OpenCV MJPG格式高速读取

更新声明：Ubuntu下测试可以直接到120fps，设定60fps读出来也是120…
大部分应用场景要求很强的实时性，所谓实时，不一定是“及时”，而是周期确定，在一般的场景还要求周期尽可能短（如伺服系统）
查阅相机手册，发现MJPG格式可以高速输出高分辨率图像，因此读取时设定这个格式。查阅很多blog，可能是版本问题，函数参数有所不同，一直没有work，特此分享自己开发环境下的设置方法。

   if (capture.isOpened())capture.release();     //decide whether capture is already opened; if so,close itcapture.open(0);           //open the default cameracapture.set(CAP_PROP_FOURCC, VideoWriter::fourcc('M', 'J', 'P', 'G')) ;capture.set(CAP_PROP_FPS, 60);capture.set(CAP_PROP_FRAME_WIDTH, ImgWidth);capture.set(CAP_PROP_FRAME_HEIGHT, ImgHeight);

与此同时，给出其他参数的设置指南，以下代码摘抄自OpenCV - videoio.hpp


/** @brief %VideoCapture generic properties identifier.Reading / writing properties involves many layers. Some unexpected result might happens along this chain.Effective behaviour depends from device hardware, driver and API Backend.@sa videoio_flags_others, VideoCapture::get(), VideoCapture::set()
*/
enum VideoCaptureProperties {CAP_PROP_POS_MSEC       =0, //!< Current position of the video file in milliseconds.CAP_PROP_POS_FRAMES     =1, //!< 0-based index of the frame to be decoded/captured next.CAP_PROP_POS_AVI_RATIO  =2, //!< Relative position of the video file: 0=start of the film, 1=end of the film.CAP_PROP_FRAME_WIDTH    =3, //!< Width of the frames in the video stream.CAP_PROP_FRAME_HEIGHT   =4, //!< Height of the frames in the video stream.CAP_PROP_FPS            =5, //!< Frame rate.CAP_PROP_FOURCC         =6, //!< 4-character code of codec. see VideoWriter::fourcc .CAP_PROP_FRAME_COUNT    =7, //!< Number of frames in the video file.CAP_PROP_FORMAT         =8, //!< Format of the %Mat objects returned by VideoCapture::retrieve().CAP_PROP_MODE           =9, //!< Backend-specific value indicating the current capture mode.CAP_PROP_BRIGHTNESS    =10, //!< Brightness of the image (only for those cameras that support).CAP_PROP_CONTRAST      =11, //!< Contrast of the image (only for cameras).CAP_PROP_SATURATION    =12, //!< Saturation of the image (only for cameras).CAP_PROP_HUE           =13, //!< Hue of the image (only for cameras).CAP_PROP_GAIN          =14, //!< Gain of the image (only for those cameras that support).CAP_PROP_EXPOSURE      =15, //!< Exposure (only for those cameras that support).CAP_PROP_CONVERT_RGB   =16, //!< Boolean flags indicating whether images should be converted to RGB.CAP_PROP_WHITE_BALANCE_BLUE_U =17, //!< Currently unsupported.CAP_PROP_RECTIFICATION =18, //!< Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently).CAP_PROP_MONOCHROME    =19,CAP_PROP_SHARPNESS     =20,CAP_PROP_AUTO_EXPOSURE =21, //!< DC1394: exposure control done by camera, user can adjust reference level using this feature.CAP_PROP_GAMMA         =22,CAP_PROP_TEMPERATURE   =23,CAP_PROP_TRIGGER       =24,CAP_PROP_TRIGGER_DELAY =25,CAP_PROP_WHITE_BALANCE_RED_V =26,CAP_PROP_ZOOM          =27,CAP_PROP_FOCUS         =28,CAP_PROP_GUID          =29,CAP_PROP_ISO_SPEED     =30,CAP_PROP_BACKLIGHT     =32,CAP_PROP_PAN           =33,CAP_PROP_TILT          =34,CAP_PROP_ROLL          =35,CAP_PROP_IRIS          =36,CAP_PROP_SETTINGS      =37, //!< Pop up video/camera filter dialog (note: only supported by DSHOW backend currently. The property value is ignored)CAP_PROP_BUFFERSIZE    =38,CAP_PROP_AUTOFOCUS     =39,CAP_PROP_SAR_NUM       =40, //!< Sample aspect ratio: num/den (num)CAP_PROP_SAR_DEN       =41, //!< Sample aspect ratio: num/den (den)CAP_PROP_BACKEND       =42, //!< Current backend (enum VideoCaptureAPIs). Read-only propertyCAP_PROP_CHANNEL       =43, //!< Video input or Channel Number (only for those cameras that support)CAP_PROP_AUTO_WB       =44, //!< enable/ disable auto white-balanceCAP_PROP_WB_TEMPERATURE=45, //!< white-balance color temperature
#ifndef CV_DOXYGENCV__CAP_PROP_LATEST
#endif};

设置格式和分辨率那些一样，不知道填什么的时候就跳转过来看一下提示~
具体的数值，就看各位具体需求了！

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