What might be causing my frame rate to be so high? Thanks in advance for any help.

Answer 1

You are getting high frame rate because you have failed to set packet duration.

• Set the time_base to higher resolution (like 1/60000) as described here:

   avcc->time_base = av_make_q(1, 60000);
  

• Set avp->duration as described here:

   AVRational avg_frame_rate = av_make_q(60, 1);   //60 fps
   avp->duration = avcc->time_base.den / avcc->time_base.num / avg_frame_rate.num * avg_frame_rate.den;    //avp->duration = 1000 (60000/60)
  

  And set the pts accordingly.

---

Complete code:

#include <iostream>
#include <opencv2/opencv.hpp>
#include <math.h>

extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/opt.h>
#include <libavutil/timestamp.h>
}

using namespace cv;

int main(int argc, char* argv[]) {
    const char* filename = "testvideo.mp4";

    AVFormatContext* avfc;
    avformat_alloc_output_context2(&avfc, NULL, NULL, filename);

    AVStream* stream = avformat_new_stream(avfc, NULL);
    AVCodec* h264 = avcodec_find_encoder(AV_CODEC_ID_H264);
    AVCodecContext* avcc = avcodec_alloc_context3(h264);

    av_opt_set(avcc->priv_data, "preset", "fast", 0);
    av_opt_set(avcc->priv_data, "crf", "20", 0);
    avcc->thread_count = 1;
    avcc->width = 1920;
    avcc->height = 1080;
    avcc->pix_fmt = AV_PIX_FMT_YUV420P;
    //Sey the time_base to higher resolution like 1/60000
    avcc->time_base = av_make_q(1, 60000); //avcc->time_base = av_make_q(1, 60);
    stream->time_base = avcc->time_base;

    if (avfc->oformat->flags & AVFMT_GLOBALHEADER)
        avcc->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

    avcodec_open2(avcc, h264, NULL);
    avcodec_parameters_from_context(stream->codecpar, avcc);

    avio_open(&avfc->pb, filename, AVIO_FLAG_WRITE);

    avformat_write_header(avfc, NULL);

    Mat frame, nothing = Mat::zeros(1080, 1920, CV_8UC1);
    AVFrame* avf = av_frame_alloc();
    AVPacket* avp = av_packet_alloc();
    int ret;

    avf->format = AV_PIX_FMT_YUV420P;
    avf->width = 1920;
    avf->height = 1080;
    avf->linesize[0] = 1920;
    avf->linesize[1] = 1920;
    avf->linesize[2] = 1920;

    for (int x = 0; x < 1000; x++) {
        frame = Mat::zeros(1080, 1920, CV_8UC1);
        circle(frame, Point(1920 / 2, 1080 / 2), (int)(250.0 * (sin(2 * M_PI * x / 1000 * 3) + 1.01)), Scalar(255), 10);

        AVRational avg_frame_rate = av_make_q(60, 1);   //60 fps

        int64_t avp_duration = avcc->time_base.den / avcc->time_base.num / avg_frame_rate.num * avg_frame_rate.den;

        avf->data[0] = frame.data;
        avf->data[1] = nothing.data;
        avf->data[2] = nothing.data;
        avf->pts = (int64_t)x * avp_duration; // avp->duration = 1000

        ret = 0;
        do {
            if (ret == AVERROR(EAGAIN)) {
                av_packet_unref(avp);
                ret = avcodec_receive_packet(avcc, avp);
                if (ret) break; // deal with error

                ////////////////////////////////////////////////////////////////
                //avp->duration was zero.
                avp->duration = avp_duration;    //avp->duration = 1000 (60000/60)

                //avp->pts = (int64_t)x * avp->duration;
                ////////////////////////////////////////////////////////////////

                av_write_frame(avfc, avp);
            } //else if(ret) deal with error
            ret = avcodec_send_frame(avcc, avf);
        } while (ret);
    }

    // flush the rest of the packets
    avcodec_send_frame(avcc, NULL);
    do {
        av_packet_unref(avp);
        ret = avcodec_receive_packet(avcc, avp);
        if (!ret)
            av_write_frame(avfc, avp);
    } while (!ret);

    av_frame_free(&avf);
    av_packet_free(&avp);

    av_write_trailer(avfc);
    avformat_close_input(&avfc);
    avformat_free_context(avfc);
    avcodec_free_context(&avcc);
    return 0;
}

---

Result of FFprobe:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'testvideo.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.76.100
  Duration: 00:00:16.65, start: 0.000000, bitrate: 456 kb/s
  Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 450 kb/s, 60.06 fps, 60 tbr, 60k tbn, 120k tbc (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]

---

Notes:

• I don't know why the fps is 60.06 and not 60.
• There is a warning message MB rate (734400000) > level limit (16711680) that I didn't fix.

Answer 2

Though the answer I accepted fixes the problem I was having, here is some more information I've figured out that may be useful:

The time_base field has some restrictions on its value (for example 1/10000 works, but 1/9999 doesn't) based on the container format, and this seems to have been the root problem I was having. When the time base was set to 1/60, the call to avformat_write_header() changed it to 1/15360. Because I had hardcoded the PTS increment to 1, this resulted in the 15360 FPS video. The strange denominator of 15360 seems to result from the given denominator being multiplied by 2 repeatedly until it reaches some minimum value. I have no idea how this algorithm works actually works. This SO question led me on to this.

By setting the time base to 1/60000 and making the PTS increment by 1000 each frame, the fast video problem was fixed. Setting the packet duration doesn't seem necessary, but is probably a good idea.

The main lesson here is to use whatever time_base libav gives you instead of assuming the value you set it to stays unchanged. @Rotem's updated code does this, and would therefore "work" with a time base of 1/60, since the PTS and packet duration will actually be based off the 1/15360 value time_base changes to.