1 OpenCV cv::Mat与FFmpeg AVFrame相互转换

最近在处理OpenCV采集摄像头图片然后使用ffmpeg编码为h264裸流，之后再将h264裸流转换为OpenCV cv::Mat进行显示的问题，在这个过程中，如何将OpenCV的cv::Mat转化为FFmpeg AVFrame在进行h264编码，以及如何将h264解码后的AVFrame转换为cv::Mat是两个核心的问题，下文将简单展示OpenCV cv::Mat与FFmpeg AVFrame相互转换的核心代码，主要使用了FFmpeg中主要用于视频像素格式转换和视频缩放的sws_getContext和sws_scale函数。

注意这里使用的ffmpeg的版本为4.1，使用更低版本注意API适配，使用更高版本注意丢弃API的更换。

1.1 OpenCV cv::Mat转换为FFmpeg AVFrame

void CvMatToAVFrame(const cv::Mat& input_mat, AVFrame* out_avframe)
{
    int image_width = input_mat.cols;
    int image_height = input_mat.rows;
    int cvLinesizes[1];
    cvLinesizes[0] = input_mat.step1();

    SwsContext* openCVBGRToAVFrameSwsContext = sws_getContext(
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_BGR24,
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_YUV420P,
        SWS_FAST_BILINEAR,
        nullptr, nullptr, nullptr
    );

    sws_scale(openCVBGRToAVFrameSwsContext,
        &input_mat.data,
        cvLinesizes,
        0,
        image_height,
        out_avframe->data,
        out_avframe->linesize);

    if (openCVBGRToAVFrameSwsContext != nullptr)
    {
        sws_freeContext(openCVBGRToAVFrameSwsContext);
        openCVBGRToAVFrameSwsContext = nullptr;
    }
}

1.2 FFmpeg AVFrame转换为OpenCV cv::Mat

cv::Mat AVFrameToCvMat(AVFrame* input_avframe)
{
    int image_width = input_avframe->width;
    int image_height = input_avframe->height;

    cv::Mat resMat(image_height, image_width, CV_8UC3);
    int cvLinesizes[1];
    cvLinesizes[0] = resMat.step1();

    SwsContext* avFrameToOpenCVBGRSwsContext = sws_getContext(
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_YUV420P,
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_BGR24,
        SWS_FAST_BILINEAR,
        nullptr, nullptr, nullptr
    );

    sws_scale(avFrameToOpenCVBGRSwsContext,
        input_avframe->data,
        input_avframe->linesize,
        0,
        image_height,
        &resMat.data,
        cvLinesizes);

    if (avFrameToOpenCVBGRSwsContext != nullptr)
    {
        sws_freeContext(avFrameToOpenCVBGRSwsContext);
        avFrameToOpenCVBGRSwsContext = nullptr;
    }

    return resMat;
}

1.3 使用示例

#include <iostream>

// ffmpeg
extern "C" {
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libavutil/avutil.h"
#include "libswscale/swscale.h"
#include <libavutil/imgutils.h>
}

// opencv
#include "opencv/cv.h"
#include "opencv2/opencv.hpp"

void CvMatToAVFrame(const cv::Mat& input_mat, AVFrame* out_avframe)
{
    int image_width = input_mat.cols;
    int image_height = input_mat.rows;
    int cvLinesizes[1];
    cvLinesizes[0] = input_mat.step1();

    SwsContext* openCVBGRToAVFrameSwsContext = sws_getContext(
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_BGR24,
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_YUV420P,
        SWS_FAST_BILINEAR,
        nullptr, nullptr, nullptr
    );

    sws_scale(openCVBGRToAVFrameSwsContext,
        &input_mat.data,
        cvLinesizes,
        0,
        image_height,
        out_avframe->data,
        out_avframe->linesize);

    if (openCVBGRToAVFrameSwsContext != nullptr)
    {
        sws_freeContext(openCVBGRToAVFrameSwsContext);
        openCVBGRToAVFrameSwsContext = nullptr;
    }
}

cv::Mat AVFrameToCvMat(AVFrame* input_avframe)
{
    int image_width = input_avframe->width;
    int image_height = input_avframe->height;

    cv::Mat resMat(image_height, image_width, CV_8UC3);
    int cvLinesizes[1];
    cvLinesizes[0] = resMat.step1();

    SwsContext* avFrameToOpenCVBGRSwsContext = sws_getContext(
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_YUV420P,
        image_width,
        image_height,
        AVPixelFormat::AV_PIX_FMT_BGR24,
        SWS_FAST_BILINEAR,
        nullptr, nullptr, nullptr
    );

    sws_scale(avFrameToOpenCVBGRSwsContext,
        input_avframe->data,
        input_avframe->linesize,
        0,
        image_height,
        &resMat.data,
        cvLinesizes);

    if (avFrameToOpenCVBGRSwsContext != nullptr)
    {
        sws_freeContext(avFrameToOpenCVBGRSwsContext);
        avFrameToOpenCVBGRSwsContext = nullptr;
    }

    return resMat;
}


int main()
{
    cv::Mat input_image = cv::imread("C:/Users/Administrator/Desktop/example.jpg");
    AVFrame* avFrame = av_frame_alloc();
    avFrame->format = AVPixelFormat::AV_PIX_FMT_YUV420P;
    avFrame->width = input_image.cols;
    avFrame->height = input_image.rows;

    // 为需要创建的YUV Frame分配内存
    if (av_frame_get_buffer(avFrame, 0) < 0)
    {
        av_frame_free(&avFrame);
        avFrame = nullptr;
        return -1;
    }


    cv::imshow("解码前", input_image);

    // OpenCV cv::Mat转换成AVFrame
    CvMatToAVFrame(input_image,avFrame);


    // 将AVFrame转换成OpenCV cv::Mat
    cv::Mat out_avFrameToMat = AVFrameToCvMat(avFrame);

    cv::imshow("解码后", out_avFrameToMat);


    cv::waitKey(0);
    cv::destroyAllWindows();

    // free memory
    if (avFrame != nullptr)
    {
        av_frame_free(&avFrame);
        avFrame = nullptr;
    }

    return 0;
}

从示例程序的结果上看，上述的转换代码是正确的