error during camera calibration in OpenCV

Question

I am doing camera calibration using opencv. I am using the same code given in "Cook book programming".

I am taking pictures from my smartphone of a chessboard. Then I am using opencv program to do camera calibration for me. Program worked for only one set of images when I have very large chess board. It does not work for other set of images and I get run time error "Assertion failed

I dont know what going wrong in my code. The code is as follows:-

int main()
{
CameraCalibrator calibrateCam;
std::vector<std::string> filelist;
char buff[100];

for(int i=0;i<21;i++)
{
    sprintf(buff,"..\\Train\\3\\%d.jpg",i+1);
    filelist.push_back(buff);
}
cv::Size boardSize(4,3);
double calibrateError;
int success;
success = calibrateCam.addChessboardPoints(filelist,boardSize);
}

class CameraCalibrator{
public:
   std::vector<std::vector<cv::Point3f>> objectPoints;
   std::vector<std::vector<cv::Point2f>> imagePoints;
   //Square Lenght
   float squareLenght;
   //output Matrices
   cv::Mat cameraMatrix; //intrinsic
   cv::Mat distCoeffs;
   //flag to specify how calibration is done
   int flag;
   //used in image undistortion
   cv::Mat map1,map2;
   bool mustInitUndistort;
public:
    CameraCalibrator(): flag(0), squareLenght(36.0), mustInitUndistort(true){};
    int addChessboardPoints(const std::vector<std::string>& filelist,cv::Size& boardSize){
        std::vector<std::string>::const_iterator itImg;
        std::vector<cv::Point2f> imageCorners;
        std::vector<cv::Point3f> objectCorners;
        //initialize the chessboard corners in the chessboard reference frame
        //3d scene points
        for(int i = 0; i<boardSize.height; i++){
            for(int j=0;j<boardSize.width;j++){
                objectCorners.push_back(cv::Point3f(float(i)*squareLenght,float(j)*squareLenght,0.0f));
            }
        }
        //2D Image points:
        cv::Mat image; //to contain chessboard image
        int successes = 0;

        for(itImg=filelist.begin(); itImg!=filelist.end(); itImg++){
            image = cv::imread(*itImg,CV_LOAD_IMAGE_GRAYSCALE);
            std::cout<<*itImg<<"\n";

            bool found = cv::findChessboardCorners(image, boardSize, imageCorners);

            cv::drawChessboardCorners(image, boardSize, imageCorners, found);                      
            cv::cornerSubPix(image, imageCorners, cv::Size(5,5),cv::Size(-1,-1),
                cv::TermCriteria(cv::TermCriteria::MAX_ITER+cv::TermCriteria::EPS,30,0.1));
            //if we have a good board, add it to our data
            if(imageCorners.size() == boardSize.area()){
                addPoints(imageCorners,objectCorners);
                successes++;
            }
        }

        return successes;
    }
    void addPoints(const std::vector<cv::Point2f>& imageCorners,const std::vector<cv::Point3f>& objectCorners){
        //2D image point from one view
        imagePoints.push_back(imageCorners);
        //corresponding 3D scene points
        objectPoints.push_back(objectCorners);
    }
    double calibrate(cv::Size &imageSize){
        mustInitUndistort = true;
        std::vector<cv::Mat> rvecs,tvecs;
        return
            cv::calibrateCamera(objectPoints, //the 3D points
                imagePoints,
                imageSize, 
                cameraMatrix, //output camera matrix
                distCoeffs,
                rvecs,tvecs,
                flag);

    }
    void remap(const cv::Mat &image, cv::Mat &undistorted){
        std::cout << cameraMatrix;
        if(mustInitUndistort){ //called once per calibration
            cv::initUndistortRectifyMap(
                cameraMatrix,
                distCoeffs,
                cv::Mat(),
                cameraMatrix,
                image.size(),
                CV_32FC1,
                map1,map2);
            mustInitUndistort = false;
        }
        //apply mapping functions
        cv::remap(image,undistorted,map1,map2,cv::INTER_LINEAR);
    }
};

In camera calibration class, it opens the image successfully but it fails on findChessboardCorners line . . .

Plz help me in that. One sample chess board image is as follows. The algorithm fails on this very 1st image to find the corners . . . . :-

Answer 1

First, in cv::findChessboardCorners your boardSize is wrong, you define it as cv::Size boardSize(4,3); when it should be cv::Size boardSize(5,4); since your board has 5 inner corners per row and 4 corners per column.

Also, you should add a check in case corners are not found. In your code, after

bool found = cv::findChessboardCorners(image, boardSize, imageCorners);

you continue and call functions like cv::drawChessboardCorners and cv::cornerSubPix which will not work if there are no corners. This part should be:

for(itImg=filelist.begin(); itImg!=filelist.end(); itImg++)
{
    image = cv::imread(*itImg,CV_LOAD_IMAGE_GRAYSCALE);
    std::cout<<*itImg<<"\n";

    bool found = cv::findChessboardCorners(image, boardSize, imageCorners);

    if (found)  // continue only if corners have been found
    {
        cv::drawChessboardCorners(image, boardSize, imageCorners, found);                      
        cv::cornerSubPix(image, imageCorners, cv::Size(5,5),cv::Size(-1,-1), cv::TermCriteria(cv::TermCriteria::MAX_ITER+cv::TermCriteria::EPS,30,0.1));

        //if we have a good board, add it to our data
        if(imageCorners.size() == boardSize.area())
        {
            addPoints(imageCorners,objectCorners);
            successes++;
        }
    }
    else    // if no corners found
    {
        std::cout<<"No corners found in image"<<std::endl;

        // Do anything else you want here
    }
}

Also, why is there a red border around your black squares? I haven't looked at how exactly cv::findChessboardCorners detects the corners, but this could very possibly lead the algorithm to fail. Can you try with a new board without it? While we're at it, the light reflection on the upper corner could play some role too, since the intensity values there for black are a very light grey. Did you try your algorithm in the sample images provided by OpenCV?