OpenCV Python - Capture Video from Camera

By using the VideoCapture() function in OpenCV library, it is very easy to capture a live stream from a camera on the OpenCV window.

This function needs a device index as the parameter. Your computer may have multiple cameras attached. They are enumerated by an index starting from 0 for built-in webcam. The function returns a VideoCapture object

cam = cv.VideoCapture(0)

After the camera is opened, we can read successive frames from it with the help of read() function

ret,frame = cam.read()

The read() function reads the next available frame and a return value (True/False). This frame is now rendered in desired color space with the cvtColor() function and displayed on the OpenCV window.

img = cv.cvtColor(frame, cv.COLOR_BGR2RGB)
# Display the resulting frame
cv.imshow('frame', img)

To capture the current frame to an image file, you can use imwrite() function.

cv2.imwrite(“capture.png”, img)

To save the live stream from camera to a video file, OpenCV provides a VideoWriter() function.

cv.VideoWriter( filename, fourcc, fps, frameSize)

The fourcc parameter is a standardized code for video codecs. OpenCV supports various codecs such as DIVX, XVID, MJPG, X264 etc. The fps anf framesize parameters depend on the video capture device.

The VideoWriter() function returns a VideoWrite stream object, to which the grabbed frames are successively written in a loop. Finally, release the frame and VideoWriter objects to finalize the creation of video.

Example

Following example reads live feed from built-in webcam and saves it to ouput.avi file.

import cv2 as cv
cam = cv.VideoCapture(0)
cc = cv.VideoWriter_fourcc(*'XVID')
file = cv.VideoWriter('output.avi', cc, 15.0, (640, 480))
if not cam.isOpened():
   print("error opening camera")
   exit()
while True:
   # Capture frame-by-frame
   ret, frame = cam.read()
   # if frame is read correctly ret is True
   if not ret:
      print("error in retrieving frame")
      break
   img = cv.cvtColor(frame, cv.COLOR_BGR2RGB)
   cv.imshow('frame', img)
   file.write(img)

   
   if cv.waitKey(1) == ord('q'):
      break

cam.release()
file.release()
cv.destroyAllWindows()
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