Draw contours of detected objects using python and opencv

Question

I am using opencv2.4.10 with python and I am trying to draw some contours of moving objects. Unfortunately I cannot figure out what am I doing wrong.

First of all I use Background Subtractor from opencv ( cv2.BackgroundSubtractorMOG2 ). It works fine, I can see my moving objects separated from the background.

After separation I use cv2.FindContours method. It also works fine and I can print contorus as list of numbers.

But then, when I am trying to draw contours on the original image frame an error occurs.

    cv2.drawContours(frame, contours, 1, (0, 255, 0), 3)
    cv2.error: ..\..\..\..\opencv\modules\imgproc\src\contours.cpp:1810: error: (-215) 0 <= contourIdx && contourIdx < (int)last in function cv::drawContours

Could you comment on this issue?

Answer 1

From the error message we can get conditions that have to be met: 0 <= contourIdx && contourIdx < (int)last

Focusing on the second part, contourIdx < (int)last , I'd guess that you may be trying to draw the second contour of a list that's only got either one or zero contours.