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

Question

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));

Solution 1

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.

Solution 2

I used the findHomography method after applying keypoint matching.

• Inliers are matched keypoints that are calculated to be true positives (correct matches);
• Outliers are matched keypoints that are calculated to be false positives (false matches).

Then you can use the mask output to extract the subset of correct matches from all matches.

• There is an example in Python 3.6 & OpenCV 3.4.1:

  good_kp = [gray_kp[m.queryIdx].pt for m in good_matches]
  correct_matched_kp = [good_kp[i] for i in range(len(good_kp)) if mask[i]]
  

Comments

[jsalvador]

To convert the values of the mask to QString I use the following for each matrix field: ` QString::number(mask.at<double>(i,j)) `

[jsalvador]

Nice, that was the mistake. Thank you very much!

[JonasVautherin]

I cannot print the value correctly by using: mask.at<uchar>(0, 0). It prints a small matrix icon with values "0 0, 0 1"... Do you know why?

[JonasVautherin]

Okay... It is just that I call findHomography() exactly as he does, and when I print cout << mask.at<uchar>(0, 0) << endl; as you proposed, I don't get a 0 or a 1. I will see to ask another question if you think it is different as here.

[Aurelius]

@user1368342 I've reproduced your problem. Cast the result to an int to see the numerical value, like so: cout << (int)mask.at<uchar>(0,0) << endl;

[JonasVautherin]

Exactly what I needed! Thanks a lot!

[rbaleksandar]

@Aurelius, you are right: stackoverflow.com/questions/24456788/… Here is the question and also the answer. It was quite trivial after all. :P