c++ - 我必须改变什么才能从数组中读取像素值？

Question

我有以下代码。我从图像中读取了一个像素 block ，我想从每个 block (数组 16*16)中获取值。但是，我收到此错误:

OpenCV Error: Assertion failed (dims <= 2 && data && (unsigned)i0 < (unsigned)size.p[0] && (unsigned)(i1*DataType<_Tp>::channels) < (unsigned)(size.p[1]*channels()) && ((((sizeof(size_t)<<28)|0x8442211)

((DataType<_Tp>::depth) & ((1 << 3) - 1))*4) & 15) == elemSize1()) in unknown function, file C:\opencv231\build\include\opencv2/core/mat.hpp, line 537

我应该更改什么才能运行我的代码？

enum Color {White, Black}; 
Color checkBlock(Mat& img, int& i, int& j, double& T)
{
    unsigned int Sum=0;
    for(int k=0;k<16;k++)
        for(int l=0;l<16;l++)
            Sum += img.at<unsigned char>(i+k,j+l);
    double Average = Sum/256;
    std::cout << Average << std::endl;
    return (Average > T) ? (White) : (Black);
}

void main()
{
    Mat img = imread("Frame.jpg",0);
    namedWindow( "Display window", CV_NORMAL );// Create a window for display.
    if(!img.data)   
        std::cout << "error";
    // STEPS TO CONVERT TO BINARY IMAGE
    // LOAD THE IMAGE 
    cv::Mat imageMat = cv::imread("Frame.jpg", CV_LOAD_IMAGE_COLOR);

    cv::Mat grayscaleMat (imageMat.size(), CV_8U);

    //Convert BGR to Gray
    cv::cvtColor(imageMat, grayscaleMat, CV_BGR2GRAY );

    //Binary image
    cv::Mat binaryMat(grayscaleMat.size(), grayscaleMat.type());

    //Apply thresholding
    cv::threshold(grayscaleMat, binaryMat, 100, 255, cv::THRESH_BINARY);

    //Show the results
    // cv::namedWindow("Output",CV_NORMAL);
    //cv::imshow("Output", binaryMat);

    // cv::waitKey(0);

    double minVal, maxVal;
    minMaxLoc(img,&minVal,&maxVal,NULL,NULL);
    double Threshold = 0.5 * (minVal + maxVal);
    int i=4,j=4;
    Size s = img.size();
    Color old_c, new_c;
    // define the position wher i will begin to read the first row from the image 
    for (j=16*55;j<=s.height;j=j+16)
        for(i=0;i<=s.width;i=i+16)
        {
            Point x(i,j);
            Point y(i+16,j+16);
            //std::cout << x << " " << y << std::endl;
            rectangle(img, x, y, Scalar(255,0,0),3);
            Color c = checkBlock(img,i,j,Threshold);
        }

Answer 1

在这一行中，您使用 i 来索引一行:

        Sum += img.at<unsigned char>(i+k,j+l);

但是这里，i的来源，明明是一个col的index。

        for(i=0;i<=s.width;i=i+16)

所以第一行应该是:

Sum += img.at<unsigned char>(j+l, i+k);

要清楚一点，at 的参数是 (row, col)，Point 的参数是 (x,y)，这有点陷阱。

还有

for (j=16*55;j<=s.height;j=j+16)
    for(i=0;i<=s.width;i=i+16)
        ...
        Point y(i+16,j+16);

应该是

for (j = 16 *  55; j < s.height - 15 ; j = j + 16)
    for(i = 0; i < s.width - 15; i = i + 16)
        ...
        Point y(i + 15, j + 15);