python - OpenCV example error - TypeError: 'NoneType' object is not subscriptable -


i'm trying run python example opencv site:

http://docs.opencv.org/trunk/d7/d8b/tutorial_py_lucas_kanade.html

import numpy np import cv2 cap = cv2.videocapture('slow.flv') # params shitomasi corner detection feature_params = dict( maxcorners = 100,                        qualitylevel = 0.3,                        mindistance = 7,                        blocksize = 7 ) # parameters lucas kanade optical flow lk_params = dict( winsize  = (15,15),                   maxlevel = 2,                   criteria = (cv2.term_criteria_eps | cv2.term_criteria_count, 10, 0.03)) # create random colors color = np.random.randint(0,255,(100,3)) # take first frame , find corners in ret, old_frame = cap.read() old_gray = cv2.cvtcolor(old_frame, cv2.color_bgr2gray) p0 = cv2.goodfeaturestotrack(old_gray, mask = none, **feature_params) # create mask image drawing purposes mask = np.zeros_like(old_frame) while(1):     ret,frame = cap.read()     frame_gray = cv2.cvtcolor(frame, cv2.color_bgr2gray)     # calculate optical flow     p1, st, err = cv2.calcopticalflowpyrlk(old_gray, frame_gray, p0, none, **lk_params)     # select points     good_new = p1[st==1]     good_old = p0[st==1]     # draw tracks     i,(new,old) in enumerate(zip(good_new,good_old)):         a,b = new.ravel()         c,d = old.ravel()         mask = cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)         frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1)     img = cv2.add(frame,mask)     cv2.imshow('frame',img)     k = cv2.waitkey(30) & 0xff     if k == 27:         break     # update previous frame , previous points     old_gray = frame_gray.copy()     p0 = good_new.reshape(-1,1,2) cv2.destroyallwindows() cap.release()

i use python 3 run example

it works 5-15 seconds videos , stops next error:

traceback (most recent call last): file "o.py", line 28, in good_new = p1[st==1] typeerror: 'nonetype' object not subscriptable

what can wrong in example?

that happens if optical flow objects (the color dots on screen) go out of frame. this- if array p1 empty, find features again , calculate optical flow. should work.

add in while loop( fills entire screen lines on time) :

if p1 none:    p0 = cv2.goodfeaturestotrack(old_gray, mask = none, **feature_params)    p1, st, err = cv2.calcopticalflowpyrlk(...., **lk_params)

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