How to calculate the black / white ratio of pixels inside a contour - c++

Question

How is it possible to calculate the black / white ratio of the pixels inside the outline of a contour (not the bounding box)?
The image is pre-processed with cv::threshold(src, img, 0, 255, cv::THRESH_BINARY | cv::THRESH_OTSU); and then inverted img = 255 - img;
I look for the retangular outline of the table (contour) via cv::RETR_EXTERNAL.. I want to calculate the black pixels inside the contour
There can be other components in the image so I can't just count all non-zero pixels
This is the original image before binarized and inverted

Answer 1

I think there's some confusion about terminology. A contour is simply a sequence of points. If you draw them as a closed polygon (e.g. with cv::drawContours), all the points inside the polygon will be white.
You can however use this mask to count the white or black pixels on your thresholded image:
cv::Mat1b bw_image = ...
std::vector<std::vector<cv::Point>> contours;
cv::findContours(bw_image, contours, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
for(size_t i=0; i<contours.size(); ++i)
{
cv::Mat1b contour_mask(bw_image.rows, bw_image.cols, uchar(0));
cv::drawContours(contour_mask, contours, i, Scalar(255), cv::FILLED);
int total_white_inside_contour = cv::countNonZero(mask);
int white_on_image_inside_contour = cv::countNonZero(bw_image & mask);
int black_on_image_inside_contour = total_white_inside_contour - white_on_image_inside_contour;
}

Answer 2

You cannot calculate the white and black ratio of a contour, because what is a contour? A group of white pixels which are connected which each other calls contour, so a contour does not contain any black pixel if it does, it calls hole inside the contour.
And also a contour does not have a specific shape.
So you can do it by Bounding Rectangle the rectangle around the contour then you will be to calculate the black and white ratio inside the rectangle.