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c++ matrix opencv save depth

OpenCV imwrite saving complete black jpeg

发布于 2020-04-11 11:49:57

I have done some pre processing for dft , and i am trying to save this image by imwrite.

My cropped image has this information

output.type()           5   

output.channels()       1

output.depth()          5

But everytime when i save it gives black output. I have checked old existing threads of stackoverflow but all seems not working for me. e.g. OpenCV2.3 imwrite saves black image

I have tried many color conversions and depth conversion as well but i dont know why it does not work.

std::vector<int> qualityType;

Mat out1,out2;

cv::cvtColor(output, out1, CV_GRAY2BGR);
//out1.convertTo(out2,CV_8U,1./256); // i tried this too 
cv::imwrite("dft1.jpg",out1,qualityType); // tried even using quality type

imshow display this image fine only problem comes when i save it.

please help

[EDIT] Maybe my dft class that i made has problem because i whenever i use dft function then the output can only work with inshow but for save it does not work.



Mat CDftRidgeAnalyses::GetRidgeAnalyses(Mat inpGray)
    Mat img = inpGray;
    int WidthPadded=0,HeightPadded=0;
    int M = getOptimalDFTSize( img.rows );
    //Create a Gaussian Highpass filter 5% the height of the Fourier transform
    double db  = 0.05 * HeightPadded;

    Mat fft = ForierTransform(img.clone(),HeightPadded,WidthPadded);

    Mat ghpf = CreateGaussianHighPassFilter(Size(WidthPadded, HeightPadded), db);
    Mat res;


    Mat mag  = GetDftToImage(res,img);

    int cx = mag.cols/2;
    int cy = mag.rows/2;
    cv::Mat croped = mag(cv::Rect(0,0,cx, cy));

    cv::threshold(mag, mag, 0.019, 1, cv::THRESH_BINARY);

    Mat bgr;

    //imshow("croped", croped);
    return croped;

Mat CDftRidgeAnalyses::ForierTransform(Mat inpGray,int M,int N)
    Mat img = inpGray;
    int i = img.channels();

    Mat padded;
    Mat img2;

    copyMakeBorder(img2, padded, 0, M - img2.rows, 0, N - img2.cols, BORDER_CONSTANT, Scalar::all(0));

    Mat element1 = Mat_<float>(padded);
    Mat element2 = Mat::zeros(padded.size(), CV_32F);
    Mat planes[] = {element1, element2};
    Mat complexImg;

    merge(planes, 2, complexImg);

    dft(complexImg, complexImg ,0, img.rows);


    return complexImg;

double CDftRidgeAnalyses::pixelDistance(double u, double v)
    return cv::sqrt(u*u + v*v);

double CDftRidgeAnalyses::gaussianCoeff(double u, double v, double d0)
    double d = pixelDistance(u, v);
    return 1.0 - cv::exp((-d*d) / (2*d0*d0));

cv::Mat CDftRidgeAnalyses::CreateGaussianHighPassFilter(cv::Size size, double cutoffInPixels)
    Mat ghpf(size, CV_32F);

    cv::Point center2((size.width*0.80), size.width/2);
    //cv::Point center2(0,0);

    for(int u = 0; u < ghpf.rows; u++)
        for(int v = 0; v < ghpf.cols; v++)
            ghpf.at<float>(u, v) = gaussianCoeff(u - center2.x, v - center2.y, cutoffInPixels);

    Mat bmp;

    int channels = ghpf.channels();
    int type = ghpf.type();
    int depth = ghpf.depth();

    int cx = ghpf.cols/2;
    int cy = ghpf.rows/2;
    Mat tmp;
    int iExactright =  (size.width*0.59);
    int iExactbottom = (size.height*0.86);
    //full   Mat q0(ghpf, Rect(69,10,400,290));

//  Mat whiteq(ghpf, Rect(0,390,270,330));

    int iMainleft=0, iMainright=0;
    int iMainBottom=0,iMainTop=0;

    Mat Quad;
    Mat ql(ghpf, Rect(190,0,270,330));

    /** Make the rectangle on middle default filter with respect to top right angle**/
    iMainBottom = (size.height*0.155);
    Quad = ghpf(Rect(iMainleft,iMainTop,iMainright+6,iMainBottom));
    Mat qTopRight(ghpf, Rect(iExactright,0, iMainright+6, iMainBottom));

    /** Make the rectangle on middle default filter with respect to top left angle**/
    Quad = ghpf(Rect(300,iMainTop,300,iMainBottom));
    Mat qTopLeft(ghpf, Rect(0,0, 300, iMainBottom));

    /** Make the rectangle on middle default filter with respect to bottom left angle**/
    iMainTop = iMainTop-iMainBottom;
    iExactbottom = size.height - iMainBottom;
    Quad = ghpf(Rect(300,iMainTop,300,iMainBottom));
    Mat qBottomLeft(ghpf, Rect(0,iExactbottom, 300, iMainBottom));

    /** Make the rectangle on middle default filter with respect to bottom right angle**/

    Quad = ghpf(Rect(iMainleft,iMainTop,iMainright+6,iMainBottom));
    Mat qBottomRight(ghpf, Rect(iExactright,iExactbottom, iMainright+6, iMainBottom));

    // remove middle rectangle [ circle ] 
    Quad = ghpf(Rect(0,iMainTop+iMainTop,size.width,iMainBottom+iMainBottom-130));
    Mat qMiddle(ghpf,Rect(0,iMainTop+150,size.width,iMainBottom+iMainBottom-130));
    qMiddle =ghpf(Rect(0,iMainTop-10,size.width,iMainBottom+iMainBottom-130));

    normalize(ghpf, ghpf, 0, 1, CV_MINMAX);

    /*Mat x;
    imshow("fftXhighpass2", x);*/
    Filter = ghpf;
    Mat padded;

    copyMakeBorder(ghpf, padded, 0, size.height - ghpf.rows, 0, size.width - ghpf.cols, BORDER_CONSTANT, Scalar::all(0));

    Mat planes[] = {Mat_<float>(padded), Mat::zeros(padded.size(), CV_32F)};
    Mat complexImg;

    merge(planes, 2, complexImg);

    return complexImg;

Mat CDftRidgeAnalyses::GetDftToImage(Mat res,Mat orgImage)

    Mat padded;
    copyMakeBorder(orgImage, padded, 0,orgImage.rows, 0, orgImage.cols, BORDER_CONSTANT, Scalar::all(0));
    Mat planes[] = {Mat_<float>(padded), Mat::zeros(padded.size(), CV_32F)};
    split(res, planes);
    magnitude(planes[0], planes[1], planes[0]);
    Mat mag = planes[0];
    mag += Scalar::all(1);
  //  log(mag, mag);

    // crop the spectrum, if it has an odd number of rows or columns
    mag = mag(Rect(0, 0, mag.cols & -2, mag.rows & -2));

    normalize(mag, mag, 1, 0, CV_MINMAX);

    return mag;

The output that i am trying to save is from

Mat org = imread("4.png",CV_LOAD_IMAGE_GRAYSCALE);
Mat re;
Mat xyz = CDftRidgeAnalyses::GetRidgeAnalyses(re);

here the matrix xyz has these values

output.type()           5   

output.channels()       1

output.depth()          5

I hope you guys now can help me more better... maybe after converion from complex filter i am losing some points????

Herbert 2012-05-26 18:20

This 'feels' like a problem with floating point numbers and integers. When your image has floating point values, the imshow() of opencv expects these values to be between 0 and 1:


I'm not quite sure what imwrite() does with floating point images, as I could not read it here:


Anyhow, imwrite might expect integer values between 0 and 255, and might simply cast floats to integers. In this case, almost everything is casted to 0 (i.g. 0.8 is casted to 0), hence your black images.

Try to convert your images to CV_U8CX. Alternatively, here is something I use to debug such opencv problems:

void printType(Mat &mat) {
         if(mat.depth() == CV_8U)  printf("unsigned char(%d)", mat.channels());
    else if(mat.depth() == CV_8S)  printf("signed char(%d)", mat.channels());
    else if(mat.depth() == CV_16U) printf("unsigned short(%d)", mat.channels());
    else if(mat.depth() == CV_16S) printf("signed short(%d)", mat.channels());
    else if(mat.depth() == CV_32S) printf("signed int(%d)", mat.channels());
    else if(mat.depth() == CV_32F) printf("float(%d)", mat.channels());
    else if(mat.depth() == CV_64F) printf("double(%d)", mat.channels());
    else                           printf("unknown(%d)", mat.channels());

void printInfo(const char *prefix, Mat &mat) {
    printf("%s: ", prefix);
    printf("dim(%d, %d)", mat.rows, mat.cols);

void printInfo(Mat &mat) {
    printf("dim(%d, %d)", mat.rows, mat.cols);

This way you can find out what your cv::Mat has in it's data-field.

PS: I did not debug your code throughly, so stay open to other probleem causes.