Getting error in capturing image through a program in Python

Question

I am new to the Python opencv. Can anybody please help me to sort out the error

import cv

cv.NamedWindow("w1", cv.CV_WINDOW_AUTOSIZE)
camera_index = 1
capture = cv.CaptureFromCAM(camera_index)

def repeat():
global capture #declare as globals since we are assigning to them now    global camera_index
frame = cv.QueryFrame(capture)
cv.ShowImage("w1", frame)
c = cv.WaitKey(100)
if(c=="n"): #in "n" key is pressed while the popup window is in focus
    camera_index += 1 #try the next camera index
    capture = cv.CaptureFromCAM(camera_index)
    if not capture: #if the next camera index didn't work, reset to 0.
        camera_index =1
        capture = cv.CaptureFromCAM(camera_index)

while True:
repeat()

this is the error what I am getting -

OpenCV Error: Null pointer (NULL array pointer is passed) in cvGetMat, file /home/paraste/OpenCV-2.3.1/modules/core/src/array.cpp, line 2382
Traceback (most recent call last):
  File "dualcamara.py", line 10, in <module>
img = cv.GetMat(cv.QueryFrame(capture), 500)
cv2.error: NULL array pointer is passed

WildCrustacean · Accepted Answer · 2012-12-31 03:26:59Z

1

It seems likely that either cv.CaptureFromCAM() or cv.QueryFrame() is failing (maybe camera_index is wrong?), and thus you get a NULL in frame which causes that error. You should check the result of those two functions and make sure they succeed (I'm just printing a message in this case, you could of course do something else):

capture = cv.CaptureFromCAM(camera_index)
if not capture:
     print "Failed to initialize capture"

frame = cv.QueryFrame(capture)
if not frame:
     print "Failed to get frame"

answered Dec 31, 2012 at 3:26

WildCrustacean

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1 Comment

parastenitk22f Over a year ago

hello, Thank you for your reply. The CaptureFromCAM function is returning some hexadecimal value but the QueryFrame function is returning none. i thought camera is not on so, i manually switched it on using cheese command but there is no change in the return value. Please let me know how to switch on the camera using inbuilt function. Even i tried using the function cv2.VideoCapture.open but it ends with the following error:AttributeError: 'builtin_function_or_method' object has no attribute 'open'

Morten Kristensen · Accepted Answer · 2013-09-08 01:48:03Z

0

cv.QueryFrame() may be returning None and you are not handling the possibility. I find that cv.QueryFrame() sometimes returns None at the beginning, so I simply put:

if frame == None:
    return

This way, even if your first call fails, or calls fail intermittently, your loop will continue and feed images as they are captured. I had the same problem on my Mac Book Pro using python 2.7 and this solved it for me.

edited Sep 8, 2013 at 1:48

Morten Kristensen

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answered Sep 8, 2013 at 1:25

user2275776

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