OpenCV stereoRectify distorts image

Question

we have a ELP 1.0 Megapixel Dual Lens Usb Stereo camera and we are trying to calibrate it using OpenCV 3.1 in C++. However, the result of the calibration is totally unusable

Answer 1

Just to sumarize if someone needs similar help, this is what I do to get the best lookable result:

Upscale the chessboard image before corner detection:

Mat resized;
resize(img[k], resized, Size(FRAME_WIDTH * 2, FRAME_HEIGHT * 2), 0.0, 0.0, INTER_LINEAR);
 findChessboardCorners(resized, boardSize, corners, CALIB_CB_ADAPTIVE_THRESH | CALIB_CB_NORMALIZE_IMAGE

Downscale the detected corners:

for (int i = 0; i < corners.size(); ++i) {
    corners[i].x /= 2.0;
    corners[i].y /= 2.0;
}

Calibrate each camera separately:

double rms = calibrateCamera(objectPoints, imagePoints[k], Size(FRAME_WIDTH, FRAME_HEIGHT), cameraMatrix[k], distCoeffs[k], rvecs, tvecs,
    CALIB_FIX_PRINCIPAL_POINT | CALIB_FIX_ASPECT_RATIO | CALIB_ZERO_TANGENT_DIST | CALIB_RATIONAL_MODEL | CALIB_FIX_K3 | CALIB_FIX_K4 | CALIB_FIX_K5);

Calibrate stereo camera:

stereoCalibrate(objectPoints, imagePoints[0], imagePoints[1], cameraMatrix[0], distCoeffs[0], cameraMatrix[1], distCoeffs[1],
    Size(FRAME_WIDTH, FRAME_HEIGHT), R, T, E, F, 
    CALIB_FIX_INTRINSIC | CALIB_SAME_FOCAL_LENGTH,
    TermCriteria(TermCriteria::COUNT + TermCriteria::EPS, 30, 0));

Compute rectification (with alpha = 0.0):

stereoRectify(cameraMatrix[0], distCoeffs[0], cameraMatrix[1], distCoeffs[1], Size(FRAME_WIDTH, FRAME_HEIGHT), 
    R, T, R1, R2, P1, P2, Q,
    CALIB_ZERO_DISPARITY, 0.0, Size(FRAME_WIDTH, FRAME_HEIGHT), &validRoI[0], &validRoI[1]);

These are the calibration result matrices

Intrinsics:

M1: !!opencv-matrix
   rows: 3
   cols: 3
   dt: d
   data: [ 2.6187262304487734e+02, 0., 1.5950000000000000e+02, 0.,
       2.6187262304487734e+02, 1.1950000000000000e+02, 0., 0., 1. ]
D1: !!opencv-matrix
   rows: 1
   cols: 5
   dt: d
   data: [ -4.6768074176991381e-01, 2.0221327568191746e-01, 0., 0., 0. ]
M2: !!opencv-matrix
   rows: 3
   cols: 3
   dt: d
   data: [ 2.6400975025525213e+02, 0., 1.5950000000000000e+02, 0.,
       2.6400975025525213e+02, 1.1950000000000000e+02, 0., 0., 1. ]
D2: !!opencv-matrix
   rows: 1
   cols: 5
   dt: d
   data: [ -4.5713211677198845e-01, 2.8855737500717565e-01, 0., 0., 0. ]

Extrinsics:

R: !!opencv-matrix
   rows: 3
   cols: 3
   dt: d
   data: [ 9.9963073433190641e-01, 4.6310793035473068e-04,
       2.7169477545556639e-02, -6.9475632716349024e-04,
       9.9996348636555088e-01, 8.5172324905818230e-03,
       -2.7164541091274301e-02, -8.5329635354663789e-03,
       9.9959455592785362e-01 ]
T: !!opencv-matrix
   rows: 3
   cols: 1
   dt: d
   data: [ -6.1830090720273198e+01, 1.6774590574449604e+00,
       1.8118983433925613e+00 ]

My another question is whether there are any special requests for variable initializations or is this enough?

Mat cameraMatrix[2] = { Mat::eye(3, 3, CV_64F), Mat::eye(3, 3, CV_64F) };
Mat distCoeffs[2], R, T, E, F, R1, R2, P1, P2, Q;

Answer 2

Hey have you tried changing the value of parameter alpha in function stereoRectify. I remember that once I also obtained such results and changing the value of alpha to 0 did the work for me. Please let me know the results you obtain with alpha = -1, alpha=0.5 and alpha =0