Python：opencv播放视频及保存视频操作

代码已上传至：https://gitee.com/tqbx/python-opencv/tree/master/Getting_started_videos

目标

• 学习读取视频，播放视频，保存视频。
• 学习从相机中捕捉帧并展示。
• 学习cv2.VideoCapture()，cv2.VideoWriter()的使用

从相机中捕捉视频

通过自带摄像头捕捉视频，并将其转化为灰度视频显示出来。

基本步骤如下：

1. 首先创建一个VideoCapture对象，它的参数包含两种：
   • 设备索引，指定摄像机的编号。
   • 视频文件的名称。
2. 逐帧捕捉。
3. 释放捕捉物。

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()返回布尔值，如果frame读取正确，为True，可以通过这个值判断视频是否已经结束。
• 有时，cap可能会初始化捕获失败，可以通过cap.isOpened()来检查其是否被初始化，如果为True那是最好，如果不是，可以使用cap.open()来尝试打开它。
• 当然，你可以使用cap.get(propId)的方式获取视频的一些属性，如帧的宽度，帧的高度，帧速等。propId是0-18的数字，每个数字代表一个属性，对应关系见底部附录。
• 既然可以获取，当然也可以尝试设置，假设想要设置帧的宽度和高度为320和240：cap.set(3,320),cap.set(4,240)。

从文件中播放视频

代码和从相机中捕获视频基本相同，不同之处在于传入VideoCapture的参数，此时传入视频文件的名称。

在显示每一帧的时候，可以使用cv2.waitKey()设置适当的时间，如果值很小，视频将会很快。正常情况下，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()

保存视频

1. 创建一个VideoWriter 对象，指定如下参数：
   • 输出的文件名，如output.avi。
   • FourCC code。
   • 每秒的帧数fps。
   • 帧的size。
2. FourCC code传递有两种方式：
   • fourcc = cv2.VideoWriter_fourcc(*'XVID')
   • fourcc = cv2.VideoWriter_fourcc('X','V','I','D')
3. FourCC是一个用于指定视频编解码器的4字节代码。

• 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()

附录

• 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)

参考阅读

Getting Started with Videos

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