Python：opencv播放視頻及保存視頻操作

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並展示。
• 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，並將其轉化爲灰度視頻顯示出來。

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 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()返回布爾值，如果frame讀取正確，爲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，如果爲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，對應關係見底部附錄。
• 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爲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，不同之處在於傳入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 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()

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是一個用於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 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