OpenCV findContour() corners index order

Question

I am working with Android OpenCV .I have a square shape marker image .I have to find out the position of four corners of marker in the live feed with their index's position. I am able to get the position of these four points in the live feed having the frame size 640*480. But the problem with the index's positions those are not fixed ,sometimes i am getting the 0th index on the top left position and sometimes 1th index at that position ... How to solve this problem or is there any alternative for findContour() method.

Answer 1

Do the objects have an intrinsic absolute orientation (i.e. a glove, or an oriented checkerboard) that may change from frame to frame, and you need to recognize it? Or is it only a matter of ensuring that the index ordering is consistent from frame to frame? If the latter, and the relative interframe motion is small, why not just track it - i.e. reassign indices by proximity with the positions of the corners at the previous frame?

Answer 2

If the shape is quadrilateral, you can use corner sorting approach:

1- Get the mass center.

2- Points that have lower y-axis than mass center are the top points, otherwise they are bottom points.

3- Given two top points, the one with lower x-axis is the top-left. The other is the top-right.

4- Given two bottom points, the one with lower x-axis is the bottom-left. The other is the bottom-right.

Reference with opencv/c++: https://opencv-code.com/tutorials/automatic-perspective-correction-for-quadrilateral-objects/