# why the left 3x3 portion of Projection matrix(P) is different from the Intrinsic camera matrix(K)?

Intrinsic camera matrix for the raw

(distorted) images. [fx 0 cx]

K = [ 0 fy cy]

[ 0 0 1]

Projects 3D points in the camera coordinate frame to 2D pixel coordinates using the focal lengths (fx, fy) and principal point (cx, cy).Projection/camera matrix

[fx' 0 cx' Tx]

P = [ 0 fy' cy' Ty]

[ 0 0 1 0]

By convention, this matrix specifies the intrinsic (camera) matrix of the processed (rectified) image.That is, the left 3x3 portion is the normal camera intrinsic matrix for the rectified image.It projects 3D points in the camera coordinate frame to 2D pixel coordinates using the focal lengths (fx', fy') and principal point (cx', cy') - these may differ from the values in K.given by http://docs.ros.org/en/noetic/api/sen...

inage width: 1920 image height:1080 canera nane: narrow stereo

cameramatrix: rows: 3 cols:3

data:[1963.58626 0. 978.96792 0. 1961.4452 548.41778 0. 0. 1.]

distortion_model: plumb bob

distortion coefficients: rOWS: 1 cols:5

data: [-0.554099, 0.343450,-0.000465. 0.001727, 0.000000]

rectification matrix: rows: 3 cols:3

data: [1. 0. 0. 0. 1. 0. 0.0. 1. ]

projection matrix rows. 3 cols:4

data: [1679.60999 0. 989.73115 0. 0.1875.7949 548.96323 0. 0. 0. 1. 0. ]

when i use the **camera_calibration cameracalibrator.py** to calibrate the camera, the result shows the left 3x3 portion of Projection matrix(P) is different from the Intrinsic camera matrix(K), why they are different?

how to caculate from Intrinsic camera matrix(K) to Projection matrix(P)?