Optical Mark Recognition with DotImage

private Rectangle FindMarker(AtalaImage img)
{
    int numRowsToSearch = img.Height / 8; // marker is within this area
    int markerSizeThreshold = img.Width / 25; // marker is at least this big
    // eventual marker position
    int top = -1;
    int left = -1;
    int right = -1;
    int bottom = -1;

    // try to find the top of the marker, by looping through each row
    for (int y = 0; y < numRowsToSearch; ++y)
    {
        if (FindLine(img, y, markerSizeThreshold, ref left, ref right))
        {
            top = y;
            break;
        }
    }
    if (top == -1)
    {
        throw new Exception("Didn't find marker");
    }

    // find marker extents
    int expectedBottom = top + (right - left) + 10;
    bottom = expectedBottom;
    for (int y = top + 1; y < expectedBottom; ++y)
    {
        int l=-1, r=-1;
        if (FindLine(img, y, markerSizeThreshold, ref l, ref r))
        {
            if (l > right+5 || r < left-5)
            {
                throw new Exception("Marker not found");
            }
            if (l < left) left = l;
            if (r > right) right = r;
        }
        else if (y - top < markerSizeThreshold)
        {
            throw new Exception("Marker not big enough");
        }
        else
        {
            bottom = y;
            break;
        }
    }

    return new Rectangle(left, top, right - left, bottom - top);
}