detecting a pallet in forklift

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Im trying to detect the pallet in forklift. but find contours cant detect the rectangle in a correct way.

how can I detect the large pallet.

I have tried hough transform but it fails of detecting the forklift rectangle, so I'm using findcontours instead.

pallet

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

int main()
{
    cv::Mat input = cv::imread("pallet.jpg");

        // convert to grayscale (you could load as grayscale instead)
        cv::Mat gray;
        cv::cvtColor(input,gray, CV_BGR2GRAY);

        // compute mask (you could use a simple threshold if the image is always as good as the one you provided)
        cv::Mat mask;
        cv::threshold(gray, mask, 0, 255, CV_THRESH_BINARY_INV | CV_THRESH_OTSU);

        // find contours (if always so easy to segment as your image, you could just add the black/rect pixels to a vector)
        std::vector<std::vector<cv::Point> > contours;
        std::vector<cv::Vec4i> hierarchy;
        cv::findContours(mask,contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

        /// Draw contours and find biggest contour (if there are other contours in the image, we assume the biggest one is the desired rect)
        // drawing here is only for demonstration!
        int biggestContourIdx = -1;
        float biggestContourArea = 0;
        cv::Mat drawing = cv::Mat::zeros( mask.size(), CV_8UC3 );
        for( int i = 0; i< contours.size(); i++ )
        {
            cv::Scalar color = cv::Scalar(0, 100, 0);
            drawContours( drawing, contours, i, color, 1, 8, hierarchy, 0, cv::Point() );

            float ctArea= cv::contourArea(contours[i]);
            if(ctArea > biggestContourArea)
            {
                biggestContourArea = ctArea;
                biggestContourIdx = i;
            }
        }

        // if no contour found
        if(biggestContourIdx < 0)
        {
            std::cout << "no contour found" << std::endl;
            return 1;
        }

        // compute the rotated bounding rect of the biggest contour! (this is the part that does what you want/need)
        cv::RotatedRect boundingBox = cv::minAreaRect(contours[biggestContourIdx]);
        // one thing to remark: this will compute the OUTER boundary box, so maybe you have to erode/dilate if you want something between the ragged lines



        // draw the rotated rect
        cv::Point2f corners[4];
        boundingBox.points(corners);
        cv::line(drawing, corners[0], corners[1], cv::Scalar(255,255,255));
        cv::line(drawing, corners[1], corners[2], cv::Scalar(255,255,255));
        cv::line(drawing, corners[2], corners[3], cv::Scalar(255,255,255));
        cv::line(drawing, corners[3], corners[0], cv::Scalar(255,255,255));

        // display
        cv::imshow("input", input);
        cv::imshow("drawing", drawing);
        cv::waitKey(0);

        cv::imwrite("rotatedRect.png",drawing);
    return 0;