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 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

User · Answer

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.

User · Answer

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   amp  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

[python] Displaying a webcam feed using OpenCV and Python

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