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
作者:天乔巴夏丶
出处:https://www.cnblogs.com/summerday152/
本文已收录至Gitee:https://gitee.com/tqbx/JavaBlog
若有兴趣,可以来参观本人的个人小站:https://www.hyhwky.com
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