[Question] What are final camera parameters after rectification?

[JeyP4]

I understood that distortion correction is performed.

I am interested in what are the cx, cy, fx, fy after rectification (/zed/zed_node/rgb/image_rect_color) ? (Let's say for VGA)

[Myzhar]

Hi @JeyP4 you can find many good information searching for "Pin hole camera model".

• (cx, cy) is a principal point that is usually at the image center
• fx, fy are the focal lengths expressed in pixel units.

[JeyP4]

Hi @JeyP4 you can find many good information searching for "Pin hole camera model".

• (cx, cy) is a principal point that is usually at the image center
• fx, fy are the focal lengths expressed in pixel units.

Thanks I am sorry, I was not clear enough in my question.

My question is in the config file I see these values. Which are required to perform undistort. After undistort, distortion coefficients will vanish. But rectified image will have its new fx, fy, cx , cy. May this wrappper publish somewhere new values?

[Myzhar]

Each image topic has its own camera parameter associated. e.g. /zed2/zed_node/rgb/camera_info and /zed2/zed_node/rgb/image_rect_color -> undistorted, but with fx, fy, cx, cy that you can find in the K parameter in the camera_info topic: e.g. K: [531.2548217773438, 0.0, 666.2540283203125, 0.0, 531.2548217773438, 349.59100341796875, 0.0, 0.0, 1.0] fx = 531.2548217773438 fy = 531.2548217773438 cx = 666.2540283203125 cy = 349.59100341796875

one example of complete camera_info topic:

---
header: 
  seq: 1
  stamp: 
    secs: 1595352953
    nsecs: 379747070
  frame_id: "zed2_left_camera_optical_frame"
height: 720
width: 1280
distortion_model: "plumb_bob"
D: [0.0, 0.0, 0.0, 0.0, 0.0]
K: [531.2548217773438, 0.0, 666.2540283203125, 0.0, 531.2548217773438, 349.59100341796875, 0.0, 0.0, 1.0]
R: [1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0]
P: [531.2548217773438, 0.0, 666.2540283203125, 0.0, 0.0, 531.2548217773438, 349.59100341796875, 0.0, 0.0, 0.0, 1.0, 0.0]
binning_x: 0
binning_y: 0
roi: 
  x_offset: 0
  y_offset: 0
  height: 0
  width: 0
  do_rectify: False
---

[JeyP4]

Thanks

Do you mean after undistort new camera_matrix (k_new) = old camera_matrix (K) ?

[Myzhar]

No, I have not said that. Indeed you can compare the camera matrix before and after calibration and you will notice that they are different. e.g. /zed2/zed_node/rgb_raw/camera_info (raw image) vs /zed2/zed_node/rgb/camera_info (undistorted image)

[JeyP4]

Ok Now it's clear. I will check tomorrow.

Thanks.