OpenCV better detection of red color?

Question

I have the following image:

I would like to detect the red rectangle using cv::inRange method and HSV color space.

int H_MIN = 0; int H_MAX = 10; int S_MIN = 70;  int S_MAX = 255; int V_MIN = 50; int V_MAX = 255;  cv::cvtColor( input, imageHSV, cv::COLOR_BGR2HSV );  cv::inRange( imageHSV, cv::Scalar( H_MIN, S_MIN, V_MIN ), cv::Scalar( H_MAX, S_MAX, V_MAX ), imgThreshold0 ); 

I already created dynamic trackbars in order to change the values for HSV, but I can't get the desired result.

Any suggestion for best values (and maybe filters) to use?

Answer 1

In HSV space, the red color wraps around 180. So you need the H values to be both in [0,10] and [170, 180].

Try this:

#include <opencv2\opencv.hpp> using namespace cv;  int main() {     Mat3b bgr = imread("path_to_image");      Mat3b hsv;     cvtColor(bgr, hsv, COLOR_BGR2HSV);      Mat1b mask1, mask2;     inRange(hsv, Scalar(0, 70, 50), Scalar(10, 255, 255), mask1);     inRange(hsv, Scalar(170, 70, 50), Scalar(180, 255, 255), mask2);      Mat1b mask = mask1 | mask2;      imshow("Mask", mask);     waitKey();      return 0; } 

Your previous result:

Result adding range [170, 180]:

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Another interesting approach which needs to check a single range only is:

• invert the BGR image
• convert to HSV
• look for cyan color

This idea has been proposed by fmw42 and kindly pointed out by Mark Setchell. Thank you very much for that.

#include <opencv2\opencv.hpp> using namespace cv;  int main() {     Mat3b bgr = imread("path_to_image");      Mat3b bgr_inv = ~bgr;     Mat3b hsv_inv;     cvtColor(bgr_inv, hsv_inv, COLOR_BGR2HSV);      Mat1b mask;      inRange(hsv_inv, Scalar(90 - 10, 70, 50), Scalar(90 + 10, 255, 255), mask); // Cyan is 90      imshow("Mask", mask);     waitKey();      return 0; }