Difference between a list of points and list of pixels.

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yes, the points are stored as floats and prev_pts and next_pts return float points. what is meant by subpixel accuracy about its location? my main goal was to draw a quiver from every prev_pts to the corresponding next_pts. But I dont know where the tip should start and end, because the locations cannot be floats.

Yes it's about it's location. Here is a good explanation.

If you are using OpenGL for example you can render in subpixel resolution, see Wikipedia.

To draw the contour points using cv::drawContours you can convert it back to integer accuracy using Mat::convertTo. It also rounds values correct.

ah, i got it regarding the subpixel. Did not know that you can dig further into the resolution. can you please give an example in your answer with respect to drawing a countour from prev_pts to next_pts. Wouldnt drawLine will be simpler, because I just need a straight line with an arrow head?

You can use cv::arrowedLine in OpenCV 3.3 (and 3.2?). Round your points to int and pass it to the function.

it seems, that the round off is not required as it is done automatically by the function.