Bird's eye view perspective transformation using OpenCV

birds-eye-view.cpp

// OpenCV imports
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

// C++ imports
#include <iostream>

// namespaces
using namespace std;
using namespace cv;
#define PI 3.1415926


int frameWidth = 640;
int frameHeight = 480;

/*
 * This code illustrates bird's eye view perspective transformation using opencv
 * Paper: Distance Determination for an Automobile Environment using Inverse Perspective Mapping in OpenCV  
 * Link to paper: https://www.researchgate.net/publication/224195999_Distance_determination_for_an_automobile_environment_using_Inverse_Perspective_Mapping_in_OpenCV
 * Code taken from: http://www.aizac.info/birds-eye-view-homography-using-opencv/
 */ 

int main(int argc, char const *argv[]) {
	
	if(argc < 2) {
      cerr << "Usage: " << argv[0] << " /path/to/video/" << endl;
      cout << "Exiting...." << endl;
      return -1;
    }

    // get file name from the command line
    string filename = argv[1];
    
    // capture object
    VideoCapture capture(filename);
  
    // mat container to receive images
    Mat source, destination;
    
    // check if capture was successful
    if( !capture.isOpened()) throw "Error reading video";


	int alpha_ = 90, beta_ = 90, gamma_ = 90;
	int f_ = 500, dist_ = 500;

	namedWindow("Result", 1);

	createTrackbar("Alpha", "Result", &alpha_, 180);
	createTrackbar("Beta", "Result", &beta_, 180);
	createTrackbar("Gamma", "Result", &gamma_, 180);
	createTrackbar("f", "Result", &f_, 2000);
	createTrackbar("Distance", "Result", &dist_, 2000);

	while( true ) {
		
		capture >> source;

		resize(source, source,Size(frameWidth, frameHeight));

		double focalLength, dist, alpha, beta, gamma; 

		alpha =((double)alpha_ -90) * PI/180;
		beta =((double)beta_ -90) * PI/180;
		gamma =((double)gamma_ -90) * PI/180;
		focalLength = (double)f_;
		dist = (double)dist_;

		Size image_size = source.size();
		double w = (double)image_size.width, h = (double)image_size.height;


		// Projecion matrix 2D -> 3D
		Mat A1 = (Mat_<float>(4, 3)<< 
			1, 0, -w/2,
			0, 1, -h/2,
			0, 0, 0,
			0, 0, 1 );

	
		// Rotation matrices Rx, Ry, Rz

		Mat RX = (Mat_<float>(4, 4) << 
			1, 0, 0, 0,
			0, cos(alpha), -sin(alpha), 0,
			0, sin(alpha), cos(alpha), 0,
			0, 0, 0, 1 );

		Mat RY = (Mat_<float>(4, 4) << 
			cos(beta), 0, -sin(beta), 0,
			0, 1, 0, 0,
			sin(beta), 0, cos(beta), 0,
			0, 0, 0, 1	);

		Mat RZ = (Mat_<float>(4, 4) << 
			cos(gamma), -sin(gamma), 0, 0,
			sin(gamma), cos(gamma), 0, 0,
			0, 0, 1, 0,
			0, 0, 0, 1	);


		// R - rotation matrix
		Mat R = RX * RY * RZ;

		// T - translation matrix
		Mat T = (Mat_<float>(4, 4) << 
			1, 0, 0, 0,  
			0, 1, 0, 0,  
			0, 0, 1, dist,  
			0, 0, 0, 1); 
		
		// K - intrinsic matrix 
		Mat K = (Mat_<float>(3, 4) << 
			focalLength, 0, w/2, 0,
			0, focalLength, h/2, 0,
			0, 0, 1, 0
			); 


		Mat transformationMat = K * (T * (R * A1));

		warpPerspective(source, destination, transformationMat, image_size, INTER_CUBIC | WARP_INVERSE_MAP);

		imshow("Result", destination);
		waitKey(100);
	}


	return 0;
}

warperspective takes only 3by3 matrix but your inputting 4by4 matrix but in research paper you wrote 3*3 matrix. One more thing is in research paper you use the world co-ordinates to get the top view but in the code you use the source image directly to warPerspective function as i am new to this area please help me with it. Thanks in Advance.

@rohithagaram: please check the step Mat transformationMat = K * (T * (R * A1)); which computes a 3-by-3 matrix which is the required transformation matrix. Hope this helps!

Hii can you please help me with the matlab implemention of this code . I m not able to find a function in matlab which works same as warp perspective.

@anujonthemove Could you upload an example input output image?

Thanks!

Hi Can you help me with the matlab implemention, too? Please help me

Hi,
What is the purpose of your Matrix A1? You mention it transforms 2D into 3D, right? How? and why you do the negative of half the width and height of the image? Isn't matrix A1 a linear transformation that implies a translation of the image?

I did this automatically by finding the vanishing point - see my paper
https://link.springer.com/article/10.1007/s00138-010-0289-5

(PDF link: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.227.8116&rep=rep1&type=pdf)

What are your units for focal length and camera distance?

Ry sinus sign seems to be wrong.

I would like to thank you for your post, but I have 2 question about it
what does the variable dist_=500 refer to ?
what are your units for the distance and focal length variables?