How frequent do you need to do camera calibration for ArUco?

Question

How important it is to do camera calibration for ArUco? What if I dont calibrate the camera? What if I use calibration data from other camera? Do you need to recalibrate if camera focuses change? What is the practical way of doing calibration for consumer application?

Answer 1

Before answering your questions let me introduce some generic concepts related with camera calibration. A camera is a sensor that captures the 3D world and project it in a 2D image. This is a transformation from 3D to 2D performed by the camera. Following OpenCV doc is a good reference to understand how this process works and the camera parameters involved in the same. You can find detailed AruCo documentation in the following document.

In general, the camera model used by the main libraries is the pinhole model. In the simplified form of this model (without considering radial distortions) the camera transformation is represented using the following equation (from OpenCV docs):

The following image (from OpenCV doc) illustrates the whole projection process:

In summary:

P_im = K・R・T ・P_world

Where:

• P_im: 2D points porojected in the image
• P_world: 3D point from the world
• K is the camera intrinsics matrix (this depends on the camera lenses parameters. Every time you change the camera focus for exapmle the focal distances fx and fy values whitin this matrix change)
• R and T are the extrensics of the camera. They represent the rotation and translation matrices for the camera respecively. These are basically the matrices that represent the camera position/orientation in the 3D world.

Now, let's go through your questions one by one:

How important it is to do camera calibration for ArUco?

Camera calibration is important in ArUco (or any other AR library) because you need to know how the camera maps the 3D to 2D world so you can project your augmented objects on the physical world.

What if I dont calibrate the camera?

Camera calibration is the process of obtaining camera parameters: intrinsic and extrinsic parameters. First one are in general fixed and depend on the camera physical parameters unless you change some parameter as the focus for example. In such case you have to re-calculate them. Otherwise, if you are working with camera that has a fixed focal distance then you just have to calculate them once.

Second ones depend on the camera location/orientation in the world. Each time you move the camera the RT matrices change and you have to recalculate them. Here when libraries such as ArUco come handy because using markers you can obtain these values automatically.

In few words, If you don't calculate the camera you won't be able to project objects on the physical world on the exact location (which is essential for AR).

What if I use calibration data from other camera?

It won't work, this is similar as using an uncalibrated camera.

Do you need to recalibrate if camera focuses change?

Yes, you have to recalculate the intrinsic parameters because the focal distance changes in this case.

What is the practical way of doing calibration for consumer application?

It depends on your application, but in general you have to provide some method for manual re-calibration. There're also method for automatic calibration using some 3D pattern.