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  • FFmpeg what exactly is the filtergraph pipeline like during transcoding ?

    8 septembre 2017, par Jeff Gong

    I have been studying the source code for FFmpeg to attempt to understand its threading model and how it processes inputs. For example, when I run a command like :

    ffmpeg -i video.mp4 -s hd720 -c:v libx264 --preset medium -c:a aac -profile:v main -r 60 -f null /dev/null

    The input itself is irrelevant, but I am trying to understand how the transcoding pipeline works. In the source code, I see that the main steps occur in the functions transcode and transcode_step.

    It seems like for a single input, a single frame is read in, decoded, encoded, and written out. The process is obviously very complex but what I am really not understanding is what FFmpeg is doing when it attempts to build out a filtergraph. For example, in transcode_step of ffmpeg.c, there is the following code that happens right after an output stream has been selected :

    if (ost->filter && !ost->filter->graph->graph) {
       if (ifilter_has_all_input_formats(ost->filter->graph)) {
           ret = configure_filtergraph(ost->filter->graph);
           if (ret < 0) {
               av_log(NULL, AV_LOG_ERROR, "Error reinitializing filters!\n");
               return ret;
           }
       }
    }

    Does this only apply if I specify a specific series of filtering options to FFmpeg, like the one in this link ? For the sample command I input above, is this code still executed ?

    One last other question I had was for the case where I run an FFmpeg instance with a single input but multiple outputs (perhaps different variants for transcoding). In this scenario, does a single phase of transcode_step take in an input frame and send that frame through decoding and encoding for only a single one of the outputs ? Or does it take a frame at a time and process this frame for each of the outputs we have specified ?

  • avformat_write_header() function call crashed when I try to save several RGB data to a output.mp4 file

    1er mai 2023, par ollydbg23

    I try to save several image data(in memory, with BGR format) to a output.mp4 file, here is the C++ code to call the ffmpeg library, the code builds correctly, but will crash when I call the ret = avformat_write_header(outFormatCtx, nullptr);, do you know how to solve this crash issue ?

    


    Thanks.

    


    #include <iostream>&#xA;#include <vector>&#xA;#include <cstring>&#xA;#include <fstream>&#xA;#include <sstream>&#xA;#include <stdexcept>&#xA;#include <opencv2></opencv2>opencv.hpp>&#xA;extern "C" {&#xA;#include <libavutil></libavutil>imgutils.h>&#xA;#include <libavcodec></libavcodec>avcodec.h>&#xA;#include <libavformat></libavformat>avformat.h>&#xA;#include <libavutil></libavutil>opt.h>&#xA;}&#xA;&#xA;using namespace std;&#xA;using namespace cv;&#xA;&#xA;int main()&#xA;{&#xA;    // Set up input frames as BGR byte arrays&#xA;    vector<mat> frames;&#xA;&#xA;    int width = 640;&#xA;    int height = 480;&#xA;    int num_frames = 100;&#xA;    Scalar black(0, 0, 0);&#xA;    Scalar white(255, 255, 255);&#xA;    int font = FONT_HERSHEY_SIMPLEX;&#xA;    double font_scale = 1.0;&#xA;    int thickness = 2;&#xA;&#xA;    for (int i = 0; i &lt; num_frames; i&#x2B;&#x2B;) {&#xA;        Mat frame = Mat::zeros(height, width, CV_8UC3);&#xA;        putText(frame, std::to_string(i), Point(width / 2 - 50, height / 2), font, font_scale, white, thickness);&#xA;        frames.push_back(frame);&#xA;    }&#xA;&#xA;&#xA;    // Populate frames with BGR byte arrays&#xA;&#xA;    // Initialize FFmpeg&#xA;    //av_register_all();&#xA;&#xA;    // Set up output file&#xA;    AVFormatContext* outFormatCtx = nullptr;&#xA;    //AVCodec* outCodec = nullptr;&#xA;    AVCodecContext* outCodecCtx = nullptr;&#xA;    //AVStream* outStream = nullptr;&#xA;    AVPacket outPacket;&#xA;&#xA;    const char* outFile = "output.mp4";&#xA;    int outWidth = frames[0].cols;&#xA;    int outHeight = frames[0].rows;&#xA;    int fps = 30;&#xA;&#xA;    // Open output file&#xA;    avformat_alloc_output_context2(&amp;outFormatCtx, nullptr, nullptr, outFile);&#xA;    if (!outFormatCtx) {&#xA;        cerr &lt;&lt; "Error: Could not allocate output format context" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    // Set up output codec&#xA;    const AVCodec* outCodec = avcodec_find_encoder(AV_CODEC_ID_H264);&#xA;    if (!outCodec) {&#xA;        cerr &lt;&lt; "Error: Could not find H.264 codec" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    outCodecCtx = avcodec_alloc_context3(outCodec);&#xA;    if (!outCodecCtx) {&#xA;        cerr &lt;&lt; "Error: Could not allocate output codec context" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;    outCodecCtx->codec_id = AV_CODEC_ID_H264;&#xA;    outCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;&#xA;    outCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;&#xA;    outCodecCtx->width = outWidth;&#xA;    outCodecCtx->height = outHeight;&#xA;    outCodecCtx->time_base = { 1, fps };&#xA;&#xA;    // Open output codec&#xA;    if (avcodec_open2(outCodecCtx, outCodec, nullptr) &lt; 0) {&#xA;        cerr &lt;&lt; "Error: Could not open output codec" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    // Create output stream&#xA;    AVStream* outStream = avformat_new_stream(outFormatCtx, outCodec);&#xA;    if (!outStream) {&#xA;        cerr &lt;&lt; "Error: Could not allocate output stream" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    // Configure output stream parameters (e.g., time base, codec parameters, etc.)&#xA;    // ...&#xA;&#xA;    // Connect output stream to format context&#xA;    outStream->codecpar->codec_id = outCodecCtx->codec_id;&#xA;    outStream->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;&#xA;    outStream->codecpar->width = outCodecCtx->width;&#xA;    outStream->codecpar->height = outCodecCtx->height;&#xA;    outStream->codecpar->format = outCodecCtx->pix_fmt;&#xA;    outStream->time_base = outCodecCtx->time_base;&#xA;&#xA;    int ret = avcodec_parameters_from_context(outStream->codecpar, outCodecCtx);&#xA;    if (ret &lt; 0) {&#xA;        cerr &lt;&lt; "Error: Could not copy codec parameters to output stream" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    outStream->avg_frame_rate = outCodecCtx->framerate;&#xA;    outStream->id = outFormatCtx->nb_streams&#x2B;&#x2B;;&#xA;&#xA;&#xA;    ret = avformat_write_header(outFormatCtx, nullptr);&#xA;    if (ret &lt; 0) {&#xA;        cerr &lt;&lt; "Error: Could not write output header" &lt;&lt; endl;&#xA;        return -1;&#xA;    }&#xA;&#xA;    // Convert frames to YUV format and write to output file&#xA;    for (const auto&amp; frame : frames) {&#xA;        AVFrame* yuvFrame = av_frame_alloc();&#xA;        if (!yuvFrame) {&#xA;            cerr &lt;&lt; "Error: Could not allocate YUV frame" &lt;&lt; endl;&#xA;            return -1;&#xA;        }&#xA;        av_image_alloc(yuvFrame->data, yuvFrame->linesize, outWidth, outHeight, AV_PIX_FMT_YUV420P, 32);&#xA;&#xA;        // Convert BGR frame to YUV format&#xA;        Mat yuvMat;&#xA;        cvtColor(frame, yuvMat, COLOR_BGR2YUV_I420);&#xA;        memcpy(yuvFrame->data[0], yuvMat.data, outWidth * outHeight);&#xA;        memcpy(yuvFrame->data[1], yuvMat.data &#x2B; outWidth * outHeight, outWidth * outHeight / 4);&#xA;        memcpy(yuvFrame->data[2], yuvMat.data &#x2B; outWidth * outHeight * 5 / 4, outWidth * outHeight / 4);&#xA;&#xA;        // Set up output packet&#xA;        av_init_packet(&amp;outPacket);&#xA;        outPacket.data = nullptr;&#xA;        outPacket.size = 0;&#xA;&#xA;        // Encode frame and write to output file&#xA;        int ret = avcodec_send_frame(outCodecCtx, yuvFrame);&#xA;        if (ret &lt; 0) {&#xA;            cerr &lt;&lt; "Error: Could not send frame to output codec" &lt;&lt; endl;&#xA;            return -1;&#xA;        }&#xA;        while (ret >= 0) {&#xA;            ret = avcodec_receive_packet(outCodecCtx, &amp;outPacket);&#xA;            if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {&#xA;                break;&#xA;            } else if (ret &lt; 0) {&#xA;                cerr &lt;&lt; "Error: Could not receive packet from output codec" &lt;&lt; endl;&#xA;                return -1;&#xA;            }&#xA;&#xA;            av_packet_rescale_ts(&amp;outPacket, outCodecCtx->time_base, outStream->time_base);&#xA;            outPacket.stream_index = outStream->index;&#xA;&#xA;            ret = av_interleaved_write_frame(outFormatCtx, &amp;outPacket);&#xA;            if (ret &lt; 0) {&#xA;                cerr &lt;&lt; "Error: Could not write packet to output file" &lt;&lt; endl;&#xA;                return -1;&#xA;            }&#xA;        }&#xA;&#xA;        av_frame_free(&amp;yuvFrame);&#xA;    }&#xA;&#xA;    // Write output trailer&#xA;    av_write_trailer(outFormatCtx);&#xA;&#xA;    // Clean up&#xA;    avcodec_close(outCodecCtx);&#xA;    avcodec_free_context(&amp;outCodecCtx);&#xA;    avformat_free_context(outFormatCtx);&#xA;&#xA;    return 0;&#xA;}&#xA;</mat></stdexcept></sstream></fstream></cstring></vector></iostream>

    &#xA;

    In-fact, I try to solve my original question here : What is the best way to save an image sequence with different time intervals in a simgle file in C++, but in that discussion, it is difficult to write a c++ code for ffmpeg library.

    &#xA;