Displaying a webcam feed using OpenCV and Python

Question

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

Answer 1

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).

Answer 2

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()

Answer 3

Try adding the line c = cv.WaitKey(10) at the bottom of your repeat() method.

This waits for 10 ms for the user to enter a key. Even if you're not using the key at all, put this in. I think there just needed to be some delay, so time.sleep(10) may also work.

In regards to the camera index, you could do something like this:

for i in range(3):
    capture = cv.CaptureFromCAM(i)
    if capture: break

This will find the index of the first "working" capture device, at least for indices from 0-2. It's possible there are multiple devices in your computer recognized as a proper capture device. The only way I know of to confirm you have the right one is manually looking at your light. Maybe get an image and check its properties?

To add a user prompt to the process, you could bind a key to switching cameras in your repeat loop:

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)
    cv.ShowImage("w1", frame)
    c = cv.WaitKey(10)
    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 = 0
            capture = cv.CaptureFromCAM(camera_index)

while True:
    repeat()

disclaimer: I haven't tested this so it may have bugs or just not work, but might give you at least an idea of a workaround.

Answer 4

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

Answer 5

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

Answer 6

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.