HSV image in openCV for color recognition

Question

I have the following BGR image (a car's front), I want to recognize its color

I converted it to HSV (I know imshow() doesn't understand the HSV and will print it as BGR)

1: Now, I want to get the hue value and know in which range it lies to recognize the color

• how to calculate the number or value of hue?
• how to specify ranges? using scalar method gives me bgr ranges

Code

int main()
{
    Mat image;
    image = imread("carcolor.png", CV_LOAD_IMAGE_COLOR);

    if (!image.data)
    {
        cout << "Could not open or find the image" << std::endl;
        return -1;
    }

    // Create a new matrix to hold the HSV image
    Mat HSV;

    // convert RGB image to HSV
    cvtColor(image, HSV, CV_BGR2HSV);

    namedWindow("Display window", CV_WINDOW_AUTOSIZE);
    imshow("Display window", image);

    namedWindow("Result window", CV_WINDOW_AUTOSIZE);
    imshow("Result window", HSV);


    vector<Mat> hsv_planes;
    split(HSV, hsv_planes);
    Mat h = hsv_planes[0]; // H channel
    Mat s = hsv_planes[1]; // S channel
    Mat v = hsv_planes[2]; // V channel

    namedWindow("hue", CV_WINDOW_AUTOSIZE);
    imshow("hue", h);
    namedWindow("saturation", CV_WINDOW_AUTOSIZE);
    imshow("saturation", s);
    namedWindow("value", CV_WINDOW_AUTOSIZE);
    imshow("value", v);



    //// red color range
    Scalar hsv_l(170, 150, 150);
    Scalar hsv_h(180, 255, 255);
    Mat bw;
    inRange(HSV, hsv_l, hsv_h, bw);
    imshow("Specific Colour", bw);
    ////

    // hue value

    //define ranges

    waitKey(0);
    return 0;
}

Answer 1

Hue is a circular range and red is exactly at the start/end of that circle, so red hue value is covered by two different ranges: [0 .. n] and [360-n .. 360]. As you used in your code, openCV additionally halfs the whole range to [0 .. 180] because of byte coverage.

So in addition to Scalar hsv_l(170, 150, 150); and Scalar hsv_h(180, 255,255); you'll need a second range.

Full code to find the range could be:

//// red color rangeS
int redRange = 10;

Scalar hsv_upper_l(180-n, 150, 150);
Scalar hsv_upper_h(180, 255, 255);
Mat red_hue_upper;
inRange(HSV, hsv_upper_l, hsv_upper_h, red_hue_upper);

Scalar hsv_lower_l(0, 150, 150);
Scalar hsv_lower_h(0+n, 255, 255);
Mat red_hue_lower;
inRange(HSV, hsv_lower_l, hsv_lower_h, red_hue_lower);

cv::Mat whole_red_hue = red_hue_lower | red_hue_upper;

imshow("Specific Colour", whole_red_hue);

Producing from this input that output:

For different colors it is easier (since they aren't placed around circle's start/end):

int hueCenter = ...; // any hue value of the color
int hueRange = 10; // or any other range
cv::Mat hueMask;
inRange(HSV, Scalar(hueCenter-hueRange, 150, 150), Scalar(hueCenter+hueRange, 255, 255), hueMask);

hope this helps.