Number of non-zero pixels in a cv::RotatedRect

Question

as the title says i'm trying to find the number of non-zero pixels in a certain area of a cv::Mat, namely within a RotatedRect.

For a regular Rect one could simply use countNonZeroPixels on a ROI. However ROIs can only be regular (non rotated) rectangles.

Another idea was to draw the rotated rectangle and use that as a mask. However openCV neither supports the drawing of rotated rectangles nor does countNonZeroPixels accept a mask.

Does anyone have a solution for how to elegantly solve this ?

Thank you !

Answer 1

Ok, so here's my first take at it.

The idea is to rotate the image reverse to the rectangle's rotation and than apply a roi on the straightened rectangle.

• This will break if the rotated rectangle is not completely within the image

• You can probably speed this up by applying another roi before rotation to avoid having to rotate the whole image...

  #include <highgui.h>
  #include <cv.h>
  
  
  // From http://stackoverflow.com/questions/2289690/opencv-how-to-rotate-iplimage
  cv::Mat rotateImage(const cv::Mat& source, cv::Point2f center, double angle)
  {
    cv::Mat rot_mat = cv::getRotationMatrix2D(center, angle, 1.0);
    cv::Mat dst;
    cv::warpAffine(source, dst, rot_mat, source.size());
    return dst;
  }
  
  int main()
  {
    cv::namedWindow("test1");
  
    // Our rotated rect
    int x = 300;
    int y = 350;
    int w = 200;
    int h = 50;
    float angle = 47;
    cv::RotatedRect rect = cv::RotatedRect(cv::Point2f(x,y), cv::Size2f(w,h), angle);
  
    // An empty image
    cv::Mat img = cv::Mat(cv::Size(640, 480), CV_8UC3);
  
    // Draw rotated rect as an ellipse to get some visual feedback
    cv::ellipse(img, rect, cv::Scalar(255,0,0), -1);
  
    // Rotate the image by rect.angle * -1
    cv::Mat rotimg = rotateImage(img, rect.center, -1 * rect.angle);
  
    // Set roi to the now unrotated rectangle
    cv::Rect roi;
    roi.x = rect.center.x - (rect.size.width / 2);
    roi.y = rect.center.y - (rect.size.height / 2);
    roi.width = rect.size.width;
    roi.height = rect.size.height;
  
    cv::imshow("test1", rotimg(roi));
    cv::waitKey(0);
  }