How to read video files using python & Opencv

Question

I am reading an avi file usinh python 2.7 and opencv2.4.I am using windows 10.My sample code is

import numpy as np
import cv2
cap = cv2.VideoCapture('videos/wa.avi')
while(cap.isOpened()):
  ret, frame = cap.read()
  gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  cv2.imshow('frame',gray)
  if cv2.waitKey(1) & 0xFF == ord('q'):
    break

cap.release()
cv2.destroyAllWindows()

When i run video is shown But the program ends with no Error

OpenCV Error: Assertion failed (size.width>0 && size.height>0) in cv::imshow, file ..\..\..\..\opencv\modules\highgui\src\window.cpp, line 261
Traceback (most recent call last):
File "C:/Users/Emmanu/PycharmProjects/VideoEventDetection/test.py", line 11, in <module>
cv2.imshow('frame',frame)
cv2.error: ..\..\..\..\opencv\modules\highgui\src\window.cpp:261: error: (-215) size.width>0 && size.height>0 in function cv::imshow

What i am doing wrong?How can i correct it?

Answer 1

The problem is in this line:

gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

This line expects frame to be a 3 channel or 4 channel Mat object but instead it got some empty Mat and that is why you are getting this assertion failed. You need to check if the frame exists in video and need to handle end of video properly.

cap.isOpened() will just check if the video file can be opened for reading but it will not return a false when end of video file is reached.

Try this

Answer 2

When you put cap.isOpened() it check that, the video is read properly, So the while loop is not working there.

But when you changed to while True it will execute without proper reading that's why it's giving an error.

Make sure than you are properly reading the video file.

Answer 3

You took the tutorial from here: Playing Video from file

This question adresses your error: open cv error: (-215) scn == 3 || scn == 4 in function cvtColor

Two things may work:

• Make sure the video is found!
• And try to use cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)

Answer 4

Well I am guessing the capture is not open. That's why your program ends instantly when you use while(cap.isOpened()): .

As stated in this doc it happens that the capture is not implicitly opened when created.

Sometimes, cap may not have initialized the capture. In that case, this code shows error. You can check whether it is initialized or not by the method cap.isOpened(). If it is True, OK. Otherwise open it using cap.open().

Try to explicitly open the capture like so :

cap = cv2.VideoCapture('videos/wa.avi')
cap.open();
while(cap.isOpened()):
...

If this does not work you will have to check for the video file path.