Vec3b usage issue and obtaining RGB values at a point in a Mat object

Question

for (int row = 0; row < curf->rows; row++)
{
    for (int col = 0; col < curf->cols; col++)
    {
        Vec3b pixcur = curf->at<cv::Vec3b>(row, col);
        cout<<"Pixel values at current point"<<endl<<pixcur.val[0]<<endl<<pixcur.val[1]<<endl<<pixcur.val[2]<<endl;    ###DEBUG STMT ###
        Vec3b pixlast = curl->at<cv::Vec3b>(row, col);
        if (abs(pixcur.val[0]-pixlast.val[0])>checkval ||
            abs(pixcur.val[1]-pixlast.val[1])>checkval ||
            abs(pixcur.val[2]-pixlast.val[2])>checkval)
        {
            numPixelsDiff++;
        }
    }
}

Here curf and curl are pointers to two Mat objects. I know they point to actual Mats because the value of curf->rows and curf->cols output actual integers.

But the output of the line I have tagged with ### DEBUG STMT ### is always junk. checkval is a variable which is set prior to entering the loops. The values in the vec3b vector is not correct.

I tried this too

for (int row = 0; row < curf->rows; row++)
{
    for (int col = 0; col < curf->cols; col++)
    {
         Vec3b pixcur = curf->at<cv::Vec3b>(row, col);
         cout<<"Pixel values at current point"<<endl<<pixcur[0]<<endl<<pixcur[1]<<endl<<pixcur[2]<<endl;    ###DEBUG STMT ###
         Vec3b pixlast = curl->at<cv::Vec3b>(row, col);
         if (abs(pixcur[0] - pixlast[0]) > checkval ||
             abs(pixcur[1] - pixlast[1]) > checkval ||
             abs(pixcur[2] - pixlast[2]) > checkval)
         {
             numPixelsDiff++;
         }
     }
}

It again only outputs junk symbols.

Am I doing something wrong here. Should I check for something else? I'm a complete and total newbie to opencv so I'll appreciate any help.

berak berak · Accepted Answer · 2014-09-09T07:59:27

avoid pointers to cv::Mat, as sooner or later you will thrash the internal refcount this way. use references, or dare to copy it (it's a shallow copy anyway).
when printing bytes, use: cout << int(pixcur[0]) . you need the cast, or cout will see a uchar, and print ascii. (or just do cout << pixcur;)
avoid per-pixel loops. in almost any case, there's a builtin function, that is doing the job better, faster, and with less chance for errors. e.g, there's absdiff() and countNonZero().

Vec3b usage issue and obtaining RGB values at a point in a Mat object

1 Answers