VideoCaptrue类对视频进行读取操作以及调用摄像头。
头文件：
#include <opencv2/video.hpp>
主要函数如下：
构造函数
C++: VideoCapture::VideoCapture();
C++: VideoCapture::VideoCapture(const string& filename);
C++: VideoCapture::VideoCapture(int device);
参数:
filename -- 打开的视频文件名。
device -- 打开的视频捕获设备id ，如果只有一个摄像头可以填0，表示打开默认的摄像头。
基本功能
打开视频文件或者设备
C++: bool VideoCapture::open(const string& filename);
C++: bool VideoCapture::open(int device);
打开一个视频文件或者打开一个捕获视频的设备（也就是摄像头）
参数:
filename -- 打开的视频文件名。
device -- 打开的视频捕获设备id ，如果只有一个摄像头可以填0，表示打开默认的摄像头。
判断打开是否成功
C++: bool VideoCapture::isOpened();
成功返回true,，否则false.
关闭视频文件或者摄像头
C++: void VideoCapture::release();
抓取下一帧
C++: bool VideoCapture::grab();//需与retrieve结合使用
C++: bool VideoCapture::retrieve(Mat& image, int channel=0);
C++: VideoCapture& VideoCapture::operator>>(Mat& image);
C++: bool VideoCapture::read(Mat& image);
获取视频属性
C++: double VideoCapture::get(int propId);
如果属性不支持，将会返回0。
参数：属性的ID。
属性的ID可以是下面的之一：
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 Currently not supported
CV_CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently)
设置属性
bool VideoCapture::set(int propertyId, double value)
成功返回true，否则返回false
参数：第一个是属性ID，第二个是该属性要设置的值。
属性ID如下：
CV_CAP_PROP_POS_MSEC Current position of the video file in milliseconds.
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 Currently unsupported
CV_CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently)
实例：获取任意一帧
1 实例及初始化：
cv::VideoCapture capture.open("Old Film Effect.mp4");
2 设置需要读取帧的位置：
capture.set(cv::CAP_PROP_POS_FRAMES,10);//设置读取第10帧
3 读取帧
Mat frame;
if (capture.read(frame))
imwrite("d:/a.bmp",frame);