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

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 !

Solution

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);
}