Vision Video

Getting started with video:
http://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_gui/py_video_display/py_video_display.html#display-video


The following example will load up a video from disk . It uses the VideoCapture object.

• Its argument can be either the device index or the name of a video file. Device usually is the index is just the number to specify which camera to use.  This line below sets the video source to use an existing file.

#------------------------------------------------------------
import numpy as np
import cv2 as cv
cap = cv.VideoCapture('output.mp4')
while(cap.isOpened()):
       ret, frame = cap.read()
       gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
       cv.imshow('frame',gray)
       if cv.waitKey(1) & 0xFF == ord('q'):
             break
cap.release()
cv.destroyAllWindows()

#------------------------------------------------------------

[link]  - tweak for VideoCapture

VideoWriter([filename, fourcc, fps, frameSize, isColor)

• filename – Name of the output video file.
• fourcc – 4-character code of codec used to compress the frames. 
• fps – Framerate of the created video stream.
• frameSize – Size of the video frames.
• isColor – If it is not zero, the encoder will expect and encode color frames, otherwise it will work with grayscale frames (the flag is currently supported on Windows only).

The read() function

• reads one frame from the video source, which in this example is the webcam. 

This returns two things:

• The actual video frame read (one frame on each loop) 
• A return code (True/False). If frame is read correctly.  It tells us if we have run out of frames, which will happen if we are reading from a file. This doesn’t matter when reading from the webcam, since we can record forever, so we will ignore it.

Reading from web camera:

#------------------------------------------------------------
while(True):  
     # Capture frame-by-frame  
     ret, frame = cap.read()  
    # frame come here  
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)  
 
    # Display the resulting frame  
    cv.imshow('frame',gray)  
    if cv.waitKey(1) & 0xFF == ord('q'):      
          break

# When everything done, release the capture
cap.release()
cv.destroyAllWindows()

#------------------------------------------------------------


Saving images from video camera input to a file:

#------------------------------------------------------------

import cv2
import numpy as np
#This line sets the video source to the default webcam, which OpenCV
cap = cv2.VideoCapture(0)
w = cap.get(cv2.CAP_PROP_FRAME_WIDTH);
h = cap.get(cv2.CAP_PROP_FRAME_HEIGHT);
fourcc = cv2.VideoWriter_fourcc(*'mp4v')
out = cv2.VideoWriter('output.mp4',fourcc, 15.0, (int(w),int(h)))

while True:
        ret,frame = cap.read()
        #---------for gray video--------
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
        out.write(frame)
        #Display the resulting frame
        cv2.imshow('myvideo',frame)
        cv2.imshow('myvideo',gray)
        cv2.waitKey(0)      
        k = cv2.waitKey(0) & 0xFF
        if k == ord('q'):    # wait for 'q' key to exit break
                break
cap.release()
out.release()
cv2.destroyAllWindows()

#------------------------------------------------------------