OpenCV Python：QueryFrame返回无

Question

I'm using OpenCV 2.4 and python 2.7.5 on a macbook. I want to display the live stream of the inbuilt camera with the following code :

import cv

cv.NamedWindow("w1", cv.CV_WINDOW_AUTOSIZE)
camera_index = 0
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)
    print type(frame)
    #cv.ShowImage("w1", frame)
    c = cv.WaitKey(10)
while True:
    repeat()

However, it seems QueryFrame does not always return an iplimage, here is what I get on the terminal:

<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'cv2.cv.iplimage'>
<type 'NoneType'>
<type 'NoneType'>
<type 'cv2.cv.iplimage'>
<type 'NoneType'>
<type 'NoneType'>
<type 'NoneType'>
<type 'cv2.cv.iplimage'>
<type 'NoneType'>
<type 'NoneType'>
<type 'cv2.cv.iplimage'>
<type 'NoneType'>

Does anybody know where the problem comes from ?

Thank you

EDIT: I noticed it takes some seconds before my camera turn on, so I put some delay before entering the "while". As for the noneType problem I have no idea why I get a correct image every 3 frames..anyway I just "fixed" it by putting a condition that checks if we get a correct image, here is the code:

import cv

cv.NamedWindow("w1", cv.CV_WINDOW_AUTOSIZE)
camera_index = 0
capture = cv.CaptureFromCAM(camera_index)
c = cv.WaitKey(5000)
def repeat():
    global capture #declare as globals since we are assigning to them now
    global camera_index
    frame = cv.QueryFrame(capture)

    if frame:

        cv.ShowImage("w1", frame)
    c = cv.WaitKey(10)
while True:
    repeat()

Answer 1

Suggestion as follow:

• camera_index = -1

• drop first frame just in case

  firstImage = copy.deepcopy(cv.QueryFrame(capture))

Share my code for your reference:

import cv2 as cv
import copy
camera_index = -1 

capture = cv.CaptureFromCAM(camera_index)

isRunning = True
firstImage = copy.deepcopy(cv.QueryFrame(capture)) #drop first frame which might empty
cv.NamedWindow("Webcam",  cv.CV_WINDOW_AUTOSIZE);

def repeat():
    global capture #declare as globals since we are assigning to them now
    global camera_index
    global isRunning
    global firstImage
    currImage = cv.QueryFrame(capture) 
    cv.ShowImage("Webcam",currImage)
    c = cv.WaitKey(10)
    if(c==27):
            isRunning = False
    if (c!=-1):
            print str(c)

while isRunning:
    repeat()