how to detect the pallet in fork lift using find contours - c++

Question

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;
}