opencv(2)---摄像头/视频读取_写入

VideoCapture类用于处理摄像头/视频读取_写入操作。

一 基本概念

VideoCapture

1.VideoCapture有三种构造方法

 /** @brief    @note In C API, when you finished working with video, release CvCapture structure with    cvReleaseCapture(), or use Ptr\<CvCapture\> that calls cvReleaseCapture() automatically in the    destructor.     */    CV_WRAP VideoCapture();    /** @overload    @param filename name of the opened video file (eg. video.avi) or image sequence (eg.    img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)    */    CV_WRAP VideoCapture(const String& filename);    /** @overload    @param filename name of the opened video file (eg. video.avi) or image sequence (eg.    img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)    @param apiPreference preferred Capture API to use. Can be used to enforce a specific reader    implementation if multiple are available: e.g. CAP_FFMPEG or CAP_IMAGES    */    CV_WRAP VideoCapture(const String& filename, int apiPreference);   /** @overload    @param index = camera_id + domain_offset (CAP_*). id of the video capturing device to open. If there is a single    camera connected, just pass 0. Advanced Usage: to open Camera 1 using the MS Media Foundation API: index = 1 + CAP_MSMF    */    CV_WRAP VideoCapture(int index);

2.两种取帧方法

   virtual VideoCapture& operator >> (CV_OUT Mat& image);

和

    CV_WRAP virtual bool read(OutputArray image);

3.常用函数

功能介绍:
1)open()—打开视频文件或者摄像头
2)isOpened()–判断读取视频文件是否正确，正确返回true
3)release()—关闭视频流文件
4)grab()—抓取下一帧的视频文件或设备
5)Retrieve()—解码并返回视频帧
6)get()—返回指定视频类的相关参数信息
7)set()—设置类信息的一个属性
4.视频文件的属性
cap.get(…)

   /** @brief Returns the specified VideoCapture property    @param propId Property identifier. It can be one of the following:     -   **CAP_PROP_POS_MSEC** Current position of the video file in milliseconds or video         capture timestamp.     -   **CAP_PROP_POS_FRAMES** 0-based index of the frame to be decoded/captured next.     -   **CAP_PROP_POS_AVI_RATIO** Relative position of the video file: 0 - start of the         film, 1 - end of the film.     -   **CAP_PROP_FRAME_WIDTH** Width of the frames in the video stream.     -   **CAP_PROP_FRAME_HEIGHT** Height of the frames in the video stream.     -   **CAP_PROP_FPS** Frame rate.     -   **CAP_PROP_FOURCC** 4-character code of codec.     -   **CAP_PROP_FRAME_COUNT** Number of frames in the video file.     -   **CAP_PROP_FORMAT** Format of the Mat objects returned by retrieve() .     -   **CAP_PROP_MODE** Backend-specific value indicating the current capture mode.     -   **CAP_PROP_BRIGHTNESS** Brightness of the image (only for cameras).     -   **CAP_PROP_CONTRAST** Contrast of the image (only for cameras).     -   **CAP_PROP_SATURATION** Saturation of the image (only for cameras).     -   **CAP_PROP_HUE** Hue of the image (only for cameras).     -   **CAP_PROP_GAIN** Gain of the image (only for cameras).     -   **CAP_PROP_EXPOSURE** Exposure (only for cameras).     -   **CAP_PROP_CONVERT_RGB** Boolean flags indicating whether images should be converted         to RGB.     -   **CAP_PROP_WHITE_BALANCE** Currently not supported     -   **CAP_PROP_RECTIFICATION** Rectification flag for stereo cameras (note: only supported         by DC1394 v 2.x backend currently)    @note When querying a property that is not supported by the backend used by the VideoCapture    class, value 0 is returned.     */    CV_WRAP virtual double get(int propId) const;

EX1:

int width=cap.get(CAP_PROP_FRAME_WIDTH);int height=cap.get(CAP_PROP_FRAME_HEIGHT);int frameRate=cap.get(CAP_PROP_FPS);int totalFrames=cap.get(CAP_PROP_FRAME_COUNT);cout<<"宽度:"<<width<<endl;cout<<"高度"<<height<<endl;cout<<"频率"<<frameRate<<endl;cout<<"视频帧数"<<totalFrames<<endl;

VideoWriter

1.构造函数
/** @brief VideoWriter constructors

The constructors/functions initialize video writers. On Linux FFMPEG is used to write videos; onWindows FFMPEG or VFW is used; on MacOSX QTKit is used. */CV_WRAP VideoWriter();/** @overload@param filename Name of the output video file.@param fourcc 4-character code of codec used to compress the frames. For example,VideoWriter::fourcc('P','I','M','1') is a MPEG-1 codec, VideoWriter::fourcc('M','J','P','G') is amotion-jpeg codec etc. List of codes can be obtained at [Video Codecs byFOURCC](http://www.fourcc.org/codecs.php) page. FFMPEG backend with MP4 container natively usesother values as fourcc code: see [ObjectType](http://www.mp4ra.org/codecs.html),so you may receive a warning message from OpenCV about fourcc code conversion.@param fps Framerate of the created video stream.@param frameSize Size of the video frames.@param isColor If it is not zero, the encoder will expect and encode color frames, otherwise itwill work with grayscale frames (the flag is currently supported on Windows only).*/CV_WRAP VideoWriter(const String& filename, int fourcc, double fps,            Size frameSize, bool isColor = true);

2.写入帧方式

1)writer.write(frame);2)writer<<frame;

3.注意事项
1)安装相应的编解码器
xvid.exe
2)生成视频是否支持彩色应与构造函数设置一致

  VideoWriter writer("D:\\out.avi",CV_FOURCC('P','I','M','1'),20,size0,false);

VideoWriter中的最后一个参数用于设置写入的视频格式是否为彩色

3)生成视频尺寸需与读取视频尺寸一致

 Size size0=Size(cap.get(CV_CAP_PROP_FRAME_WIDTH),cap.get(CV_CAP_PROP_FRAME_HEIGHT));    //定义了一个写入视频的对象 VideoWriter writer("D:\\out.avi",CV_FOURCC('P','I','M','1'),20,size0,false);

二 实例代码

读取视频

视频读取本质上就是读取帧，原因是视频是由帧组成的
1.实现功能:读取视频，并按照一定的频率进行播放

#include "mainwindow.h"#include<iostream>#include <opencv2/opencv.hpp>using namespace cv;using namespace std;MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent){    VideoCapture cap;    cap.open("D:\\1.avi");    //VideoCapture cap("1.avi");    if(!cap.isOpened())         return;     Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         if(frame.empty())             break;         imshow("video",frame);         waitKey(20);     }     cap.release();}MainWindow::~MainWindow(){}

读取摄像头

1. 简单读取
mainwindow.cpp

#include "mainwindow.h"#include<iostream>#include <opencv2/opencv.hpp>using namespace cv;using namespace std;MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent){    VideoCapture cap;    cap.open(0);    if(!cap.isOpened())         return;     Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         if(frame.empty())           break;         imshow("video",frame);         //用于关掉窗口         if(waitKey(20)>0)             break;     }     cap.release();     //销毁所有窗口     destroyAllWindows();}MainWindow::~MainWindow(){}

2. 图像处理

 Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         //Canny(frame,frame,30,200);         cvtColor(frame,frame,CV_BGR2HSV);         if(frame.empty())           break;         imshow("video",frame);         if(waitKey(20)>0)             break;     }

写入视频

1.未经处理直接写入

MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent){    VideoCapture cap;    Size size0=Size(320,240);    //定义了一个写入视频的对象    VideoWriter writer("D:\\out.avi",CV_FOURCC('P','I','M','1'),15,size0,true);   // VideoWriter writer("D:\\out.avi",-1,15,size0,true);    cap.open("D:\\1.avi");    if(!cap.isOpened())         return;     Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         //Canny(frame,frame,30,200);         //cvtColor(frame,frame,CV_BGR2HSV);         if(frame.empty())           break;         writer<<frame;         //writer.write(frame);         imshow("video",frame);         if(waitKey(20)>0)             break;     }     cout<<"Write endl"<<endl;     cap.release();     destroyAllWindows();}

2.经过图像处理再写入

MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent){    VideoCapture cap;    Size size0=Size(320,240);    //定义了一个写入视频的对象    VideoWriter writer("D:\\out.avi",CV_FOURCC('P','I','M','1'),20,size0,false);   // VideoWriter writer("D:\\out.avi",-1,15,size0,true);    cap.open("D:\\1.avi");    if(!cap.isOpened())         return;     Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         //Canny(frame,frame,30,200);         cvtColor(frame,frame,CV_BGR2GRAY);         if(frame.empty())           break;         writer<<frame;         //writer.write(frame);         imshow("video",frame);         if(waitKey(20)>0)             break;     }     cout<<"Write endl"<<endl;     cap.release();     destroyAllWindows();}

3.将摄像头读入的写入视频文件

MainWindow::MainWindow(QWidget *parent)    : QMainWindow(parent){    VideoCapture cap;    cap.open(0);    Size size0=Size(cap.get(CV_CAP_PROP_FRAME_WIDTH),cap.get(CV_CAP_PROP_FRAME_HEIGHT));    //定义了一个写入视频的对象    VideoWriter writer("D:\\out.avi",CV_FOURCC('P','I','M','1'),20,size0,false);   // VideoWriter writer("D:\\out.avi",-1,15,size0,true);    if(!cap.isOpened())         return;     Mat frame;     while(1){         //cap>>frame;         cap.read(frame);         //Canny(frame,frame,30,200);         cvtColor(frame,frame,CV_BGR2GRAY);         if(frame.empty())           break;         writer<<frame;         //writer.write(frame);         imshow("video",frame);         if(waitKey(20)>0)             break;     }     cout<<"Write endl"<<endl;     cap.release();     destroyAllWindows();}

三 知识点详解

读取视频

1.读取视频的两种方法
A.

   VideoCapture cap;   cap.open(“1.avi”);

B.

VideoCapture cap(“1.avi”);

2.循环显示每一帧

while(1){   Mat frame;  //定义Mat变量，用来存储每一帧   cap>>frame; //读取当前fd帧方法一 //cap.read(frame); //读取当前帧方法二   imshow(“视频显示”, frame); //显示一帧画面   waitKey(30); //延时30ms}

3.注意点
1)打开的视频可以是本地视频如:

VideoCapture cap("1.avi");

也可以是网络视频如：

VideoCapture cap("http://www.laganiere.name/bike.avi");

2)读取视频一定要加异常判断
A.打开视频失败(各种原因)
B. 取帧失败异常(程序结尾判断)
ex1:

if(!cap.isOpened()) //检查打开是否成功   return;

ex2:

cap>>frame;if(!frame.empty()) //判断当前 帧是否为空{   imshow("video", frame);   waitKey(30);}else  break;

4. waitKey
1)设置显示图像的频率
用于设置窗口video每隔20ms刷新一次

waitKey(20);

2)设置按键按下的事件
当按键为ESC时，退出运行

 if(waitKey(30)==27)               break;

当按下任意按键时，退出运行
原因是：没有按键按下，返回值为-1;有按键按下，返回值都大于0

if(waitKey(30)>0)   break;

读取摄像头

1.读取摄像头的方法

VideoCapture cap;cap.open(int index);

索引 index
1)如果默认笔记本/台式机只有一个USB摄像头,Index=0; 如果有2个，一般Index为0和1，根据具体情况区分，摄像头接入和断开会改变Index值
2)如果接入2个以上，但只想用指定的一个，可以在设备管理器中禁用其他，同时Index设置为0.
2.WaitKey
用waitKey()返回值判断退出预览

 imshow("video", frame); if(waitKey(30)>0)//无按键按下返回-1    break;

可以指定按键退出：

    if(waitKey(30)==27) //Esc键退出    break;