opencv打开摄像头与DShow相关

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由于Opencv默认的videocapture打开摄像头时，使用的默认方式，即直接填写id即可，如

cv::videocapture cap;

cap.open(1);

但是，如果项目中与dshow相关，dshow显示的摄像头顺序与opencv默认的打开顺序不同，则可以使用第二个参数与之对应，即

cv::videocapture cap;

cap.open(1,cv::CAP_DSHOW);

open的打开方式，具体内容可查看open函数本身

 CV_WRAP virtual bool open(int index, int apiPreference = CAP_ANY);

enum VideoCaptureAPIs {
       CAP_ANY          = 0,            //!< Auto detect == 0
       CAP_VFW          = 200,          //!< Video For Windows (obsolete, removed)
       CAP_V4L          = 200,          //!< V4L/V4L2 capturing support
       CAP_V4L2         = CAP_V4L,      //!< Same as CAP_V4L
       CAP_FIREWIRE     = 300,          //!< IEEE 1394 drivers
       CAP_FIREWARE     = CAP_FIREWIRE, //!< Same value as CAP_FIREWIRE
       CAP_IEEE1394     = CAP_FIREWIRE, //!< Same value as CAP_FIREWIRE
       CAP_DC1394       = CAP_FIREWIRE, //!< Same value as CAP_FIREWIRE
       CAP_CMU1394      = CAP_FIREWIRE, //!< Same value as CAP_FIREWIRE
       CAP_QT           = 500,          //!< QuickTime (obsolete, removed)
       CAP_UNICAP       = 600,          //!< Unicap drivers (obsolete, removed)
       CAP_DSHOW        = 700,          //!< DirectShow (via videoInput)
       CAP_PVAPI        = 800,          //!< PvAPI, Prosilica GigE SDK
       CAP_OPENNI       = 900,          //!< OpenNI (for Kinect)
       CAP_OPENNI_ASUS  = 910,          //!< OpenNI (for Asus Xtion)
       CAP_ANDROID      = 1000,         //!< Android - not used
       CAP_XIAPI        = 1100,         //!< XIMEA Camera API
       CAP_AVFOUNDATION = 1200,         //!< AVFoundation framework for iOS (OS X Lion will have the same API)
       CAP_GIGANETIX    = 1300,         //!< Smartek Giganetix GigEVisionSDK
       CAP_MSMF         = 1400,         //!< Microsoft Media Foundation (via videoInput)
       CAP_WINRT        = 1410,         //!< Microsoft Windows Runtime using Media Foundation
       CAP_INTELPERC    = 1500,         //!< RealSense (former Intel Perceptual Computing SDK)
       CAP_REALSENSE    = 1500,         //!< Synonym for CAP_INTELPERC
       CAP_OPENNI2      = 1600,         //!< OpenNI2 (for Kinect)
       CAP_OPENNI2_ASUS = 1610,         //!< OpenNI2 (for Asus Xtion and Occipital Structure sensors)
       CAP_GPHOTO2      = 1700,         //!< gPhoto2 connection
       CAP_GSTREAMER    = 1800,         //!< GStreamer
       CAP_FFMPEG       = 1900,         //!< Open and record video file or stream using the FFMPEG library
       CAP_IMAGES       = 2000,         //!< OpenCV Image Sequence (e.g. img_%02d.jpg)
       CAP_ARAVIS       = 2100,         //!< Aravis SDK
       CAP_OPENCV_MJPEG = 2200,         //!< Built-in OpenCV MotionJPEG codec
       CAP_INTEL_MFX    = 2300,         //!< Intel MediaSDK
       CAP_XINE         = 2400,         //!< XINE engine (Linux)
     };