OpenCV Access RGB values in a MAT object

Question

I am attempting to use OpenCV to grab frames from a webcam and convert it to aHSV(Hue,Saturation,Value) Mat object and threshold it.

When I print threshold image pixel values its giving me [0,0,0] for all the pixels, even black pixel values are also [0,0,0]. And I need to do some calculations if the selected pixel is black; how can I access the pixel values?.

    imgOriginal=frame from camera 


    Mat imgHSV;

    cvtColor(imgOriginal, imgHSV, COLOR_BGR2HSV); //Convert the captured frame from BGR to HSV

    Mat imgThresholded;
    inRange(imgHSV, Scalar(iLowH, iLowS, iLowV), Scalar(iHighH, iHighS, iHighV), imgThresholded); //Threshold the image

    //morphological opening (remove small objects from the foreground)
    erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)) );
    dilate( imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)) ); 

    //morphological closing (fill small holes in the foreground)
    dilate( imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)) ); 
    erode(imgThresholded, imgThresholded, getStructuringElement(MORPH_ELLIPSE, Size(5, 5)) );


    //************************************



    std::vector<cv::Vec3b> pixels(imgThresholded.rows * imgThresholded.cols);
    cv::Mat m(imgThresholded.rows, imgThresholded.cols, CV_8UC3, &pixels[0]);
    imgThresholded.copyTo(m);


    for(int i =0;i<1000;i++)
    cout<<pixels[0];
    if(pixels[0][0]==black)
    // do some calculations!

Bull Bull · Accepted Answer · 2014-09-11T00:20:19

for(int i =0;i<1000;i++)
    cout<<pixels[0];

will just print the first pixel 1000 times. I think you meant:

Vec3b black(0, 0, 0);

for(int i =0;i<1000;i++)
{
    cout << pixels[i];
    if pixels[i] == black)
    {
       /* ... */
    }
}

But why bother copying the pixels to a std::vector? You could do this

Vec3b black(0, 0, 0);

Mat img(imgThresholded); // just to make a short name

for(int y = 0; y < img.rows; ++y)
{
    Vec3b* row = img.ptr<Vec3b>(y);
    for(int x = 0; x < img.cols; ++x)
    {
        Vec3b& pixel = row[x];
        if(pixel == black)
        {
            /* ... */
        }
    }
 }

OpenCV Access RGB values in a MAT object

1 Answers