Calibrate camera to determine intrinsic params

Question

I have been struggling to get right calibration done using openCV with c++. Below I have included my code snippet for its' calculation, would greatly appreciate if you can point out what I am doing wrong, or how I can improve it.

// Initialize and reset calibration params
        void InitCaliberation()
        {
            //numBoards = 0;
            numCornersHor = horizontalCorners;
            numCornersVer = verticalCorners;
            numSquares = horizontalCorners * verticalCorners;

            board_sz = Size(horizontalCorners, verticalCorners);
            frame_sz = Size(frameWidth, frameHeight);
            sqSizeInmm = sqSizemm; //25 mm
            object_points.clear();
            image_points.clear();
            corners.clear();

            finishedCalberation = false;
        }

//Process frames
bool CheckCheckerboardFrame(Mat image, bool debug=false)
        {
            vector<Point3f> obj;
            Mat grayImage;
            cvtColor(image, grayImage, COLOR_BGR2GRAY);

            for (int j = 0; j < numSquares; j++)
                obj.push_back(Point3f(sqSizeInmm * j / numCornersHor, sqSizeInmm * j%numCornersHor, 0.0f));
            bool found = findChessboardCorners(grayImage, board_sz, corners, CALIB_CB_ADAPTIVE_THRESH | CALIB_CB_FILTER_QUADS);

            if (found)
            {
                //sub-pixel accurate location
                cornerSubPix(grayImage, corners, Size(11, 11), Size(-1, -1), TermCriteria(TermCriteria::MAX_ITER | TermCriteria::EPS, 30, 0.1));
                if(debug)
                    drawChessboardCorners(image, board_sz, corners, found);
                else {
                    image_points.push_back(corners);
                    object_points.push_back(obj);
                }
            }

            return found;
        }

// Calculate params
void FinishCaliberation()
        {
            calibrateCamera(object_points, image_points, frame_sz, intrinsic, distCoeffs, R, T);
            finishedCalberation = true;
            cout <<"\nIntrinsic: "<< intrinsic.size<<endl;

            for (int i = 0; i < intrinsic.rows; i++)
            {
                for (int j = 0; j < intrinsic.cols; j++)
                    cout << intrinsic.at<float>(i, j) << " ";
                cout << endl;
            }

            cout << "\nDist coeff: " << distCoeffs.size << endl;
            for (int i = 0; i < distCoeffs.rows; i++)
            {
                for (int j = 0; j < distCoeffs.cols; j++)
                    cout << distCoeffs.at<float>(i, j) << " ";
                cout << endl;
            }
            //cout<<
        }

Below is the output that I am getting with openCV.

And this is the expected output. I have calculated this calibration from matlab.

I am not trying to match the results! but want to understand why Fy and Cx values are 0, and why intrinsic.at(0,1) is non-zero? and what am I doing wrong here?

here is the minimal code of the calibration process: http://collabedit.com/3vay2 if anyone's interested.

Answer 1

Finally figured out the issue. Thanks to - @Micka

so I was getting garbage values because of data type mismatch. Opencv expects double precision, while I was using float type for Mat params. This was also the reason for unexpected values like non-zero value at intrinsic.at(0,1).

Which actually made me doubtful of my implementation. But converting it to double fixes that as well. Although I am not sure how the double value of 0 is being casted to 6.05179 in float. May have to do with hidden implementation of OpenCV.