cvQueryFrame返回null

Question

我正在尝试使用Windows上的CodeBlocks运行此OpenCV教程代码：

#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/imgproc/imgproc.hpp>

using namespace std;
using namespace cv;

// Function Headers
void detectAndDisplay( Mat frame );

// Global variables
String face_cascade_name = "haarcascade_frontalface_alt.xml";
String eyes_cascade_name = "haarcascade_eye_tree_eyeglasses.xml";
CascadeClassifier face_cascade;
CascadeClassifier eyes_cascade;
string window_name = "Capture - Face detection";
RNG rng(12345);

// @function main
int main( int argc, const char** argv ){
    CvCapture* capture;
    Mat frame;

    //-- 1. Load the cascades
    if( !face_cascade.load( face_cascade_name ) ){ std::cout << "--(!)Error loading Face cascade\n"; return -1; };
    if( !eyes_cascade.load( eyes_cascade_name ) ){ std::cout << "--(!)Error loading Eyes cascade\n"; return -1; };

   //-- 2. Read the video stream
    capture = cvCaptureFromCAM( 0 );
    if( capture ){
        while( true ){
            frame = cvQueryFrame( capture );

            //-- 3. Apply the classifier to the frame
            if( !frame.empty() ){
                detectAndDisplay( frame );
            }
            else{
                std::cout << " --(!) No captured frame -- Break!"; break;
            }
            int c = waitKey(10);
            if( (char)c == 'c' ) { break; }
        }
    }
    return 0;
}

// @function detectAndDisplay
void detectAndDisplay( Mat frame ){

    std::vector<Rect> faces;
    Mat frame_gray;

    cvtColor( frame, frame_gray, CV_BGR2GRAY );
    equalizeHist( frame_gray, frame_gray );

    //-- Detect faces
    face_cascade.detectMultiScale( frame_gray, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );

    for( size_t i = 0; i < faces.size(); i++ ){
        Point center( faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5 );
        ellipse( frame, center, Size( faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );

        Mat faceROI = frame_gray( faces[i] );
        std::vector<Rect> eyes;

        //-- In each face, detect eyes
        eyes_cascade.detectMultiScale( faceROI, eyes, 1.1, 2, 0 |CV_HAAR_SCALE_IMAGE, Size(30, 30) );

        for( size_t j = 0; j < eyes.size(); j++ ){
            Point center( faces[i].x + eyes[j].x + eyes[j].width*0.5, faces[i].y + eyes[j].y + eyes[j].height*0.5 );
            int radius = cvRound( (eyes[j].width + eyes[j].height)*0.25 );
            circle( frame, center, radius, Scalar( 255, 0, 0 ), 4, 8, 0 );
        }
    }
    //-- Show what you got
    imshow( window_name, frame );
 }

但输出是“ - （！）没有捕获的帧 - 打破！” 在执行过程中，相机闪光灯闪烁，然后显示输出。