cv2.line and cv2.circle return None

Question

I am trying to implement the Lucas-Kanade algorithm for optical flow on Python using cv2.calcOpticalFlowPyrLK. However, when I try to draw a line to connect the current and previous points as well as the point itself, the functions return None.

Here is the code

#draw the overlaying tracking img
for i,(new,old) in enumerate(zip(good_new,good_old)):
    a,b = new.ravel() #tmp new value
    c,d = old.ravel() #tmp old value

    #draws a line connecting the old point with the new point
    mask = cv2.line(mask,(a,b),(c,d),color[i].tolist(),2) #returns None... why??
    #draws the new point
    frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1) #returns None... why??

img = cv2.add(frame,mask)

#show on window
cv2.imshow("frame",img)

a,b,c,d are numpy.float32

frame is my value read from the webcam

mask is init to an array of zeros

color is a random array

This is the error message I get when I try to imshow

error: /Users/vagrant/pisi-64bit/tmp/opencv-2.4.5-2/work/opencv-2.4.5/modules/core/src/array.cpp:2482: error: (-206) Unrecognized or unsupported array type in function cvGetMat

I really appreciate the help!

Answer 1

According to the OpenCV docs, cv2.line() and cv2.circle() always return None. http://docs.opencv.org/modules/core/doc/drawing_functions.html

These methods are in place methods, so they alter the first argument that you pass to them. See this Stack Overflow question that describes these types of functions: How are Python in-place operator functions different than the standard operator functions?

Your fixed code should look something like this:

#draw the overlaying tracking img
for i,(new,old) in enumerate(zip(good_new,good_old)):
    a,b = new.ravel() #tmp new value
    c,d = old.ravel() #tmp old value

    #draws a line connecting the old point with the new point
    cv2.line(mask,(a,b),(c,d),color[i].tolist(),2) #returns None... why??
    #draws the new point
    cv2.circle(frame,(a,b),5,color[i].tolist(),-1) #returns None... why??

img = cv2.add(frame,mask)