Archive for November, 2012

The code  snippet shown below  is  for simple image stitching of two images in OpenCV . It can easily be modified to stitch multiple images together and create a Panorama.

OpenCV also has a stitching module which helps in achieving this task and which is more robust than this. The code presented here will help in understanding the major steps involved in image stitching algorithm. I am using OpenCV 2.4.3 and Visual studio 2010.  This code is based on the  openCV tutorial  available here.

The main parts of stitching algorithm are –  1) Finding Surf descriptors in both images 2) Matching the surf descriptors between two images . 3) Using  RANSAC to estimate the homography matrix using the matched surf descriptors. 4) Warping the images based on the homography matrix.

Input images :    

Stitched Output:

Code:

#include <stdio.h>
#include <iostream>

#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/nonfree.hpp"
#include "opencv2/calib3d/calib3d.hpp"
#include "opencv2/imgproc/imgproc.hpp"

using namespace cv;

void readme();

/** @function main */
int main( int argc, char** argv )
{
 if( argc != 3 )
 { readme(); return -1; }

// Load the images
 Mat image1= imread( argv[2] );
 Mat image2= imread( argv[1] );
 Mat gray_image1;
 Mat gray_image2;
 // Convert to Grayscale
 cvtColor( image1, gray_image1, CV_RGB2GRAY );
 cvtColor( image2, gray_image2, CV_RGB2GRAY );

imshow("first image",image2);
 imshow("second image",image1);

if( !gray_image1.data || !gray_image2.data )
 { std::cout<< " --(!) Error reading images " << std::endl; return -1; }

//-- Step 1: Detect the keypoints using SURF Detector
 int minHessian = 400;

SurfFeatureDetector detector( minHessian );

std::vector< KeyPoint > keypoints_object, keypoints_scene;

detector.detect( gray_image1, keypoints_object );
 detector.detect( gray_image2, keypoints_scene );

//-- Step 2: Calculate descriptors (feature vectors)
 SurfDescriptorExtractor extractor;

Mat descriptors_object, descriptors_scene;

extractor.compute( gray_image1, keypoints_object, descriptors_object );
 extractor.compute( gray_image2, keypoints_scene, descriptors_scene );

//-- Step 3: Matching descriptor vectors using FLANN matcher
 FlannBasedMatcher matcher;
 std::vector< DMatch > matches;
 matcher.match( descriptors_object, descriptors_scene, matches );

double max_dist = 0; double min_dist = 100;

//-- Quick calculation of max and min distances between keypoints
 for( int i = 0; i < descriptors_object.rows; i++ )
 { double dist = matches[i].distance;
 if( dist < min_dist ) min_dist = dist;
 if( dist > max_dist ) max_dist = dist;
 }

printf("-- Max dist : %f \n", max_dist );
 printf("-- Min dist : %f \n", min_dist );

//-- Use only "good" matches (i.e. whose distance is less than 3*min_dist )
 std::vector< DMatch > good_matches;

for( int i = 0; i < descriptors_object.rows; i++ )
 { if( matches[i].distance < 3*min_dist )
 { good_matches.push_back( matches[i]); }
 }
 std::vector< Point2f > obj;
 std::vector< Point2f > scene;

for( int i = 0; i < good_matches.size(); i++ )
 {
 //-- Get the keypoints from the good matches
 obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
 scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
 }

// Find the Homography Matrix
 Mat H = findHomography( obj, scene, CV_RANSAC );
 // Use the Homography Matrix to warp the images
 cv::Mat result;
 warpPerspective(image1,result,H,cv::Size(image1.cols+image2.cols,image1.rows));
 cv::Mat half(result,cv::Rect(0,0,image2.cols,image2.rows));
 image2.copyTo(half);
 imshow( "Result", result );

 waitKey(0);
 return 0;
 }

/** @function readme */
 void readme()
 { std::cout << " Usage: Panorama < img1 > < img2 >" << std::endl; }

Running the code :

Build the code and pass in the two images to be stitched as arguments to the generated exe. Sometimes if the stitching  output is not proper reversing the order of the two images when you pass to the exe would help.