What do the values of the mask parameter returned by findHomography represent?

c++ opencv computer-vision homography ransac
jsalvador picture jsalvador · Apr 4, 2013 · Viewed 10.1k times · Source

I am using the function findHomography of OpenCV with the RANSAC method in order to find the homography that relates two images linked with a set of keypoints.

The main issue is that I haven't been able to find anywhere yet what are the values of the mask matrix that the function outputs.

The only information that I know is that 0 values are outliers, and non zero values are inliers. But what does it mean the inliers value? Does anyone know?

Piece of code where I call findHomography:

cv::Mat H12;
cv::Mat mask;

H12 = cv::findHomography(FvPointsIm1, FvPointsIm2, mask, CV_RANSAC, 5); 
ui->Debug_Label->setText(Mat2QString(mask));

Answer

Aurelius picture Aurelius · Apr 4, 2013

The mask returned by findHomography is an 8-bit, single-channel cv::Mat (or std::vector<uchar>, if you prefer) containing either 0 or 1 indicating the outlier status.

EDIT: You access each element of the mask by calling .at<double>, which is leading to the confusing output. You should be using .at<uchar>, which will interpret the matrix value correctly.

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