How To Count Non-zero Pixels Inside A Contour Opencv

I am developing OMR scanner android application using opencv library. I have detected my circles inside the sheet as contours and now I want to get filled circle contours from all

Solution 1:

I reworked my own code and found this solution. Hope it might help.

for (int contourIdx = 0; contourIdx < questionSortedR.size(); contourIdx++) {
        //creating rectangle around identified contour
        Rect rectCrop = boundingRect(questionSortedR.get(contourIdx));
        //creating crop of that contour from actual image
        Mat imageROI= thresh.submat(rectCrop);
        //apply countnonzero method to that cropint total = countNonZero(imageROI);
        double pixel =total/contourArea(questionSortedR.get(contourIdx))*100;
        //pixel is in percentage of area that is filledif(pixel>=100 && pixel<=130){
            //counting filled circles
            count++;
        }

    }

Solution 2:

I propose an alternative to the accepted answer: instead of counting pixels inside a bounding rectangle, paint the contour into a mask, then mask the original image and count the pixels inside it. I was counting black pixels on a white background, where the contour kept several pixels on the edge, so your mileage may vary. Here is my code in Python:

mask = np.zeros(bw_image.shape, np.uint8)
cv.drawContours(mask, [contour], 0, 255, -1)
inverted = cv.bitwise_not(bw_image)
masked = cv.bitwise_not(cv.bitwise_and(inverted, inverted, mask = mask))

# Grab masked image inside contour
x, y, w, h = cv.boundingRect(contour)
pixels = masked[y:y+h, x:x+w]

# Check if black is only a line, in which case whiteness is 1
kernel = np.ones((3, 3), np.uint8)
dilated = cv.dilate(pixels, kernel, iterations = 1)
whiteness = np.sum(dilated) / (255 * w * h)