why couldn't I access matrix element in openCV?

Question

I am trying to access openCV matrix element which comes from converting video frame into grayscale. My code is like this:

int C = 3; 
cv::Mat grayPrevious;
cv::cvtColor ( previousFrame, grayPrevious, CV_RGB2GRAY  );
double* grayPtr = grayPrevious.ptr <double> ();

double mean [600][800][3];

///... // set some values to mean here

for ( int i = 0; i < 600; i ++ )
{
    for ( int j = 0; j < 800; j ++ )
    {
        for ( int m = 0; m < C; m ++ )
        {
            diff [i][j][m]= abs(grayPtr[ 800* (i-1)+j] - mean[i][j][m]);
        }

    }
}

But it crashes in the for loop. Is this something related to datatypes? How can I change it?

Besides, my second question is, I tried to see the elements of grayPrevious matrix on the screen using "grayPrevious.at", but it shows values like " -5.235125e+30" for "double", or "-11242433" for "int". This is also confusing me, I was expecting to see values between 0 and 255.

Sassa · Accepted Answer · 2012-10-05T15:42:21.760

2

You should use uchar instead of int or double since you are dealing with grayscale images. (You can cast the uchar to print: cout<<(int)grayPrevious.at<uchar>(i,j);)

Is there any reason you are trying to access pixels directly "the hard way"? You could do grayprevious.at<uchar>(i,j) (assuming your image is 800x600) which is easier to debug and then use direct pixel access.

Edit: I think it should be:

uchar* grayPtr = grayPrevious.ptr <uchar> (); and then grayPtr[grayPrevious.cols*i+j];

You could also do grayPrevious.data[grayPrevious.cols*i+j]; (Still direct access without ptr)

edited Oct 05 '12 at 15:42

answered Oct 05 '12 at 15:21

Sassa

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I am using this way because "grayPrevious.at" method is doing range checking, and it is slow time consuming to me. – E_learner Oct 05 '12 at 15:24

score 1 · Answer 2 · answered Oct 05 '12 at 15:17

1

I seriously doubt this line maybe the problem

 for ( int m = 0; m < C; m ++ )

Check your value of C when your loop is running and crashes, maybe for mean, trying to access an element outside of the array bounds. Most probably it is more than 3. And why aren't you using pre increment operators. Mayn't be relevant in this example, but is good practice to be using preincrement operators rather than post increment operators.

answered Oct 05 '12 at 15:17

DumbCoder

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@ederman Is **grayPtr and grayPrevious** initialized at all ? Your absurd values maybe because it hasn't been initialized properly. – DumbCoder Oct 05 '12 at 15:22
@DumbCoder, why is preincrement better than post increment? – CallMeNorm Oct 05 '12 at 15:24
@CallMeNorm http://stackoverflow.com/questions/24901/is-there-a-performance-difference-between-i-and-i-in-c – DumbCoder Oct 05 '12 at 15:25
@DumbCoder: yes, my grayPrevious is initialized with zeros. – E_learner Oct 05 '12 at 15:28

why couldn't I access matrix element in openCV?

2 Answers2