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Thread: thresholding and pixel coordinate extraction in OpenCV

  1. #1
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    thresholding and pixel coordinate extraction in OpenCV

    Hi everyone.

    Im currently using OpenCV for a project at school, and we are not supposed to use the already builtin thresholding function, as that would be too easy i assume.

    Im trying to do two things; perform thresholding, and to fill an array with all x-coordinates, and another with the y-coordinates, for object pixels. it doesn't work however, and i do not understand why.

    here my code:
    Code:
    #include <iostream>
    #include <string>
    #include <opencv2/core/core.hpp>
    #include <opencv2/highgui/highgui.hpp>
    #include <opencv2/imgproc/imgproc.hpp>
    
    using namespace std;
    using namespace cv;
    
    Mat blobDetection(Mat binaryIMG){ //function to do objectextraction
    int binaryIMG_Height = binaryIMG.rows, binaryIMG_Width = binaryIMG.cols, pixValue, blobSize = 0, objectSize = 0;
    Mat objectImage;
    
    //find the amount of object pixels (white)
    for(int x = 0; x < binaryIMG_Width; x++){
        for(int y = 0; y < binaryIMG_Height; y++){
        pixValue = binaryIMG.at<unsigned char>(x,y);
        if(pixValue > 0){
        blobSize++;
        }
        }
    }
    
    // initialize arrays for holding coordinate values
    int blobXCoordinates[blobSize], blobYCoordinates[blobSize];
    
    //fill in the coordinate values
    for(int x = 0; x < binaryIMG_Width; x++){
        int i = 0;
        int xCoordinate, yCoordinate;
        for(int y = 0; y < binaryIMG_Height; y++){
        pixValue = binaryIMG.at<unsigned char>(x,y);
        if(pixValue == 255){
        blobXCoordinates[i] = x;
        blobYCoordinates[i] = y;
        i++;
        }
        }
    }
    
    for(int cycle = 0; cycle <= blobSize; cycle++){
    cout << blobXCoordinates[cycle] << " " << blobYCoordinates[cycle] << endl;
    }
    
    //should ideally return these values
    }
    
    Mat thresholdIMG(Mat inputImg, int lowerValue, int highValue){
    int width = inputImg.cols, height = inputImg.rows, pixValue;
    Mat outputImg;
    
    //thresholding function to set all pixl values to either 0 or 255 dependant on the threshold values set
    for(int x = 0; x <= width; x++){
        for(int y = 0; y <= height; y++){
        pixValue = inputImg.at<unsigned char>(x,y);
        if(pixValue >= lowerValue && pixValue <= highValue){
        outputImg.at<unsigned char>(x,y) = 255;
        }else{
        outputImg.at<unsigned char>(x,y) = 0;
        }
        }
    
    }
    return outputImg;
    
    }
    
    int main(int argc, char** argv)
    {
        VideoCapture capture;
    
        if (argc == 2)
            capture.open(argv[1]);
        else
            capture.open(0); //0 for built in webcam, 1 for external
    
        if (!capture.isOpened())
        {
            cout << "Cannot open video device or file!" << endl;
            return -1;
        }
    
        while(true){
    Mat frame, gray_frame, grayThresh;
    
    
    namedWindow("gray video", CV_WINDOW_AUTOSIZE);
    
    //capture footage
    capture >> frame;
    if(frame.empty())
    break;
    
    //convert to 8 bit grayscale
    cvtColor(frame, gray_frame, CV_BGR2GRAY);
    
    //call the thresholding function
    grayThresh = thresholdIMG(gray_frame, 170, 255);
    
    imshow("gray video", grayThresh);
    
    //hit q to end program
    if ((char)waitKey(1) == 'q')
    break;
    
    
    }
    
    }
    without extracting coordinates, but just trying to show the thresholded image, it give me; "segmentation fault (core dumped)"

    can anyone tell me what im misunderstanding here?

    the thresholding function sort of works, if the return part is in the for-x loop, in which case it can thresh hold something like 4/5 of the top line.

    i really am out of ideas, any help will be happily welcomed
    "...It takes a special kind of incompetence to not be able to make a browser that you coded for an OS that you coded not work across all of your OS versions." - Camfered Edge, on theverge, said in regards to microsofts criticism of google updates

  2. #2
    Join Date
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    Re: thresholding and pixel coordinate extraction in OpenCV

    Quote Originally Posted by rasmus91 View Post
    without extracting coordinates, but just trying to show the thresholded image, it give me; "segmentation fault (core dumped)"

    can anyone tell me what im misunderstanding here?

    the thresholding function sort of works, if the return part is in the for-x loop, in which case it can thresh hold something like 4/5 of the top line.
    For each of your 3 for loops that use <=, consider whether this is correct or whether < should be used instead.

  3. #3
    Join Date
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    Re: thresholding and pixel coordinate extraction in OpenCV

    Did this.

    had some help from a student whos been here longer than me, it works now. thanks for your time
    "...It takes a special kind of incompetence to not be able to make a browser that you coded for an OS that you coded not work across all of your OS versions." - Camfered Edge, on theverge, said in regards to microsofts criticism of google updates

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