Linux下OpenCV打开USB接口的UVC摄像头的方法以及索引号为什么要是202的原因

一般来说，USB的UVC摄像头在LINUX平台下通过V4L驱动来使用。OpenCV打开USB的UVC摄像头一般也通过V4L驱动来打开。打开方法网上都有，一般就是通过VideoCapture的open（int index）函数来打开，其中index设为202。如下：

VideoCapture cap;Mat frame;if(!cap.open(202))    cout<<"camera error!"<<endl;while(1){    cap>>frame;//do what you want here//put your code here}

注意这个打开的索引号，只有设置为202才能打开UVC摄像头，设置成其他的往往都无法开UVC摄像头，和windows平台不一样。那么，为什么UVC摄像头的索引需要设为202呢？我们可以看下Opencv的源码，就可以明白。

CV_IMPL CvCapture * cvCreateCameraCapture (int index){    // interpret preferred interface (0 = autodetect)    int pref = (index / 100) * 100;    // remove pref from index    index -= pref;    // local variable to memorize the captured device    CvCapture *capture = 0;    switch (pref)    {    default:        // user specified an API we do not know        // bail out to let the user know that it is not available        if (pref) break;#ifdef HAVE_MSMF    case CV_CAP_MSMF:        if (!capture)            capture = cvCreateCameraCapture_MSMF(index);        if (pref) break;#endif#ifdef HAVE_TYZX    case CV_CAP_STEREO:        if (!capture)            capture = cvCreateCameraCapture_TYZX(index);        if (pref) break;#endif    case CV_CAP_VFW:#ifdef HAVE_VFW        if (!capture)            capture = cvCreateCameraCapture_VFW(index);#endif#if defined HAVE_LIBV4L || defined HAVE_CAMV4L || defined HAVE_CAMV4L2 || defined HAVE_VIDEOIO        if (!capture)            capture = cvCreateCameraCapture_V4L(index);#endif#ifdef HAVE_GSTREAMER        if (!capture)            capture = cvCreateCapture_GStreamer(CV_CAP_GSTREAMER_V4L2,                                                reinterpret_cast<char *>(index));        if (!capture)            capture = cvCreateCapture_GStreamer(CV_CAP_GSTREAMER_V4L,                                                reinterpret_cast<char *>(index));#endif        if (pref) break; // CV_CAP_VFW    case CV_CAP_FIREWIRE:#ifdef HAVE_DC1394_2        if (!capture)            capture = cvCreateCameraCapture_DC1394_2(index);#endif#ifdef HAVE_DC1394        if (!capture)            capture = cvCreateCameraCapture_DC1394(index);#endif#ifdef HAVE_CMU1394        if (!capture)            capture = cvCreateCameraCapture_CMU(index);#endif#if defined(HAVE_GSTREAMER) && 0        // Re-enable again when gstreamer 1394 support will land in the backend code        if (!capture)            capture = cvCreateCapture_GStreamer(CV_CAP_GSTREAMER_1394, 0);#endif        if (pref) break; // CV_CAP_FIREWIRE#ifdef HAVE_MIL    case CV_CAP_MIL:        if (!capture)            capture = cvCreateCameraCapture_MIL(index);        if (pref) break;#endif#if defined(HAVE_QUICKTIME) || defined(HAVE_QTKIT)    case CV_CAP_QT:        if (!capture)            capture = cvCreateCameraCapture_QT(index);        if (pref) break;#endif#ifdef HAVE_UNICAP    case CV_CAP_UNICAP:        if (!capture)            capture = cvCreateCameraCapture_Unicap(index);        if (pref) break;#endif#ifdef HAVE_PVAPI    case CV_CAP_PVAPI:        if (!capture)            capture = cvCreateCameraCapture_PvAPI(index);        if (pref) break;#endif#ifdef HAVE_OPENNI    case CV_CAP_OPENNI:        if (!capture)            capture = cvCreateCameraCapture_OpenNI(index);        if (pref) break;#endif#ifdef HAVE_OPENNI2    case CV_CAP_OPENNI2:        if (!capture)            capture = cvCreateCameraCapture_OpenNI2(index);        if (pref) break;#endif#ifdef HAVE_XIMEA    case CV_CAP_XIAPI:        if (!capture)            capture = cvCreateCameraCapture_XIMEA(index);        if (pref) break;#endif#ifdef HAVE_AVFOUNDATION    case CV_CAP_AVFOUNDATION:        if (!capture)            capture = cvCreateCameraCapture_AVFoundation(index);        if (pref) break;#endif#ifdef HAVE_GIGE_API    case CV_CAP_GIGANETIX:        if (!capture)            capture = cvCreateCameraCapture_Giganetix(index);        if (pref) break; // CV_CAP_GIGANETIX#endif    }    return capture;}

上面是OPENCV的打开摄像头的中间一层的源码。注意其中开头的代码

int pref = (index / 100) * 100;    // remove pref from index    index -= pref;

可见，pref是索引号中百的倍数，index变为了原index的个位数。而此函数根据pref来选择底层驱动打开摄像头索引号，个位数是最终的摄像头索引号。

然后我们再看

enum{    CV_CAP_ANY      =0,     // autodetect    CV_CAP_MIL      =100,   // MIL proprietary drivers    CV_CAP_VFW      =200,   // platform native    CV_CAP_V4L      =200,    CV_CAP_V4L2     =200,    CV_CAP_FIREWARE =300,   // IEEE 1394 drivers    CV_CAP_FIREWIRE =300,    CV_CAP_IEEE1394 =300,    CV_CAP_DC1394   =300,    CV_CAP_CMU1394  =300,    CV_CAP_STEREO   =400,   // TYZX proprietary drivers    CV_CAP_TYZX     =400,    CV_TYZX_LEFT    =400,    CV_TYZX_RIGHT   =401,    CV_TYZX_COLOR   =402,    CV_TYZX_Z       =403,    CV_CAP_QT       =500,   // QuickTime    CV_CAP_UNICAP   =600,   // Unicap drivers    CV_CAP_DSHOW    =700,   // DirectShow (via videoInput)    CV_CAP_MSMF     =1400,  // Microsoft Media Foundation (via videoInput)    CV_CAP_PVAPI    =800,   // PvAPI, Prosilica GigE SDK    CV_CAP_OPENNI   =900,   // OpenNI (for Kinect)    CV_CAP_OPENNI_ASUS =910,   // OpenNI (for Asus Xtion)    CV_CAP_ANDROID  =1000,  // Android - not used    CV_CAP_ANDROID_BACK =CV_CAP_ANDROID+99, // Android back camera - not used    CV_CAP_ANDROID_FRONT =CV_CAP_ANDROID+98, // Android front camera - not used    CV_CAP_XIAPI    =1100,   // XIMEA Camera API    CV_CAP_AVFOUNDATION = 1200,  // AVFoundation framework for iOS (OS X Lion will have the same API)    CV_CAP_GIGANETIX = 1300,  // Smartek Giganetix GigEVisionSDK    CV_CAP_INTELPERC = 1500, // Intel Perceptual Computing    CV_CAP_OPENNI2 = 1600,   // OpenNI2 (for Kinect)    CV_CAP_GPHOTO2 = 1700,    CV_CAP_GSTREAMER = 1800, // GStreamer    CV_CAP_FFMPEG = 1900,    // FFMPEG    CV_CAP_IMAGES = 2000     // OpenCV Image Sequence (e.g. img_%02d.jpg)};

CV_CAP_VFW,CV_CAP_V4L,CV_CAP_V4L2都是200。所以，看到下面部分，只有当输入的索引号为200~299时，opencv才会用V4L相关的驱动去打开摄像头。

而在LINUX平台上，USB摄像头的节点一般为/dev/video2。因此索引号一般为2。因此，最终输入OPENCV的索引号应当为202，OpenCV才能正确的打开USB的UVC摄像头。