Writing x264 from OpenCV 3 with FFmpeg on Linux

Using the FFmpeg library to encode video streams from OpenCV 3 to x264 (H.264 encoder) on Linux involves multiple steps. The process can be broadly divided into the following stages:

1. Environment Setup: Ensure that OpenCV and FFmpeg libraries, including the x264 encoder, are installed on the Linux system.
2. Writing Code: Use C++ with OpenCV API to capture video frames, then use FFmpeg's libav* series libraries to encode the frames into x264 format.
3. Compilation and Execution: Compile the C++ program and run it on Linux, ensuring the video is correctly encoded and stored.

Detailed Steps:

1. Environment Setup:

• First, install OpenCV and FFmpeg on the Linux system. Use package managers such as apt (Debian/Ubuntu) or yum (Fedora) for installation.

  bash
  sudo apt-get install libopencv-dev
  sudo apt-get install ffmpeg libavcodec-dev libavformat-dev libswscale-dev

2. Writing Code:

• Create a C++ program that uses OpenCV to capture video frames and then uses FFmpeg's API to encode these frames into x264. The following is a simplified code example:

cpp
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>
extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
}

int main() {
    cv::VideoCapture cap(0); // Open default camera
    if (!cap.isOpened()) {
        return -1;
    }

    AVCodec *codec;
    AVCodecContext *c= NULL;
    int i, ret, x, y, got_output;
    FILE *f;
    AVFrame *frame;
    AVPacket pkt;
    uint8_t endcode[] = { 0, 0, 1, 0xb7 };

    avcodec_register_all();

    codec = avcodec_find_encoder(AV_CODEC_ID_H264);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    c->bit_rate = 400000;
    c->width = 640;
    c->height = 480;
    c->time_base = (AVRational){1, 25};
    c->gop_size = 10;
    c->max_b_frames = 1;
    c->pix_fmt = AV_PIX_FMT_YUV420P;

    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    frame->format = c->pix_fmt;
    frame->width  = c->width;
    frame->height = c->height;

    ret = av_image_alloc(frame->data, frame->linesize, c->width, c->height, c->pix_fmt, 32);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate raw picture buffer\n");
        exit(1);
    }

    // OpenCV and FFmpeg integration section
    cv::Mat image;
    while (cap.read(image)) {
        // Convert OpenCV's Mat to FFmpeg's AVFrame
        // Color space conversion is required here

        av_init_packet(&pkt);
        pkt.data = NULL;    // packet data will be allocated by the encoder
        pkt.size = 0;

        fflush(stdout);
        ret = avcodec_encode_video2(c, &pkt, frame, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }
        if (got_output) {
            fwrite(pkt.data, 1, pkt.size, f);
            av_packet_unref(&pkt);
        }
    }

    // Add encoding termination marker
    avcodec_encode_video2(c, &pkt, NULL, &got_output);
    if (got_output) {
        fwrite(pkt.data, 1, pkt.size, f);
        av_packet_unref(&pkt);
    }
    fclose(f);

    avcodec_close(c);
    av_free(c);
    av_freep(&frame->data[0]);
    av_frame_free(&frame);
    return 0;
}

3. Compilation and Execution:

• Compile the above code using g++, linking OpenCV and FFmpeg libraries.

  bash
  g++ -o video_capture video_capture.cpp `pkg-config --cflags --libs opencv4` -lavcodec -lavformat -lavutil -lswscale

• Run the program:

  bash
  ./video_capture

Notes:

• The code example omits details of error handling and resource release; in actual applications, these should be added to ensure program robustness.
• Color space conversion is necessary because OpenCV typically uses BGR format, while x264 requires YUV format.