OpenCV+Python Part 1–Working with Videos

Learn how to load, display and save videos. I’ll explain this using the code snippets.The following program captures a video from the camera (I am using the in-built webcam of my laptop) and displays it.

import numpy as np
import cv2

x = cv2.VideoCapture(0)

while(True):
# Capture frame-by-frame
ret, frame = x.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
x.release()
cv2.destroyAllWindows()

The first thing that we need to do is to create a Video Capture object ‘x’.The argument passed to it is either the Device Index(a number to specify which camera) or the name of a video file. Normally since only one camera is connected to the system a 0 is passed. To select a second camera you can pass a 1 and so on.

cap.read() checks if the frame is read correctly and returns a boolean value.

Next lets play a video from a file.

First of all–
Go to : OpenCV\3rdparty\ffmpeg\
Copy the dll files opencv_ffmpeg.dll or opencv_ffmpeg_64.dll (depending on your system architecture) and paste them into C:\Python27\

Now rename both these to opencv_ffmpeg24x.dll or opencv_ffmpeg24x_64.dll where x is the version of opencv you are using. For example I am using OpenCV 2.4.6 so I renamed them
opencv_ffmpeg246.dll or opencv_ffmpeg246_64.dll.

Then using the following code snippet you can play any video from the current directory.

import numpy as np
import cv2

cap = cv2.VideoCapture('video.mp4')

while(cap.isOpened()):
ret, frame = cap.read()
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
cv2.imshow('frame',gray)
if cv2.waitKey(1) & 0xFF == ord('q'):
break

cap.release()
cv2.destroyAllWindows()

Now the final step is to save a video from a cam.
The code captures from a Camera, flips every frame vertically and saves the video.

import numpy as np
import cv2

cap = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object
fourcc = cv2.cv.CV_FOURCC(*'XVID')
out = cv2.VideoWriter('output.avi',fourcc, 20.0, (640,480))

while(cap.isOpened()):
ret, frame = cap.read()
if ret==True:
frame = cv2.flip(frame,0)

# write the flipped frame
out.write(frame)

cv2.imshow('frame',frame)
if cv2.waitKey(1) & 0xFF == ord('q'):
break
else:
break

# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()


For images we can easily used the function cv2.imwrite().However with videos it gets a bit tough.
Now along with a VideoCapture object we create a VideoWriter object where the arguments are the following:
1.)The name of the output file.
2.)Then we specify the FourCC code.FourCC is a 4-byte code used to specify the video codec.
Download the codec file for windows from — FourCC Codec.
3.)The number of frames per second(fps).
4.)The frame size.

That is all in this post..! All the best.

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