Calibration Files
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Theia Axiom - v2025
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The camera calibration file contains the calibrations for all of the cameras using a structure similar to XML. An example calibration file is provided with the sample data. Key elements and attributes of the calibration file are outlined here. Note that Qualisys calibrations exported from QTM as TXT files and Vicon calibrations (XCP) can also be used.
Top level element with no attributes.
Grouping element that holds <camera> child elements. It has no attributes.
Element that holds the calibration information for a single camera. Required attributes are: active, serial, and viewrotation. Required child elements are: <transform> and <intrinsic>.
active: 1 if the camera is used, 0 if unused.
serial: The camera ID. Must be unique to the camera and match the camera ID used to name the video subfolders.
viewrotation: Rotation of the camera. 0 if upright, 180 if upside down, 90 or 270 if sideways. This is calculated from the rotation matrix of the calibration.
Translation and rotation components of the transformation from global to camera coordinates (p→cam=Rp→global+t→). Required attributes are x, y, z and r11 through r33.
x, y, z: Elements of the translation component (t→) of the transformation. Expressed in mm.
rij: Elements of row i and column j of the rotation component (R) of the transformation.
Intrinsic camera parameters. Required attributes are: focallength, sensorMinU, sensorMaxU, sensorMinV, sensorMaxV, focalLengthU, focalLengthV, centerPointU, centerPointV, skew, radialDistortion1-3, and tengentialDistortion1-2.
focallength: Focal length of the lens in mm.
sensorMinU: Minimum u coordinate of the sensor in pixels.
sensorMaxU: Maximum u coordinate of the sensor in pixels.
sensorMinV: Minimum v coordinate of the sensor in pixels.
sensorMaxV: Maximum v coordinate of the sensor in pixels.
focalLengthU: Focal length along the u axis in pixels.
focalLengthV: Focal length along the v axis in pixels.
centerPointU: Principal point u coordinate in pixels.
centerPointV: Principal point v coordinate in pixels.
skew: Skew coefficient. Non-zero if the image axes are not perpendicular. Note that non-zero skew is currently unsupported.
radialDistortion1-3: Radial distortion coefficients.
tangentialDistortion1-2: Tangential distortion coefficients.
Accurate 3D reconstruction in markerless motion capture requires both intrinsic and extrinsic calibration of all cameras in the system. These two types of calibration serve different but complementary purposes. The same calibration chessboard is used for both the Intrinsic and Extrinsic calibration.
Sony RX0 II intrinsic parameters are built-in within Theia and do not need to be performed. All other camera systems that will use an extrinsic chessboard calibration will require custom instrinsics.
Intrinsic calibration characterizes the internal properties of each camera, including:
Focal length
Sensor center point
Skew
Lens distortion
Extrinsic calibration defines the position and orientation of each camera within the global 3D coordinate system. This is essential for aligning multiple cameras into a unified space so they can work together to triangulate 3D points.
Extrinsic calibration requires:
Chessboard
Single-camera views are insufficient to determine the board’s 3D position and orientation, therefore multi-camera visibility is critical and the best results are obtained when three or more cameras can see the chessboard pattern throughout the calibration trial.
Intrinsic calibration ensures each camera produces geometrically correct images by accounting for internal optical characteristics and enables projection from 2D pixels coordinates to 3D rays.
Extrinsic calibration aligns the position and orientation of each camera relative to a global coordinate system. It requires multiple cameras to view the chessboard simultaneously to resolve the calibration board's 3D pose.
Element that holds the calibration results metrics, as described in .
More information about camera calibration can be found from online resources such as .
This calibration corrects image distortion and ensures that pixel locations correspond accurately to 3D rays extending from the camera into space. Intrinsic calibration is performed for all cameras, and the resulting parameters are unique to each camera. These trials can be performed and processed individually or recorded and processed together as in this . The known dimensions of the chessboard allow these parameters to be calculated and any lens effects to be corrected. Details on performing the lens calibration can be found in the
Details on performing the chessboard calibration can be found in the
