Optical motion capture relies on having a good quality estimate of the calibration for each of the cameras in the system. The calibration consists of both the pose (position and rotation) of the camera, and internal parameters such as the field of view, and those related to any radial lens distortion. We generally refer to the "camera calibration" as referring to that computed via the wand wave process, which is an estimation of the true camera calibration (ie, the true poses and projection functions of the cameras).
To ensure a Vicon system has a good camera calibration (defined as one which is close to the true camera calibration), we have previously recommended performing calibration at regular intervals, for example, daily, based on observations of system performance. While this generally provides a good calibration, this strategy has the following disadvantages:
- The calibration may not have changed, which makes the recalibration redundant.
- The calibration may require additional attention between the regular calibrations.
Evoke provides tools that enable you to reduce recalibrations and to ensure that some calibration issues are fixed or mitigated between recalibrations.
To achieve longer periods between complete recalibrations, you have to rely on a combination of:
- Camera calibration being stable
- Camera calibration monitoring plus manual fixing of any issues
- Camera calibration automatic healing
Evoke contains tools to both monitor the performance of the system, and in particular the health of the camera calibration, and to help mitigate camera calibration issues. Before exploring those tools, it is worth examining what is meant by calibration stability and the types of issues related to the calibration.
Causes of camera calibration issues
Camera calibration issues can come in a variety of forms and have different causes. Some of the common issues are:
- Poor initial calibration
- Camera internal elements shifting as they reach operating temperature
- One or more cameras moved to a different location
- One or more cameras knocked and position or rotation changed
- One or more cameras' mounts slipping
- Rig expanding/contracting due to environmental temperature changes
Small, incremental changes in the camera calibration are referred to as "calibration drift" or "cameras drifting out of calibration".
You can use both automatic and manual tools to handle these issues without requiring a full recalibration (see Fix issues with camera calibration).
Whether you are taking advantage of automatic healing tools, using the manual tools, or just want the system to indicate that it may be time for a full recalibration, it is important to understand the monitoring utilities that report issues related to camera calibration. To find out more, see Monitor system health.