OpenCV findContours issue

Question

I have the following code which performs background subtraction and then uses findContours to draw a boundary around the foreground object.

// 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();

}

The image is just a blank white background but findContours() is returning a contour with the 4 edges of the image. This ends up being the largest contour found, negating my logic in the code.Is there anyway to fix this ? I want it to return a null vector when the screen is blank.

Also, can this code be improved in anyway to improve efficiency ?

Answer 1

Your background should be black (0) and any object you want to contour should be white( or >= 1). You have it reversed and that's why FindContours detects the background as a contour and not the object.