程式碼已上傳至：https://gitee.com/tqbx/python-opencv/tree/master/Getting_started_videos

目標

學習讀取視訊，播放視訊，儲存視訊。
學習從相機中捕捉幀並展示。
學習cv2.VideoCapture()，cv2.VideoWriter()的使用

從相機中捕捉視訊

通過自帶攝像頭捕捉視訊，並將其轉化為灰度視訊顯示出來。

基本步驟如下：

1.首先建立一個VideoCapture物件，它的引數包含兩種：

• 裝置索引，指定攝像機的編號。
• 視訊檔案的名稱。
  

2.逐幀捕捉。

3.釋放捕捉物。

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 frame is read correctly ret is True
  if not ret:
    print("Can't receive frame (stream end?). Exiting ...")
    break
  # Our operations on the frame come here
  gray = cv.cvtColor(frame,cv.COLOR_BGR2GRAY)
  # Display the resulting frame
  cv.imshow('frame',gray)
  if cv.waitKey(1) == ord('q'):
    break
# When everything done,release the capture
cap.release()
cv.destroyAllWindows()

其他：

• cap.read()返回布林值，如果frame讀取正確，為True，可以通過這個值判斷視訊是否已經結束。
• 有時，cap可能會初始化捕獲失敗，可以通過cap.isOpened()來檢查其是否被初始化，如果為True那是最好，如果不是，可以使用cap.open()來嘗試開啟它。
• 當然，你可以使用cap.get(propId)的方式獲取視訊的一些屬性，如幀的寬度，幀的高度，幀速等。propId是0-18的數字，每個數字代表一個屬性，對應關係見底部附錄。
• 既然可以獲取，當然也可以嘗試設定，假設想要設定幀的寬度和高度為320和240：cap.set(3,320),cap.set(4,240)
  。
  

從檔案中播放視訊

程式碼和從相機中捕獲視訊基本相同，不同之處在於傳入VideoCapture的引數，此時傳入視訊檔案的名稱。

在顯示每一幀的時候，可以使用cv2.waitKey()設定適當的時間，如果值很小，視訊將會很快。正常情況下，25ms就ok。

import numpy as np
import cv2

cap = cv2.VideoCapture('vtest.avi')

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()

儲存視訊

1.建立一個VideoWriter 物件，指定如下引數：

• 輸出的檔名，如output.avi。
• FourCC code。
• 每秒的幀數fps。
• 幀的size。
  

2.FourCC code傳遞有兩種方式：

• fourcc = cv2.VideoWriter_fourcc(*'XVID')
• fourcc = cv2.VideoWriter_fourcc('X','V','I','D')
  

3.FourCC是一個用於指定視訊編解碼器的4位元組程式碼。

• In Fedora: DIVX,XVID,MJPG,X264,WMV1,WMV2. (XVID is more preferable. MJPG results in high size video. X264 gives very small size video)
• In Windows: DIVX (More to be tested and added)
• In OSX : (I don't have access to OSX. Can some one fill this?)

import numpy as np
import cv2

cap = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object
fourcc = cv2.VideoWriter_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()

附錄

• CV_CAP_PROP_POS_MSEC Current position of the video file in milliseconds or video capture timestamp.
• CV_CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.
• CV_CAP_PROP_POS_AVI_RATIO Relative position of the video file: 0 - start of the film,1 - end of the film.
• CV_CAP_PROP_FRAME_WIDTH Width of the frames in the video stream.
• CV_CAP_PROP_FRAME_HEIGHT Height of the frames in the video stream.
• CV_CAP_PROP_FPS Frame rate.
• CV_CAP_PROP_FOURCC 4-character code of codec.
• CV_CAP_PROP_FRAME_COUNT Number of frames in the video file.
• CV_CAP_PROP_FORMAT Format of the Mat objects returned by retrieve() .
• CV_CAP_PROP_MODE Backend-specific value indicating the current capture mode.
• CV_CAP_PROP_BRIGHTNESS Brightness of the image (only for cameras).
• CV_CAP_PROP_CONTRAST Contrast of the image (only for cameras).
• CV_CAP_PROP_SATURATION Saturation of the image (only for cameras).
• CV_CAP_PROP_HUE Hue of the image (only for cameras).
• CV_CAP_PROP_GAIN Gain of the image (only for cameras).
• CV_CAP_PROP_EXPOSURE Exposure (only for cameras).
• CV_CAP_PROP_CONVERT_RGB Boolean flags indicating whether images should be converted to RGB.
• CV_CAP_PROP_WHITE_BALANCE_U The U value of the whitebalance setting (note: only supported by DC1394 v 2.x backend currently)
• CV_CAP_PROP_WHITE_BALANCE_V The V value of the whitebalance setting (note: only supported by DC1394 v 2.x backend currently)
• CV_CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently)
• CV_CAP_PROP_ISO_SPEED The ISO speed of the camera (note: only supported by DC1394 v 2.x backend currently)
• CV_CAP_PROP_BUFFERSIZE Amount of frames stored in internal buffer memory (note: only supported by DC1394 v 2.x backend currently)
  

參考閱讀

Getting Started with Videos

作者：天喬巴夏丶
出處：https://www.cnblogs.com/summerday152/
本文已收錄至Gitee：https://gitee.com/tqbx/JavaBlog
若有興趣，可以來參觀本人的個人小站：https://www.hyhwky.com

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