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));

Are you asking about the meaning of the inlier/outlier distinction, or about the numerical values in the mask? — Aurelius, Apr 04 '13 at 15:25
Outliers are 0. Inliers get values, most of them very small. They order of magnitude is between e^-200 and e^-400. — jsalvador, Apr 04 '13 at 15:46
Could you post the code where you call `findHomography` and then examine the mask values? It would be nice to know the type of your mask input. — Aurelius, Apr 04 '13 at 15:58

Aurelius · Accepted Answer · 2013-04-04T16:39:06.280

15

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.

edited Apr 04 '13 at 16:39

answered Apr 04 '13 at 16:25

Aurelius

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To convert the values of the mask to QString I use the following for each matrix field: ` QString::number(mask.at(i,j)) ` – jsalvador Apr 04 '13 at 16:33
Nice, that was the mistake. Thank you very much! – jsalvador Apr 04 '13 at 17:00
1

I cannot print the value correctly by using: `mask.at(0, 0)`. It prints a small matrix icon with values "0 0, 0 1"... Do you know why? – JonasVautherin Apr 15 '13 at 14:59
Okay... It is just that I call findHomography() exactly as he does, and when I print `cout << mask.at(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. – JonasVautherin Apr 15 '13 at 15:15
3

@user1368342 I've reproduced your problem. Cast the result to an `int` to see the numerical value, like so: `cout << (int)mask.at(0,0) << endl;` – Aurelius Apr 15 '13 at 15:23
@Aurelius, you are right: http://stackoverflow.com/questions/24456788/opencv-how-to-get-inlier-points-using-findhomography-findfundamental-and-ra/24456789#24456789 Here is the question and also the answer. It was quite trivial after all. :P – rbaleksandar Jul 02 '14 at 18:20

score 2 · Answer 2 · edited Jul 11 '18 at 07:03

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]]

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

2 Answers2

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