how to change the black pixels in the below image to red pixels using opencv libraries with c++ -

i want change black pixels in image red pixels, such ball should white , red. want use opencv libraries , code in c++. have tried converting image rgb.

common approach threshold image, in case each pixel intensity less threshold considered being black , recolored red. 1 way find threshold (that divides image's pixel 2 classes ("more black" , "more white") otsu thresholding:

int main() {     cv::mat input = cv::imread("../inputdata/ball_thresholding.jpg");      cv::mat gray;     cv::cvtcolor(input,gray,cv_bgr2gray);      cv::mat mask;     // compute inverse thresholding (dark areas become "active" pixel in mask) otsu thresholding:     double graythres = cv::threshold(gray, mask, 0, 255, cv_thresh_binary_inv | cv_thresh_otsu);      // color masked pixel red:     input.setto(cv::scalar(0,0,255), mask);      // compute median filter remove whitish black parts , darker white parts      cv::imshow("input", input);     cv::waitkey(0);     return 0; } 

giving mask:

and result:

for image, threshold computed otsu 127, means each grayscale pixel intensity of 127 or less (or less 127, i'm not sure) recolored red.

if want keep shading effect withing black/red region, can remove input.setto(cv::scalar(0,0,255), mask); lind , replace by:

// keep shading:     for(int j=0; j<input.rows; ++j)         for(int i=0; i<input.cols; ++i)         {             if(mask.at<unsigned char>(j,i))             {                 input.at<cv::vec3b>(j,i)[2] = 255;             }         } 

which result int: