How should I create an array of matrices with openCV

Question

I want to create an array of matrices for segment the image.

Here is what I do for creating array, and it shows an error of "EXC_I386_GPFLT".

How can I fix it or What should I do to achieve my purpose?

#include "opencv2/core/core.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "iostream"

using namespace cv;
using namespace std;

int main( )
{

    Mat img;
    img = imread("/Users/koike1979/Documents/0306/trucka.bmp", CV_LOAD_IMAGE_COLOR);
    namedWindow( "Original image", CV_WINDOW_AUTOSIZE );
    imshow( "Original image", img );

    Mat H[2]= {Mat(20,20,CV_8UC1),Mat(20,20,CV_8UC1)};

    for (int i=0; i<200; i++)
        for (int j=0; j<200; j++)
    {

    Vec3b intensity2 = img.at<Vec3b>(i ,j);
    int blue = intensity2.val[0];
    int green = intensity2.val[1];
    int red = intensity2.val[2];
        H[0].at<uchar>(i,j)=(blue+green+red)/3;
            }
    namedWindow( "Modify pixel", CV_WINDOW_AUTOSIZE );
    imshow( "Modify pixel", H[0] );
    waitKey(0);

    return 0;
}

Are you aware that you're are using indices out of bound for your matrices? H[0] is 20x20, while you're accessing values up to 200x200. Aside from that, and the fact that you can code this in a better way, this code works for me — Miki, Mar 09 '16 at 13:31

score 1 · Answer 1 · edited May 23 '17 at 11:45

https://stackoverflow.com/a/19652248/4518710

What's your main purpose of creating an array of matrices?
Your code seems to have some issues,

First of all, you need to access coordinate of Mat NOT by hardcoding.
Use Mat::rows and Mat::cols instead.
original:

for (int i=0; i<200; i++)
    for (int j=0; j<200; j++)
    {
    }

modified:

int rows = img.rows;
if(rows > 200)
    rows = 200;
int cols = img.cols;
if(cols > 200)
    cols = 200;

for (int i=0; i<rows; i++)
    for (int j=0; j<cols; j++)
    {
    }

Second, use vector<> feature of C++ instead of array itself.
original:

Mat H[2]= {Mat(20,20,CV_8UC1),Mat(20,20,CV_8UC1)};

modified:

std::vector<Mat> H;
H.push_back(Mat(20, 20, CV_8UC1));
H.push_back(Mat(20, 20, CV_8UC1));

Third, directly casting int to uchar can cause potential side effects.
Use static_cast instead.
original:

H[0].at<uchar>(i,j)=(blue+green+red)/3;

modified:

H[0].at<uchar>(i,j) = static_cast<uchar>((blue+green+red)/3);

Zoltán · Answer 2 · 2016-03-09T11:51:25.323

You might use an std::vector

Mat img;
img = imread("./res/mydhm.png", CV_LOAD_IMAGE_COLOR);
namedWindow("Original image", CV_WINDOW_AUTOSIZE);
imshow("Original image", img);

//Mat H[2] = { Mat(img.cols, img.row, img.type), Mat(img.cols, img.row, img.type) };
std::vector<Mat> H; // use vector
H.push_back(Mat(img.cols, img.rows, img.type()));
H.push_back(Mat(img.cols, img.rows, img.type()));

for (int i = 0; i<img.cols; i++)
{
    for (int j = 0; j<img.rows; j++)
    {
        Vec3b intensity2 = img.at<Vec3b>(Point(i, j));
        int blue = intensity2.val[0];
        int green = intensity2.val[1];
        int red = intensity2.val[2];
        H[0].at<Vec3b>(Point(i, j)) = (uchar)(blue + green + red) / 3;
    }
}
namedWindow("Modify pixel", CV_WINDOW_AUTOSIZE);
imshow("Modify pixel", H[0]);
waitKey(0);

don't forget to #include<vector>

H[0].at(Point(i, j)) = (uchar)(blue + green + red) / 3; this will generate problem — Humam Helfawi, Mar 09 '16 at 11:52
row and col seems to be switched. If you use Point() with such i,j, you need to access pixel by H[0].at(Point(j,i)) or just by H[0].at(i,j) — Suhyeon Lee, Mar 09 '16 at 11:56

How should I create an array of matrices with openCV

2 Answers2