Why Isn't Inrange Function Detecting Blue Color When I Have Given It The Entire Possible Hue Range For The Blue Color?

On the website colorizer.org, they have an HSV range of H=0-360, S=0-100, V=0-100. We are also aware that the HSV range in OpenCV is H=0-180, S=0-255, V=0-255. I wanted to select a

Solution 1:

using this code does work for me (C++):

    cv::Mat input = cv::imread("../inputData/HSV_RGB.jpg");

    //assuming your image to be in RGB format after loading:
    cv::Mat hsv;
    cv::cvtColor(input,hsv,CV_RGB2HSV);

    // hue range:
    cv::Mat mask;
    inRange(hsv, cv::Scalar(85, 50, 40), cv::Scalar(135, 255, 255), mask);

    cv::imshow("blue mask", mask);

I used this input image (saved and loaded in BGR format although it in fact is a RGB image, that's why we have to use RGB2HSV instead of BGR2HSV):

resulting in this mask:

The difference to your code is that I used CV_RGB2HSV instead of CV_RGB2HSV_FULL. Flag CV_RGB2HSV_FULL uses the whole byte to store the hue values, so range 0 .. 360 degrees will be scaled to 0 .. 255 instead of 0 .. 180 as in CV_RGB2HSV

I could verify this by using this part of the code:

// use _FULL flag:
    cv::cvtColor(input,hsv,CV_RGB2HSV_FULL);
    // but scale the hue values accordingly:double hueScale = 2.0/1.41176470588;
    cv::Mat mask;
    // scale hue values:
    inRange(hsv, cv::Scalar(hueScale*85, 50, 40), cv::Scalar(hueScale*135, 255, 255), mask);

giving this result:

For anyone who wants to test with the "right" image:

Here's the input converted to BGR: If you want to use that directly you have to switch conversion from RGB2HSV to BGR2HSV. But I thought it would be better to show the BGR version of the input, too...