System calibration is a process by which Vicon Nexus calibrates the system based on specialized calibration objects (whose dimensions and relative marker positions are known). It consists of several procedures:
- Camera calibration The camera calibration process calculates the physical position and orientation of each Vicon camera in the capture volume based on the movement of the calibration object. Nexus uses this information to determine each camera's physical position and orientation in the capture volume, to correct for any lens distortion, and to set internal camera parameters.
- Set volume origin During this process, Nexus measures the position of the calibration object and uses this information to identify the origin of the world (center of the capture volume) and its horizontal and vertical axes. These volume origin and axes are referred to as the global coordinate system. The global origin coordinates are always (0,0,0). The global axes coordinates are given in the form (x, y, z), where x is a horizontal axis, y is the horizontal axis perpendicular to x, and z is the vertical axis. If the floor of your volume is uneven, you can also use the Set Floor Plane option as part of this process.
For basic step-by-step instructions on how to complete these procedures, see Calibrate a Vicon system in the Vicon Nexus User Guide.
To enable you to perform these procedures, the System Preparation Tools pane contains the following sections (to show additional options, click Show Advanced in the relevant section).
Video Calibration Setup section
Enables you to activate video calibration mode by clicking the Activate button. Note that if you haven't saved the current configuration, you will be prompted to do so before you can proceed.
You automatically activate calibration mode when you click Start in the Mask Cameras section, the Aim Cameras section or the Calibrate Cameras section.
To exit calibration mode, click the Deactivate button in the Video Calibration Setup section.
Mask Cameras section
Enables you to automatically create cameras masks to obscure all reflections visible to the Vicon cameras. By enabling the Advanced properties, you can choose to mask all cameras or just selected cameras.
For more information on how to mask cameras, see Mask unwanted reflections in the Vicon Nexus User Guide.
Aim Cameras section
Enables you to check the positioning of Vicon cameras around the capture volume with real-time feedback. You normally perform this step before you begin calibration check.
For more information on how to aim cameras, see Aim Vicon cameras in the the Vicon Nexus User Guide.
Calibrate Cameras section
Enables you to calibrate the Vicon cameras to determine their positions, orientations, and lens properties, which enables Nexus to produce accurate 3D and 2D (video) data from motion data captured throughout the capture volume. The Advanced properties contain the following options:
- Wand The calibration object to be used during the dynamic stage of the camera calibration process.
Default: Active Wand v2 (both optical cameras and digital video cameras are calibrated with an Active Wand)
- Calibration Type Specify whether to use a Full Calibration or a quicker Calibration Refinement. (If you have not already calibrated or aimed all cameras, you cannot perform a calibration with the Calibration Refinement option.) For more information on calibration options, see Understand camera calibration refinement.
- Cameras to Calibrate Can be All Cameras or Selected Cameras (that is, the cameras selected in the System Resources tree). The selection is applied when the Stop button in the Calibrate Cameras section is clicked.
Default: All Cameras
- Refinement frames The minimum coverage (in number of frames) required per camera in the final phase of the refine camera calibration process.
Default: 1000. For more information, see Understand camera calibration refinement.
- Wand Ratio Tolerance (Advanced setting) Tolerance of the distance between the markers on the wand (expressed as a ratio), to enable it to be labeled in 2D.
- Wand Straightness Tolerance Tolerance in alignment of wand markers (relating to the maximum angle allowable between the markers), to enable it to be labeled in 2D.
- DV Calibration frames The minimum coverage (in number of frames) required per DV camera for the calibration to autostop (if Auto Stop is selected).
- Auto Stop Whether or not Nexus is to automatically stop the camera calibration process when sufficient data has been collected.
For more information on how to calibrate cameras (both optical and video), see Calibrate Vicon cameras in the Vicon Nexus User Guide.
Set Volume Origin section
Enables you to define the global origin and the axes of the world (in the context of the capture volume).
The Advanced options are:
- L-Frame The calibration object to be used for setting the volume origin.
Default: 5 Marker Wand & L-Frame
- Start/Set Origin and Cancel buttons Enable you to start, set the origin and cancel setting the origin.
- One Marker Enables you to set the origin to the selected marker.
- Three Markers Enables you to set the origin to the selected three markers.
- Set Floor Plane Enables you to set the position of the floor plane in Nexus by using markers in the volume to automatically define it. For more information, see Calibrate the floor plane in the Vicon Nexus User Guide.
Static Video Calibration section
If you are not using an Active Wand, you use this section to calibrate digital video cameras included in your Nexus system. This enables 3D overlay from Vicon cameras to be displayed with the 2D video from the digital video cameras.
Manage Camera Calibration section
Contains the following controls:
- Auto number cameras Ensure you have aimed the cameras (see Aim Vicon cameras) before clicking this button. Automatically numbers the currently connected Vicon cameras in ascending order, according to their position in the capture volume. Automatic numbering starts with the camera furthest from the volume origin. The cameras are then numbered in a clockwise direction around the volume. If your cameras are positioned at different levels, the cameras in the level that contains the most cameras are numbered first.
- Reset Removes all non-existing cameras, clears the calibrated position for existing cameras, and reverts all calibration parameters to their defaults. This enables you to recalibrate the system from a clean starting point.
- Load Navigate to and select a previously saved camera calibration (.xcp file) defining settings for the Vicon cameras and any supported digital video cameras in your Nexus system.
Camera Calibration Feedback section
Enables you to view system calibration processing progress and status information. This section contains the following controls:
- Progress bar This bar displays a percentage indicating the progress of the overall camera calibration process.
- Camera This column contains the device ID for each Vicon camera being calibrated.
- Wand Count For each Vicon camera, this value identifies the number of frames it has captured containing the calibration object. Initially, the entry for the number of wand frames is displayed in red; the entry turns green when Nexus has acquired enough wand data to calibrate that camera, typically 1000 frames.
By default, the calibration process stops when the camera with the lowest frame count reaches the number of frames specified in the Refinement frames parameter in the Calibrate Cameras section.
- World Error Displays the calibration error in millimeters. World error is calculated per camera from the Image Error in pixels and the distance of the camera to the center of the volume. Cameras further away, with the same image error, display a larger world error.
- Image Error This value (RMS distance in camera pixels) indicates the accuracy of the 3D reconstruction of the markers. This value represents the difference between the 2D image of each marker on the camera sensor and the 3D reconstructions of those markers projected back to the camera's sensor. Acceptable values depend on factors such as the size of the capture volume and the camera lens type.
Additional features in this section enable you to export your calibration, to identify calibration issues or to ensure that the current calibration conforms to an established standard.
Export a calibration as a CSV file
You can export the current calibration as a .csv file. This helps you to monitor your calibration standards over time.
To export a calibration as a CSV file:
- In the Camera Calibration Feedback section, right-click and then click Export.
- Enter a file name, ensure the file extension is .csv and click Save.
You can compare the current calibration results with the results from the previous calibration.
To compare the current and last calibrations:
- In the Camera Calibration Feedback section, right-click and then select Show Differences.
The World Error and Image Error columns display the differences between the two calibrations.
If the results from the latest calibration are better (ie produce smaller errors) than the previous one, the columns are displayed in green to yelllow.
If the results from the latest calibration are worse (ie produce larger errors) than the previous one, the columns are displayed in red to orange.
Set calibration error threshold
You can specify a maximum calibration error, above which cameras are flagged as having an error that is higher than the threshold that you have set.
To set a calibration error threshold:
- At the bottom of the the Camera Calibration Feedback section, set the error threshold (World or Image).
After calibration, in the Camera Calibration Feedback section, cameras with an error greater than this value are displayed with a yellow warning icon instead of the normal blue camera icon. The default setting (0.000) turns off any warnings.
In the following example, the error threshold is set to 0.5mm, so that the three cameras with a calibration error greater than this are displayed with a warning icon:
If you are working with a large number of cameras, it is useful to be able to sort the results to enable you to quickly identify problematic cameras. You can now sort the columns in the Camera Calibration Feedback section by clicking on the required column heading.
The default sorting is on the camera number (low to high):
To make it easy to find cameras with the highest errors, you can sort on World Error or Image Error, displaying the cameras with the highest errors at the top of the column:
Understand camera calibration refinement
Calibration refinement provides a fast, reliable way to fine-tune an existing camera calibration, for example, as part of your daily calibration workflow before beginning the day's captures.
Full Calibration consists of an initialization phase, followed by a multi-pass process to optimize the camera positions.
Calibration Refinement uses exactly the same process as full calibration, but without the initialization phase. It provides a reliable way to refine existing calibration data to produce a calibration that is as good as a full calibration of the same system, but is much faster.
As Calibration Refinement operates on existing data, you must have loaded a full calibration into Nexus before running the refinement calibration.
To save time while maintaining accuracy, you can perform both full and refinement calibration on any selected camera(s), as well as on all cameras.
Under most circumstances, the default value for Refinement frames produces good results. If you need to improve the results, particularly if you are using larger numbers of cameras, try increasing this value.
Note that the value specified for Refinement frames applies to the number of frames used:
- By Auto Stop
- In the refinement phase of a Full Calibration
- When running a Calibration Refinement
When to use a refinement calibration
The following table gives guidance on when to use each type of calibration:
|Scenario||Type of calibration|
|A full calibration of all cameras has recently been performed, but since then, several cameras have been repositioned to another part of the volume.||Full Calibration on just the moved cameras, with a short wand wave that concentrates on the moved cameras.|
|A full calibration of all cameras has recently been performed, but during the trial, one camera was accidentally slightly bumped.||Full Calibration on the bumped camera, with a short wand wave that concentrates on the bumped camera.|
|Since yesterday's full calibration, environmental factors may have caused small changes in the camera positions and it is necessary to re-calibrate them accurately and quickly.||Calibration Refinement of all cameras, with a normal length wand wave that includes all cameras.|