Python : Testing Video in OpenCV using python

Question

I just installed opencv3 on my macOS sierra 10.12.3. I tried some examples and it worked out but when I try the following example it throws me some exceptions. My python version is 2.7.
I cannot figure out the problem. Can anybody help me with the issue? My python code is:

import cv2
import numpy as np
cap = cv2.VideoCapture("vtest.avi")

ret, frame1 = cap.read()
prvs = cv2.cvtColor(frame1,cv2.COLOR_BGR2GRAY)
hsv = np.zeros_like(frame1)
hsv[...,1] = 255

while(1):
    ret, frame2 = cap.read()
    next = cv2.cvtColor(frame2,cv2.COLOR_BGR2GRAY)

    flow = cv2.calcOpticalFlowFarneback(prvs,next, None, 0.5, 3, 15, 3, 5, 1.2, 0)

    mag, ang = cv2.cartToPolar(flow[...,0], flow[...,1])
    hsv[...,0] = ang*180/np.pi/2
    hsv[...,2] = cv2.normalize(mag,None,0,255,cv2.NORM_MINMAX)
    rgb = cv2.cvtColor(hsv,cv2.COLOR_HSV2BGR)

    cv2.imshow('frame2',rgb)
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break
    elif k == ord('s'):
        cv2.imwrite('opticalfb.png',frame2)
        cv2.imwrite('opticalhsv.png',rgb)
    prvs = next

cap.release()
cv2.destroyAllWindows()

The error message is:

    OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /tmp/opencv3-20170324-1646-1ehj5xu/modules/imgproc/src/color.cpp, line 9748
Traceback (most recent call last):
  File "/Users/Rouzbeh/BoxSync/Spring2017/TrafficProject/test/test.py", line 6, in <module>
    prvs = cv2.cvtColor(frame1,cv2.COLOR_BGR2GRAY)
cv2.error: /tmp/opencv3-20170324-1646-1ehj5xu/modules/imgproc/src/color.cpp:9748: error: (-215) scn == 3 || scn == 4 in function cvtColor

Update :
I had some problem with ffmpeg formatting. However, it shows the video but at the end still throws this exception.

Answer 1

That's because at the end of video, frame1 is empty. You should do:

while(1):
    ret, frame2 = cap.read()
    if not ret:
        break
    next = cv2.cvtColor(frame2,cv2.COLOR_BGR2GRAY)
    # rest of code here
    pass    

Answer 2

cap = cv2.VideoCapture("vtest.avi")
print cap.isOpened()

you can check cv2.VideoCapture() function Working properly

maybe your OpenCV doesn't support ffmpeg