OpenCV findContours問題

我有下面的代碼執行背景減法，然後使用findContours圍繞前景對象繪製邊界。OpenCV findContours問題

// frame - Input frame from a camera. 
// output - Output frame to be displayed. 
    void process(cv:: Mat &frame, cv:: Mat &output) { 

    cv::cvtColor(frame, gray, CV_BGR2GRAY); 

    // initialize background to 1st frame 
    if (background.empty()) 
     gray.convertTo(background, CV_32F); 

    // convert background to 8U 
    background.convertTo(backImage,CV_8U); 

    // compute difference between current image and background 
    cv::absdiff(backImage,gray,foreground); 

    // apply threshold to foreground image 
    cv::threshold(foreground,output,threshold,255,cv::THRESH_BINARY_INV); 

    // accumulate background 
    cv::accumulateWeighted(gray, background, learningRate, output); 

    // Find regions of interest 
    std::vector<std::vector<cv::Point> > v; // Detected foreground points 

    cv::findContours(output,v,CV_RETR_LIST,CV_CHAIN_APPROX_NONE); 

    // Sort to find the entry with the most points at the beginning. 
      // This is done to overcome noisy input. 
    std::sort(v.begin(), v.end(), DescendingCompare); 

    cv::Mat drawing = frame; 

    std::vector<std::vector<cv::Point>> contours_poly(1); 

    // Determine an approximate polygon for v[0] which is the largest contour 
    cv::approxPolyDP(cv::Mat(v[0]), contours_poly[0], 3, false); 

    // Draw polygonal contour 
    cv::Scalar color = cv::Scalar(0,0,255); 
    cv::drawContours(drawing, contours_poly, 0, color, 2, 8, std::vector<cv::Vec4i>(), 0, cv::Point()); 

    // Show in a window 
    output = drawing; 
    v.clear(); 

}

的圖像僅僅是一個空白白色背景，但findContours（）被返回與圖像的4個邊緣的輪廓。這最終成爲找到的最大輪廓，否定了我在代碼中的邏輯。無論如何，要解決這個問題嗎？我希望它在屏幕爲空時返回空矢量。