error: (-28:Unknown error code -28) The input arrays should have at least 4 corresponding point sets to calculate Homography in function 'findHomography'

[utility-aagrawal]

Hi,

Would you know why I am getting this error? I see in the code here that you check for minimum 4 pairs of points before findHomography but still getting the error:

norfair/norfair/camera_motion.py

Line 220 in 009a1b1

if not (

File "/home/aagrawal/face_detection/real_videos/open_source_face_detection.py", line 232, in main
coord_transformation = motion_estimator.update(frame) #, mask)
File "/home/aagrawal/face_detection/face_venv/lib/python3.10/site-packages/norfair/camera_motion.py", line 396, in update
update_prvs, coord_transformations = self.transformations_getter(
File "/home/aagrawal/face_detection/face_venv/lib/python3.10/site-packages/norfair/camera_motion.py", line 216, in call
homography_matrix, points_used = cv2.findHomography(
cv2.error: OpenCV(4.9.0) /io/opencv/modules/calib3d/src/fundam.cpp:385: error: (-28:Unknown error code -28) The input arrays should have at least 4 corresponding point sets to calculate Homography in function 'findHomography'

[aguscas]

The reason might be because that bug was fixed not long ago, but the fix is still not part of the latest release. Maybe try installing Norfair from the master branch:

pip install git+https://github.com/tryolabs/norfair.git@master#egg=norfair

Tell me if this fixes the issue.

[utility-aagrawal]

Thanks a lot, @aguscas !

Installing from master seemed to work. I do get the following warning though:

Does this mean that transformation will be skipped for the frame in case of this issue but everything else will work fine?

[aguscas]

Everything should work fine. That means that the coord_transformation will not be updated in that frame (the MotionEstimator will return the latest transformation it could estimate). As long as that doesn't happen often it shouldn't be a problem with the coord_transformation, since typically a single frame shouldn't affect that much at the coordinate_transformation.

[utility-aagrawal]

Makes sense! Closing this issue. Again, thanks so much for your help!

[aguscas]

If you want to avoid having that warning (so that you always estimate the transformation), you can also try setting the max_points argument of the MotionEstimator to a higher value (so that it tries sampling more points), or the min_distance argument to a smaller value (so that we allow to sample points that are closer to each other).

[utility-aagrawal]

For now, it's not an issue for me but I'll experiment with those two parameters. Thanks a lot!