What does Mat::checkVector do in OpenCV?

Question

I tried to use the following function in OpenCV (C++)

calcOpticalFlowPyrLK(prev_frame_gray, frame_gray, points[0], points[1], status, err, winSize, 3, termcrit, 0, 0.001);

and I get this error

OpenCV Error: Assertion failed ((npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0) in calcOpticalFlowPyrLK,
file /home/rohit/OpenCV_src/opencv-2.4.9/modules/video/src/lkpyramid.cpp, line 845
terminate called after throwing an instance of 'cv::Exception'
what():  /home/rohit/OpenCV_src/opencv-2.4.9/modules/video/src/lkpyramid.cpp:845:
error: (-215) (npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0 in function calcOpticalFlowPyrLK

Both of the following return -1

frame_gray.checkVector(2, CV_32F, true)
prev_frame_gray.checkVector(2, CV_32F, true)

I wanted to know what checkVector actually does because it is leading to the assertion error as you can see above.

Please provide the definition of the input variables/parameters of `calcOpticalFlowPyrLK` — Antonio, May 06 '15 at 21:09
I forgot to add the definitions. But, It's solved now. Basically, I hadn't initialized points[0] and hence the error. I would still like to know what checkVector does as I couldn't find it in the documentation. — rohit-biswas, May 06 '15 at 21:52

score 15 · Accepted Answer · edited Feb 18 '17 at 02:32

15

The official OpenCV's doc says:

cv::Mat::checkVector() returns N if the matrix is 1-channel (N x ptdim) or ptdim-channel (1 x N) or (N x 1); negative number otherwise

OpenCV considers some data types equivalent in case of some functions i.e. objectPoints of cv::solvePnP() can be:

1xN/Nx1 1-channel cv::Mat
3xN/Nx3 3-channel cv::Mat
std::vector<cv::Point3f>

With checkVector you can make sure that you are passing the correct representation of your data.

edited Feb 18 '17 at 02:32

MM.

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answered May 07 '15 at 09:04

Kornel

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What is the definition of 'N' here? Cannot find it in the documentation. – Udaya Jan 04 '17 at 07:30
it's a bit confusing, does this means that the data is continues? – TripleS Nov 16 '17 at 08:37
N just stands for a list of "N" points, 1, 2, 3, ... whatever the count of N points happens to be. – user1270710 Nov 14 '18 at 23:34

score 0 · Answer 2 · answered Jul 17 '19 at 17:53

I had a similar issue with cv2.projectPoints function (-215:Assertion failed) because openCV was expecting a nx3 matrix and i was passing an 1D array of length 3. Try:

points[0].reshape(-1,3)

As argument to the function. It changes the shape (3,) to shape (1,3).

What does Mat::checkVector do in OpenCV?

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