Opencv video

Read video from camera

 Usually , We have to use cameras to capture real-time images .
 Provides a very simple interface .
 Let's capture a video from the camera ( I use the webcam built into my laptop ) ,
 Convert it to grayscale video and display it . Just a simple task to start .

 To capture video , You need to create one  VideoCapture  object .
 Its parameters can be the device index or the name of the video file .
 The device index is the number that specifies which camera .
 Under normal circumstances , A camera will be connected ( Like in my case ).
 So I simply pass 0( or -1).
 You can pass on 1 To choose a second camera , And so on .
 After that , You can capture... Frame by frame . But in the end , Don't forget to release the prisoners .

To use

cap = cv.VideoCapture(0, cv.CAP_DSHOW)

import numpy as np
import cv2 as cv
cap = cv.VideoCapture(0)
if not cap.isOpened():
    print("Cannot open camera")
    exit()
while True:
    #  Capture frame by frame 
    ret, frame = cap.read()
    #  If the frame is read correctly ,ret by True
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break
    #  Our operations on the framework are here 
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
    #  Show result frame e
    cv.imshow('frame', gray)
    if cv.waitKey(1) == ord('q'):
        break
#  After completing all operations , Release the catcher 
cap.release()
cv.destroyAllWindows()

**cap.read()** Returns a Boolean value (True/ False).
If the frame is read correctly , It will be True.
therefore , You can check the end of the video by checking this return value .

Sometimes ,cap Capture may not have been initialized .
under these circumstances , This code shows an error .
You can go through cap.isOpened() Method to check if it is initialized . If it is True, So be sure . otherwise , Use cap.open() To open it .

 You can still use it cap.get(propId) Method to access some functions of the video ,
 among propId yes 0 To 18 A number between .
 Each number represents the properties of the video （ If applicable to this video ）,
 And you can see the full details here ：cv::VideoCapture::get().
 Some of these values can be used cap.set(propId,value) Make changes .
value It's the new value you want .

for example , I can get through

cap.get(cv.CAP_PROP_FRAME_WIDTH)
cap.get(cv.CAP_PROP_FRAME_HEIGHT)

Check the width and height of the frame . By default , Its resolution is 640x480. But I want to change it to 320x240. Just use and .

cap.set(cv.CAP_PROP_FRAME_WIDTH,320) 
cap.set(cv.CAP_PROP_FRAME_HEIGHT,240).

Be careful If something goes wrong , Please make sure to use any other camera application ( for example Linux Medium Cheese) Can use the camera normally .

Play video from file

It's the same as capturing from a camera , Just change the camera index with the video file name . in addition , When displaying frames , Please use the right time cv.waitKey(). If it's too small , Then the video will be very fast , And if it's too big , Then the video will become very slow （ Um. , This is how slow motion is shown ）. Under normal circumstances 25 Millisecond is enough .

import numpy as np
import cv2 as cv
cap = cv.VideoCapture('vtest.avi')
while cap.isOpened():
    ret, frame = cap.read()
    #  If the frame is read correctly ,ret by True
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
    cv.imshow('frame', gray)
    if cv.waitKey(1) == ord('q'):
        break
cap.release()
cv.destroyAllWindows()

Be careful Make sure that the correct... Is installed ffmpeg or gstreamer edition . Sometimes , Use video capture (Video Capture) It's a headache , The main reason is that it was installed incorrectly ffmpeg / gstreamer.

Save the video

fourcc = cv.VideoWriter_fourcc('X', 'V', 'I', 'D')
out = cv.VideoWriter(r'C:\Users\to_bo\Desktop\2.avi',
					 fourcc, 10, (640, 480))
#cv.VideoWriter(filename', fourcc, fps, size)

So we capture a video , Frame by frame processing , We want to save this video . For the image , It's very simple , Just use cv.imwrite(). There is still work to be done here .

This time we create a VideoWriter object . We should specify the output filename ( for example : output.avi). Then we should specify FourCC Code ( See the next paragraph ). Then transfer the number of frame rates and frame sizes . The last one is the color mark . If True, Encoder expects color frame , Otherwise it works with grayscale frames .

FourCC：http://en.wikipedia.org/wiki/FourCC Is used to specify the video codec 4 Byte code . The list of available codes can be found in fourcc.org in :http://www.fourcc.org/codecs.php find . It depends on the platform . Following codecs works well for me .
stay Fedora in ：DIVX,XVID,MJPG,X264,WMV1,WMV2.
（ Best use XVID.MJPG It will generate large-scale video .X264 It will generate a very small size video ）
stay Windows in ：DIVX（ Yet to be tested and added ）
stay OSX in ：MJPG（.mp4）,DIVX（.avi）,X264（.mkv）.
FourCC Code as MJPG Of cv.VideoWriter_fourcc（‘M’,‘J’,‘P’,‘G’）or cv.VideoWriter_fourcc（*‘MJPG’） Pass on .

 use XVID

Under the code captured from the camera , Flip each frame vertically and save .

import numpy as np
import cv2 as cv
cap = cv.VideoCapture(0)
#  Define codecs and create VideoWriter object 
fourcc = cv.VideoWriter_fourcc(*'XVID')
out = cv.VideoWriter('output.avi', fourcc, 20.0, (640,  480))
while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break
    frame = cv.flip(frame, 0)
    #  Write the flipped frame 
    out.write(frame)
    cv.imshow('frame', frame)
    if cv.waitKey(1) == ord('q'):
        break
#  Release everything when you're done 
cap.release()
out.release()
cv.destroyAllWindows()

---

and then

---

Open local video

import cv2 as cv

cap = cv.VideoCapture(r'C:\Users\Desktop\1.mp4')

while cap.isOpened():
    ret, frame = cap.read()
    cv.namedWindow('frame', cv.WINDOW_NORMAL)
    cv.imshow('frame', frame)
    if cv.waitKey(1) & 0xFF == 27:
        break

cap.release()
cv.destroyAllWindows()

Save video from camera

import cv2 as cv

cap = cv.VideoCapture(0, cv.CAP_DSHOW)
fourcc = cv.VideoWriter_fourcc('X', 'V', 'I', 'D')
out = cv.VideoWriter(r'C:\Users\Desktop\2.avi', fourcc, 10, (640, 480))
if not cap.isOpened():
    print('Can not open camera')
    exit(0)

while True:
    ret, frame = cap.read()
    if not ret:
        print("Can not receive frame (stream end?). Exiting ...")
        break
    out.write(frame)
    cv.imshow('camera', frame)

    if cv.waitKey(1) & 0xFF == 27:
        break

cap.release()
out.release()
cv.destroyAllWindows()

opencv Study —VideoCapture Class Basics
【OpenCV】Python Video reading and saving
2_2_ Introduction to video