# Camera intrinsic matrix (3 by 3 matrix) has a negative value and only values on main diagonal?

Discussion in 'Computer Vision' started by zimmer550king101, Oct 6, 2022.

1. ### zimmer550king101

Joined:
May 26, 2021
Posts:
9
I recorded a simulation by moving around with my camera and viewing a bunch of static objects. The camera intrinsic parameters were recorded as camera_intrinsic and extrinsic parameters were recorded as translation and rotation inside ego of the capture JSON (please correct me if this is wrong).

Now, I am experimenting with some triangulation methods (localizing objects using their 2D bounding box and the camera's projection matrix). Below is how I set up everything to get the projection matrix:

Code (CSharp):
1.
2. import numpy as np
3. from scipy.spatial.transform import Rotation as Rot
4. int_mat = np.array([
5. capture.sensor.camera_intrinsic[0],
6. capture.sensor.camera_intrinsic[1],
7. capture.sensor.camera_intrinsic[2]
8. ])
9. r = Rot.from_quat(capture.ego.rotation)
10. rot_mat = r.as_matrix()
11. t = np.array([capture.ego.translation]).transpose()
12. ext_mat = np.hstack((rot_mat, t))
13. proj_mat = int_mat @ ext_mat
When I input the projection matrix and 2d bounding box into my custom algorithm, I noticed the triangulation was well-off. It was then that I noticed the matrix of intrinsic parameters only had values on the main diagonal and one of them is actually negative. I come from a computer vision background but not a computer graphics one. Can anyone here guide me on how to convert the intrinsic matrix provided by Unity Perception into a normal one with fx, fy, cx, cy, and skew?

I looked at the 3D Ground Truth Bounding Boxes inside the Perception_Statistics notebook but the authors seem to be considering the intrinsic parameter matrix as the projection matrix (Isn't the projection matrix supposed to be 3 by 4 due to the matrix multiplication of the 3 by 3 intrinsic parameter matrix with the 3 by 4 extrinsic parameter matrix?).

Furthermore, I also had a look at a similar question but the accepted answer there seems to assume the projection matrix is already provided while one of the answers is trying to use the FOV even though I don't see it in the data I have collected through Unity Perception.

For reference, below are the two relevant parts of the capture JSON file I am using:

Code (CSharp):
1. "sensor": {
2.         "sensor_id": "7fcdda27-3029-4bb9-83f3-9d3eac23a1a1",
3.         "ego_id": "6a1ebd8b-1417-49a0-befc-8892801a9aa3",
4.         "modality": "camera",
5.         "translation": [
6.           0.0,
7.           0.0,
8.           0.0
9.         ],
10.         "rotation": [
11.           0.0,
12.           0.0,
13.           0.0,
14.           1.00000012
15.         ],
16.         "camera_intrinsic": [
17.           [
18.             0.705989,
19.             0.0,
20.             0.0
21.           ],
22.           [
23.             0.0,
24.             1.73205078,
25.             0.0
26.           ],
27.           [
28.             0.0,
29.             0.0,
30.             -1.0006001
31.           ]
32.         ]
33.       },
34.       "ego": {
35.         "ego_id": "6a1ebd8b-1417-49a0-befc-8892801a9aa3",
36.         "translation": [
37.           2.32,
38.           1.203,
39.           2.378
40.         ],
41.         "rotation": [
42.           -0.0229570474,
43.           0.976061463,
44.           -0.173389584,
45.           -0.129279226
46.         ],
47.         "velocity": null,
48.         "acceleration": null
49.       }