BGR to HSV and vice versa

Question

The documentation does not seem to explain what the expected input range for the HSV-> BGR conversion is. Here is a sample code where I try to return the original BGR value after converting it to HSV. Does anyone know what kind of scaling is expected?

#include <iostream> #include <opencv2/opencv.hpp> cv::Vec3b HSVtoBGR(const cv::Vec3f& hsv) { cv::Vec3f hsvAdjusted = hsv; // If we use this directly, the output is (0,0,1) which is very wrong //hsvAdjusted[0] *= 360.; // If we do this to bring all of the values into the range (0,1), the output is (0,1,0), which is also very wrong // If we do this to bring all of the values into the range (0,255), the output is (0,0,200), which is still very wrong hsvAdjusted[1] *= 255./360.; hsvAdjusted[1] *= 255.; hsvAdjusted[2] *= 255.; cv::Mat_<cv::Vec3f> hsvMat(hsvAdjusted); cv::Mat_<cv::Vec3f> bgrMat; cv::cvtColor(hsvMat, bgrMat, CV_HSV2BGR); cv::Vec3b bgr = static_cast<cv::Vec3b>(bgrMat(0,0)); return bgr; } /** Input 0 <= B <= 255, 0 <= G <= 255, 0 <= R <= 255 * Output 0 <= H <= 360, 0 <= S <= 1, 0 <= V <= 1 */ cv::Vec3f BGRtoHSV(const cv::Vec3b& bgr) { cv::Mat3f bgrMat(static_cast<cv::Vec3f>(bgr)); bgrMat *= 1./255.; cv::Mat3f hsvMat; cv::cvtColor(bgrMat, hsvMat, CV_BGR2HSV); cv::Vec3f hsv = hsvMat(0,0); return hsv; } int main() { // Create a BGR color cv::Vec3b bgr(5, 100, 200); std::cout << "bgr: " << bgr << std::endl; // Convert BGR to HSV cv::Vec3f hsv = BGRtoHSV(bgr); std::cout << "hsv: " << hsv << std::endl; // outputs // (29.23, .976, .7843), which seems correct // Convert back from HSV to BGR cv::Vec3b bgr2 = HSVtoBGR(hsv); std::cout << "bgr2: " << bgr2 << std::endl; return 0; } 

Answer 1

You have scaling issues in the HSVtoBGR function.

You need to multiply by the result 255 after the conversion, not before. Remember that Mat uses different conversions like CV_32F and CV_8U .

This should work as expected:

 cv::Vec3b HSVtoBGR(const cv::Vec3f& hsv) { cv::Mat_<cv::Vec3f> hsvMat(hsv); cv::Mat_<cv::Vec3f> bgrMat; cv::cvtColor(hsvMat, bgrMat, CV_HSV2BGR); bgrMat *= 255; // Upscale after conversion // Conversion to Vec3b is handled by OpenCV, no need to static_cast return bgrMat(0); } 

Answer 2

Run the code below separately. You can control the RGB values ​​and get the corresponding HSV scaling.

 #include<iostream> #include<opencv2/core/core.hpp> #include<opencv2/imgproc/imgproc.hpp> #include<opencv2/highgui/highgui.hpp> using namespace std; using namespace cv; char RGB_window[30] = "RGB Window"; char HSV_window[30] = "HSV Window"; Mat src,hsv; static void onMouse( int event, int x, int y, int f, void* ){ Mat image=src.clone(); Vec3b rgb=image.at<Vec3b>(y,x); int B=rgb.val[0]; int G=rgb.val[1]; int R=rgb.val[2]; Mat HSV; Mat RGB=image(Rect(x,y,1,1));//capture that pixel in its own ROI cvtColor(RGB, HSV,CV_BGR2HSV); Vec3b hsv=HSV.at<Vec3b>(0,0); int H=hsv.val[0]; int S=hsv.val[1]; int V=hsv.val[2]; char name[30]; sprintf(name,"B=%d",B); putText(image,name, Point(150,40) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"G=%d",G); putText(image,name, Point(150,80) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"R=%d",R); putText(image,name, Point(150,120) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"H=%d",H); putText(image,name, Point(25,40) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"S=%d",S); putText(image,name, Point(25,80) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"V=%d",V); putText(image,name, Point(25,120) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,255,0), 2,8,false ); sprintf(name,"X=%d",x); putText(image,name, Point(25,300) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,0,255), 2,8,false ); sprintf(name,"Y=%d",y); putText(image,name, Point(25,340) , FONT_HERSHEY_SIMPLEX, .7, Scalar(0,0,255), 2,8,false ); //namedWindow("Ref HSV",WINDOW_NORMAL); Mat ref(50,50,CV_8UC3,Scalar(H,S,V)); //imwrite("hsv.jpg",image); imshow( RGB_window, image ); //imshow( "Ref HSV",ref); } int main() { //VideoCapture cap(0); static int Bs=0,Gs=0,Rs=0; namedWindow("colourCtrl"); //src = imread("bgr.png",1); for(;;) { //cap>>src; createTrackbar("B","colourCtrl",&Bs,255); createTrackbar("G","colourCtrl",&Gs,255); createTrackbar("R","colourCtrl",&Rs,255); Mat refRGB(500,500,CV_8UC3,Scalar(Bs,Gs,Rs)); src=refRGB; cvtColor(src,hsv,CV_BGR2HSV); imshow(RGB_window,src); imshow(HSV_window,hsv); setMouseCallback( RGB_window, onMouse, 0 ); setMouseCallback( HSV_window, onMouse, 0 ); char c=waitKey(10); if(c=='b') {break;} } return 0; }