无法检索电影opencv的第一帧

我试着拍电影的第一帧。但使用视频捕获opencv似乎是不可能的。 为了确保我不能得到第一个视频，我做了一个所有帧编号。您可以检查保存的视频是否具有正确的编号。 无论我做什么，我都无法取回第一帧。 检查此项的代码：

看电影6帧 写入6帧，每帧打印索引 阅读新制作的电影 显示相邻的两个图像 如何获得电影的第一帧？ 这是一个bug还是在检索代码中的帧时出错了

请编译代码并运行。没有争论，就产生了一部小电影。标准的输出注释。或者指定电影名称作为参数

另请参见下一个问题，比较VideoWrite和videoRead OpenCv之后的两幅图像

   #include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <stdlib.h>
#include <stdio.h>
#include <iostream>

using namespace cv;
using namespace std;

    std::vector<Mat> ReadImages(string  VideoName)
    {
    // read movie from disk
        VideoCapture capture( VideoName);
        cout << VideoName;
        if (!capture.isOpened())
            cout  << "Could not open the output video for write: " << VideoName << endl;

        int NumFrames = capture.get(CV_CAP_PROP_FRAME_COUNT);
        std::vector<Mat> FramesRead(NumFrames);
        //  int NumFrames = capture.get(CV_CAP_PROP_FRAME_COUNT);

        cout << endl;
        for(int fr = 0; fr < NumFrames; ++fr){
            // read frame in 3 different ways:
            cout << "Position of pointer in Movie: " << capture.get(CAP_PROP_POS_FRAMES) << endl;

            switch (3) {
                case 1:
                    capture.grab();
                    capture.retrieve(FramesRead[fr]);// >> frame;       // get frame
                    break;
                case 2:
                    capture.set ( CV_CAP_PROP_POS_FRAMES , fr);       // set position
                    capture.read(FramesRead[fr]);   // get frame
                    break;
                default:
                    capture >> FramesRead[fr];       // get frame
                    break;
            }
            // show read frame
            imshow("Reader",FramesRead[fr]);
            cout << "Frame read should indicate a '" << fr << "'" << " and " << fr+1 << " Faces" << endl;
            cout << "if number in Image is not the same as written here: VideoReader does not retrieve first image! \n";
                    if(waitKey(-1)==1) break;
        }
        cout << "finished reading Movie\n";
        return FramesRead;
    }

    string WriteImages(std::vector<Mat> frames, string VideoName)
    {
        // write movie to disk and put number in every image according to frame number
        Size S = Size((int) frames[0].cols,    // get input size
                      (int) frames[0].rows);

        VideoWriter outputVideo;
        // use a uncompressed codec sheme
        string Codec = "raw ";//"tiff";//,  "mp4v", "MJPG"};
        // Open the output
        String VideoName2 = VideoName + "_" + Codec + "_" + "Write.avi";
        cout << VideoName2 << endl;
        outputVideo.open( VideoName2, VideoWriter::fourcc(Codec[0],Codec[1],Codec[2],Codec[3]),1, S, false);
        if (!outputVideo.isOpened())
        {
            cout  << "Could not open the output video for write: " << VideoName << endl;
            return "-1";
        }
        else
        {
         // write images and put frame number in images
            for(int fr=0;fr < frames.size() ; ++fr) //Show the image captured in the window and repeat
            {
                imshow("writer",frames[fr]);
                waitKey(1);
                putText(frames[fr], to_string(fr), Point(200, 100), FONT_HERSHEY_SIMPLEX, 1, Scalar(255,255,255));
                outputVideo << frames[fr].clone();
            }
            return VideoName2;
        }
    }

    std::vector<Mat> Face_Color_Movie(int NumFrames)
    {
        // create a movie with colored faces
        const int w = 450;

        std::vector<Mat> Frames(NumFrames);
        Mat tmp = Mat::zeros(w, w, CV_8UC3);
        for(int i=0;  i< NumFrames;i++)
            tmp.copyTo(Frames[i]);
        Scalar white = Scalar(0,0,255);

        for(int fr=0;fr < NumFrames; fr++)
        {

            for( int i = 0; i < fr+1; i++ )
            {
                int dx = (i%2)*250 - 30;
                int dy = (i/2)*150;
                if (i == 0)
                    white = Scalar(0,0,255);
                else if(i == 1)
                    white = Scalar(255,0,0);
                else
                    white = Scalar(0,255,0);


                const Scalar black = Scalar(0);

                if( i == 0 )
                {
                    for( int j = 0; j <= 10; j++ )
                    {
                        double angle = (j+5)*CV_PI/21;
                        line(Frames[fr], Point(cvRound(dx+100+j*10-80*cos(angle)),
                                               cvRound(dy+100-90*sin(angle))),
                             Point(cvRound(dx+100+j*10-30*cos(angle)),
                                   cvRound(dy+100-30*sin(angle))), white, 1, 8, 0);
                    }
                }
                //    imshow("Faces", Frames[fr]);waitKey(1);
                ellipse( Frames[fr], Point(dx+150, dy+100), Size(100,70), 0, 0, 360, white, -1, 8, 0 );
                //    imshow("Faces", Frames[fr]);waitKey(1);
                ellipse( Frames[fr], Point(dx+115, dy+70), Size(30,20), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+185, dy+70), Size(30,20), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+115, dy+70), Size(15,15), 0, 0, 360, white, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+185, dy+70), Size(15,15), 0, 0, 360, white, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+115, dy+70), Size(5,5), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+185, dy+70), Size(5,5), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+150, dy+100), Size(10,5), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+150, dy+150), Size(40,10), 0, 0, 360, black, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+27, dy+100), Size(20,35), 0, 0, 360, white, -1, 8, 0 );
                ellipse( Frames[fr], Point(dx+273, dy+100), Size(20,35), 0, 0, 360, white, -1, 8, 0 );
            }
            imshow("Faces", Frames[fr]);
            waitKey(1);
        }
        return Frames;
    }


    std::vector<Mat> Load_Movie(int NumFrames,string VideoName)
    {
        // reads NUMFRAMES from a specified movie from disk

        VideoCapture capture(VideoName);


        if (!capture.isOpened())
            cout  << "Could not open the output video for write: " << VideoName << endl;

        int ex = static_cast<int>(capture.get(CV_CAP_PROP_FOURCC));
        cout << "Codec: "<< ex << endl;

        std::vector<Mat>  Frames(NumFrames);

        for(int fr = 0; fr < NumFrames; ++fr)
        {
            capture >> Frames[fr];       // get frame
            imshow("test",Frames[fr]);
            waitKey(30);
        }
        return Frames;
    }

    int main( int argc, char** argv )
    {
        int NumFrames = 6;
        std::vector<Mat> Frames;

        // create a movie
        if (argc < 2)
            // A movie with colored faces is generated
            Frames = Face_Color_Movie(NumFrames);
        //   Frames = Face_Movie(NumFrames);
        else{
            // use an existing movie

            string VideoName = "Megamind_bugy.avi";  // use your own movie here
            VideoName = argv[1];
            Frames = Load_Movie(NumFrames,VideoName);
        }

        //--------------------------------
        // write the frames to disk
        string VideoName2 =  WriteImages(Frames, "Video_Faces.avi");
        // read the frames from disk (again)
        std::vector<Mat> FramesRead =  ReadImages( VideoName2);

        // show the frames and subtract to see if they are really the same?
        for (int fr=0;fr <NumFrames; ++fr)
        {
            //-------------------------------
            // first frame of read movie is missing!!!
            // Original image;   fr+1 because VideoReader skips the first frame!!!
            imshow("Frame before writing to disk",Frames[fr]);
            // Image read from disk (first frame missing)
            imshow("Frame after reading from Disk",FramesRead[fr]);
            cout << "-------------------------------\n";
            cout << "Shows Frame before writing to disk and frames after reading from disk\n";
            cout << "Image windows may lay on top of each other\n";

            moveWindow("Frame after reading from Disk",505,46);
            if(waitKey(-1)==1) break;
        }
    }

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试着放cout如果我这样做，我得到的第一帧和第二帧显示。下一个循环迭代：1，显示第3帧。顺便说一下：第一帧中应该打印0，第二帧1，依此类推。我试过不同的编解码器，没人有主意？这对我来说仍然是个大难题！请让我知道你的想法