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

目標

學習讀取視頻，播放視頻，保存視頻。學習從相機中捕捉幀并展示。學習cv2.VideoCapture()，cv2.VideoWriter()的使用

從相機中捕捉視頻

通過自帶攝像頭捕捉視頻，并將其轉化為灰度視頻顯示出來。

基本步驟如下：

1.首先創建一個VideoCapture對象，它的參數包含兩種：

設備索引，指定攝像機的編號。 視頻文件的名稱。

2.逐幀捕捉。

3.釋放捕捉物。

import numpy as npimport cv2 as cvcap = 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 capturecap.release()cv.destroyAllWindows()

其他：

cap.read()返回布爾值，如果frame讀取正確，為True，可以通過這個值判斷視頻是否已經結束。 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()來嘗試打開它。 當然，你可以使用cap.get(propId)的方式獲取視頻的一些屬性，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，對應關系見底部附錄。 既然可以獲取，當然也可以嘗試設置，假設想要設置幀的寬度和高度為320和240：cap.set(3,320), cap.set(4,240)。

從文件中播放視頻

代碼和從相機中捕獲視頻基本相同，不同之處在于傳入VideoCapture的參數，此時傳入視頻文件的名稱。

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

import numpy as npimport cv2cap = 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’): breakcap.release()cv2.destroyAllWindows()

保存視頻

1.創建一個VideoWriter 對象，指定如下參數：

輸出的文件名，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是一個用于指定視頻編解碼器的4字節代碼。

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 npimport cv2cap = cv2.VideoCapture(0)# Define the codec and create VideoWriter objectfourcc = 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 finishedcap.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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