How should I create an array of matrices with openCV - c++

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;
}

Answer 1

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);

Answer 2

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>