I was doing some work in OpenCV 2.4.8 and I noticed a strange behaviour when detecting keypoints using ORB: `ORB::detect()` gives a different result if I use an all-permissive mask (all set to non-zero values). I was expecting the same result, but the keypoints got smaller, like if the pyramid used to detect keypoints had less levels, even if I used the same default parameters.
Does anyone know if this is the desired behaviour? And if it is, why?
Here it goes a minimal working example:
#include
#include
void orb_without_mask(const cv::Mat & img)
{
cv::ORB orb;
std::vector keypoints;
orb.detect(img, keypoints);
// Decriptors not needed, but I compute it anyway because keypoints vector
// may change based on descriptor computation (be reordered or have
// elements removed)
cv::Mat descriptors;
orb.compute(img, keypoints, descriptors);
cv::Mat output_img;
cv::drawKeypoints(img,
keypoints,
output_img,
cv::Scalar::all(-1),
cv::DrawMatchesFlags::DRAW_RICH_KEYPOINTS);
cv::imshow("orb_mwe", output_img);
cv::waitKey(0);
}
// Same as before, but with a mask of the same size as the image that accepts
// everything
void orb_with_mask(const cv::Mat & img)
{
cv::ORB orb;
std::vector keypoints;
cv::Mat mask(cv::Mat::ones(img.size(),CV_8U)); // declare mask here
orb.detect(img, keypoints, mask); // use mask here
cv::Mat descriptors;
orb.compute(img, keypoints, descriptors);
cv::Mat output_img;
cv::drawKeypoints(img,
keypoints,
output_img,
cv::Scalar::all(-1),
cv::DrawMatchesFlags::DRAW_RICH_KEYPOINTS);
cv::imshow("orb_mwe", output_img);
cv::waitKey(0);
}
int main(int argc, char const *argv[])
{
cv::namedWindow("orb_mwe", cv::WINDOW_NORMAL);
cv::Mat img = cv::imread( argv[1] );
orb_without_mask(img);
orb_with_mask(img);
return 0;
}
And the results I got:
↧