如何知道OpenCV中的图像是否为RGB？

Question

我用OpenCV库用c ++编写了一个程序.该程序记录来自网络摄像头的视频,然后将其分成帧.我想知道帧是否在RGB中,因为我想访问每个像素的RGB属性.捕获的编解码器是CV_FOURCC('M','J','P','G').如何在RGB色彩空间中获取帧？

int main() {

Mat image;
VideoCapture cap(0);
cap.set(CV_CAP_PROP_FPS, 10);

if ( !cap.isOpened() ) {
    cout << "ERROR : Cannot open the video file"<<endl;
    return -1;
}
namedWindow("MyWindow", CV_WINDOW_AUTOSIZE);

double dWidth   = cap.get(CV_CAP_PROP_FRAME_WIDTH);
double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT);
cout << "Frame size :" << dWidth << "x" << dHeight << endl;
Size frameSize(static_cast<int>(dWidth), static_cast<int>(dHeight));
VideoWriter oVideoWriter("E:/myVideo.avi", CV_FOURCC('M', 'J', 'P', 'G'), 10, frameSize, true);

if (!oVideoWriter.isOpened()) {
    cout << "ERROR : Failed to write the video"<<endl;
    return - 1;
}

while (1) {
    Mat image;
    bool bSuccess = cap.read(image);

    if (!bSuccess) {
        cout << "ERROR : Cannot read a frame from video file" << endl;
        break;
    }
    oVideoWriter.write(image);
    imshow("MyWindow", image);

    if (waitKey(10) == 27) {

        saveImages();


            cout << "ESC key is pressed by user" << endl;
            break


    }
}
return 0;
}




int saveImages() {  



CvCapture *capture = cvCaptureFromFile("E:/myVideo.avi");
if(!capture) 
{
    cout<<"!!! cvCaptureFromAVI failed (file not found?)"<<endl;
    return -1; 
}

int fps = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FPS);


IplImage* frame = NULL;
int frame_number = 0;
char key = 0;   

while (key != 'q') 
{

    frame = cvQueryFrame(capture);       
    if (!frame) 
    {
        cout<<"!!! cvQueryFrame failed: no frame"<<endl;
        break;
    }       

    char filename[100];
    strcpy(filename, "frame_");

    char frame_id[30];
    _itoa(frame_number, frame_id, 10);
    strcat(filename, frame_id);
    strcat(filename, ".jpg");

    printf("* Saving: %s\n", filename);

    if (!cvSaveImage(filename, frame))
    {
        cout<<"!!! cvSaveImage failed"<<endl;
        break;
    }

    frame_number++;

    key = cvWaitKey(1000 / fps);
}


cvReleaseCapture(&capture);

return 0;
}

Answer 1

当OpenCV从磁盘,摄像机或视频文件加载彩色图像(即3通道)时,图像数据将以BGR格式存储.这是一个简单的测试,你可以做:

/* Code using the C++ API */

cv::VideoCapture cap(0);
if (!cap.isOpened()) {
    std::cout << "!!! Failed to open webcam" << std::endl;
    return -1;
}

if (!cap.read(frame)) {
    std::cout << "!!! Failed to read a frame from the camera" << std::endl;
    return -1;
}

bool is_colored = false;
if (frame.channels() == 3) {
    is_colored = true;
}

// Do something with is_colored
// ...

除非你有一个奇怪的相机,框架将始终着色(因此,存储为BGR).

当cv::imwrite()调用(C++ API)或cvSaveImage()(C API)时,OpenCV会采用适当的魔术技巧来确保数据以与请求的输出格式(JPG,PNG,AVI等)兼容的方式保存,并且在此过程中它会自动转换如果需要,数据到RGB.

然而,如果由于某种原因你需要将图像转换为RGB,你可以调用:

cv::Mat img_rgb;
cv::cvtColor(frame, img_rgb, CV_BGR2RGB);  

请注意,OpenCV有一个C API和一个C++ API,它们不应混合使用:

• 如果您使用,IplImage那么坚持使用其余的C API.

• 如果您决定使用cv::Mat,请继续使用C++ API.

有不同的方法来访问a的像素cv::Mat,这里有一个:

unsigned char* pixels = (unsigned char*)(frame.data);
for (int i = 0; i < frame.rows; i++)
{
    for (int j = 0; j < frame.cols; j++)
    {
        char b = pixels[frame.step * j + i] ;
        char g = pixels[frame.step * j + i + 1];
        char r = pixels[frame.step * j + i + 2];
    }
}