[python] Displaying a webcam feed using OpenCV and Python

I have been trying to create a simple program with Python which uses OpenCV to get a video feed from my webcam and display it on the screen.

I know I am partly there because the window is created and the light on my webcam flicks on, but it just doesn't seem to show anything in the window. Hopefully someone can explain what I'm doing wrong.

import cv

cv.NamedWindow("w1", cv.CV_WINDOW_AUTOSIZE)
capture = cv.CaptureFromCAM(0)

def repeat():

    frame = cv.QueryFrame(capture)
    cv.ShowImage("w1", frame)


while True:
    repeat()

On an unrelated note, I have noticed that my webcam sometimes changes its index number in cv.CaptureFromCAM, and sometimes I need to put in 0, 1 or 2 even though I only have one camera connected and I haven't unplugged it (I know because the light doesn't come on unless I change the index). Is there a way to get Python to determine the correct index?

If you only have one camera, or you don't care which camera is the correct one, then use "-1" as the index. Ie for your example capture = cv.CaptureFromCAM(-1).

Try the following. It is simple, but I haven't figured out a graceful way to exit yet.

import cv2.cv as cv
import time

cv.NamedWindow("camera", 0)

capture = cv.CaptureFromCAM(0)

while True:
    img = cv.QueryFrame(capture)
    cv.ShowImage("camera", img)
    if cv.WaitKey(10) == 27:
        break
cv.DestroyAllWindows()

An update to show how to do it in the recent versions of OpenCV:

import cv2

cv2.namedWindow("preview")
vc = cv2.VideoCapture(0)

if vc.isOpened(): # try to get the first frame
    rval, frame = vc.read()
else:
    rval = False

while rval:
    cv2.imshow("preview", frame)
    rval, frame = vc.read()
    key = cv2.waitKey(20)
    if key == 27: # exit on ESC
        break

cv2.destroyWindow("preview")
vc.release()

It works in OpenCV-2.4.2 for me.

change import cv to import cv2.cv as cv See also the post here.

As in the opencv-doc you can get video feed from a camera which is connected to your computer by following code.

import numpy as np
import cv2
cap = cv2.VideoCapture(0)
while(True):
    # Capture frame-by-frame
    ret, frame = cap.read()
    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    # Display the resulting frame
    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break
# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

You can change cap = cv2.VideoCapture(0) index from 0 to 1 to access the 2nd camera.
Tested in opencv-3.2.0