OpenCV C++ 学习笔记（四）：图像/视频的输入输出（highgui模块 高层GUI和媒体I/O）

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图片读取

cv::Mat imread( const String& filename, int flags = IMREAD_COLOR );

enum ImreadModes {IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.IMREAD_GRAYSCALE            = 0,  //!< If set, always convert image to the single channel grayscale image (codec internal conversion).IMREAD_COLOR                = 1,  //!< If set, always convert image to the 3 channel BGR color image.IMREAD_ANYDEPTH             = 2,  //!< If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.IMREAD_ANYCOLOR             = 4,  //!< If set, the image is read in any possible color format.IMREAD_LOAD_GDAL            = 8,  //!< If set, use the gdal driver for loading the image.IMREAD_REDUCED_GRAYSCALE_2  = 16, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/2.IMREAD_REDUCED_COLOR_2      = 17, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.IMREAD_REDUCED_GRAYSCALE_4  = 32, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/4.IMREAD_REDUCED_COLOR_4      = 33, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.IMREAD_REDUCED_GRAYSCALE_8  = 64, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/8.IMREAD_REDUCED_COLOR_8      = 65, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.IMREAD_IGNORE_ORIENTATION   = 128 //!< If set, do not rotate the image according to EXIF's orientation flag.
};

创建窗口

void namedWindow(const String& winname, int flags = WINDOW_AUTOSIZE);

图片显示

void imshow(const String& winname, InputArray mat);

输入窗口名称和要显示的图像

若窗体未创建，会自动进行创建

CV_EXPORTS_W int waitKey(int delay = 0);

控制图片的展示时间，如设置delay=0，则表示一直展示，按SPACE停止展示。如设置delay不为0，则表示停留delay毫秒。

图片保存

bool imwrite( const String& filename, InputArray img,const std::vector<int>& params = std::vector<int>());

视频输入输出

explicit VideoCapture::VideoCapture(const String& filename, int apiPreference = CAP_ANY);
explicit VideoCapture::VideoCapture(const String& filename, int apiPreference, const std::vector<int>& params);
explicit VideoCapture::VideoCapture(int index, int apiPreference = CAP_ANY);
explicit VideoCapture::VideoCapture(int index, int apiPreference, const std::vector<int>& params);

cv::VideoWriter videoWriter;VideoWriter::VideoWriter(const String& filename, int fourcc, double fps,Size frameSize, bool isColor = true);
VideoWriter::VideoWriter(const String& filename, int fourcc, double fps, const Size& frameSize,const std::vector<int>& params);
VideoWriter::VideoWriter(const String& filename, int apiPreference, int fourcc, double fps,const Size& frameSize, const std::vector<int>& params);//构造函数
VideoWriter(const string& filename, 	 //输出文件名int fourcc, 				//编码形式。使用CV_FOURCC()宏double fps,					//输出视频帧率cv::Size frameSize,			 //单帧图片大小 bool isColor = true);			 //是否保存为彩色图像，false可传入灰度图像
//Open方法
virtual bool open(const string& filename, int fourcc, double fps,Size frameSize, bool isColor = true);
//写入帧
virtual void write(const Mat& image);
virtual VideoWriter& operator << (const Mat& image);//原帧无压缩
videoWriter.open("d:/output.avi", CV_FOURCC('D', 'I', 'B', ' '), 30, cv::Size(1024, 1024), false);	
//MPEG-4 编码压缩
videoWriter.open("d:/output1.avi", CV_FOURCC('D', 'I', 'V', 'X'), 30, cv::Size(1024, 1024), false);	

参数说明：

• filename

  • 影片档案名称（例如video.avi）
  • 图片序列（例如img_00.jpg, img_01.jpg, img_02.jpg, …）
  • 视频流的网址。每个视频流或IP摄像机源均具有其自己的URL方案。请参考源流的文档以了解正确的URL。

• index

  • 要打开的视频捕获设备的ID。要打开默认摄像头，只需传递0。
  • 当apiPreference为CAP_ANY时，使用camera_id + domain_offset（CAP_ *）向后兼容有效。

• apiPreference（not important）

  • 首选使用的Capture API后端。如果有多个可用的读取器实现，则可以用于实施特定的读取器实现。
  • 设置读取的摄像头编号，默认CAP_ANY=0,自动检测摄像头。多个摄像头时，使用索引0,1,2，…进行编号调用摄像头。 apiPreference = -1时单独出现窗口，选取相应编号摄像头。

• fps：帧率

• frameSize：输出视频中每一帧的尺寸

• fourcc
  用于编码视频文件的编码器，通过VideoWriter::fourcc函数获得

CV_WRAP static int fourcc(char c1, char c2, char c3, char c4);

代码	含义
VideoWriter::fourcc('M', 'P', '4', 'V')	MPEG-4编码，输出文件拓展名mp4
VideoWriter::fourcc( 'X' , '2' , '6' , '4' )	MPEG-4编码，输出文件拓展名mp4
VideoWriter::fourcc('P','I','M','1')	MPEG-1编码，输出文件拓展名avi
VideoWriter::fourcc('X','V','I','D')	MPEG-4编码，输出文件拓展名avi
VideoWriter::fourcc('I','4','2','0')	YUV编码，输出文件拓展名avi
VideoWriter::fourcc('T','H','E','O')	ogg vorbis编码，输出文件拓展名ogv
VideoWriter::fourcc('F',L','V','1')	flash video编码，输出文件拓展名flv

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示例

VideoCapture video("demo.mp4");
Mat fps;
video.read(fps);
VideoWriter video_out("demo_out.avi", VideoWriter::fourcc('P','I','M','1'), 30, fps.size());while (1){Mat fps;video>>fps;//video.read(fps);fps>>video_out;//video_out.write(fps);imshow("video", fps);waitKey(10);//控制帧率}