How to adjust rotation matrix by a known angle

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Hi,

I have calibrated a camera and the pitch angle considered was 0. I have the rotation mat & translation vector from using solvePnp and Rodrigues. Now, if I tilt the camera, the pitch angle changes.. how can I adjust the 3x3 rotation matrix to take into account the new pitch angle? Or the rotation vector.. The pitch angle is expressed in degrees or radians.

Basically I build up an ipm image using projectPoints function that takes the previously obtained matrixes: camera mat, rotation and translation vectors from the calibration. I simply want to modify the rotation mat to take into account the new pitch angle of the camera (mobile phone). I have rotated the points 'manually' before applying the projectPoints but the computation takes too much time.

Any ideas? Thanks!