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

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

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

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

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

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 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

[python] Displaying a webcam feed using OpenCV and Python

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