# Stereo re-calibration using OpenCV's findEssentialMat(), recoverPose()

I’m trying to re-calibrate a stereo pair, which was previously calibrated using standard chess board images. Over time, temperature changes etc. shift the baseline/rotation of the two cameras. I'm trying to recover stereo calibration using a single left/right camera image of a pattern (like a chess board).

tl;dr: Recover ‘extrinsic’ params for a stereo pair using a single left/right calibration pattern image. Assume camera ‘intrinsics’ stay the same.

What seems like a straightforward series of steps, is giving incorrect results:

```
// Get corresponding points in 'pixel' coordinates
vector<Point2f> points_left = ...;
vector<Point2f> points_right = ...;
undistortPoints(points_left, points_left, cameraMatrix[0], distCoeffs[0], noArray(), cameraMatrix[0]);
undistortPoints(points_right, points_right, cameraMatrix[1], distCoeffs[1], noArray(), cameraMatrix[1]);
auto E = findEssentialMat(points_left, points_right, cameraMatrix[0], RANSAC, 0.999, 1.0);
// We want to recover R, t
Mat R_new, t_new;
recoverPose(E, points_left, points_right, cameraMatrix[0], R_new, t_new);
// Compute new R1, R2, P1, P2, Q, as well has adjusted ‘valid ROIs’ r1, r2
Mat R1, R2, P1, P2, Q;
Rect r1, r2;
stereoRectify(cameraMatrix[0], distCoeffs[0], cameraMatrix[1], distCoeffs[1],
image_size, R_new, t_new, R1, R2, P1, P2, Q,
CALIB_ZERO_DISPARITY, 0, image_size, &r1, &r2);
```

Problem 1) Even though the cameras have moved very slightly, the values of R, t are quite different from original R, t values computed during initial calibration. Especially ‘t’ looks like it's a unit vector! There's no information about this in OpenCV documentation for recoverPose().

Problem 2) I tried using ‘normalized’ output from undistortPoints(), by giving noArray() as the last param in undistortPoints() instead of ‘cameraMatrix’. Then using focal length = 1, principle point = (0,0) in findEssentialMat(). But to no avail, the final result is still wrong.

Am I missing a step somewhere? Are my inputs to findEssentialMat() correct?

Hi, have you figured out in some way to do that? I'm interested in the same things and we can may help each other to find a solution!