VidSync can measure positions, distances, and events in a single plane using one camera. This is useful when the points being measured remain close to a floor, wall, tank bottom, water surface, or another plane whose coordinates can be defined. Ants moving across a floor and fish viewed from above in shallow water are typical examples.
A 2-D project uses the same video-navigation, distortion-correction, Object and Event, annotation, and export tools as a 3-D project. The calibration is simpler: a flat grid establishes a direct mapping between screen positions and coordinates on one physical plane.
The planar assumption
Each screen click defines a line of sight from the camera. A 2-D calibration reports the point where that line intersects the calibrated plane. A target above or below the plane is therefore assigned a displaced position on the plane. The resulting bias depends on the target’s distance from the plane and the viewing geometry.
Place the calibration grid at the level of the landmarks that will be measured. For fish viewed from above, for example, decide whether positions refer to the tank bottom, the water surface, or a representative swimming depth. Pilot recordings can include known points or lengths at the expected range of depths. These checks show whether the planar approximation meets the study’s requirements.
Use 3-D calibration when depth is itself a measurement or when subjects move far enough from one plane to produce unacceptable 2-D bias.
Prepare a planar calibration grid
Use a flat, rigid grid with clearly visible nodes whose coordinates are known. The grid should cover the image region where measurements will be made, with nodes distributed across that region. Define the origin, axis directions, and units before entering the coordinates. Those definitions determine the coordinates and units in the output.
Record the grid in the same optical state as the study video: the same camera, focus, zoom or effective focal length, stabilization and cropping, resolution, housing or port, and camera position. Keep the camera and grid motionless for the calibration frame. Changes to these conditions can invalidate the distortion correction or planar calibration.
Lens-distortion correction works the same way for 2-D and 3-D measurements and remains especially important near the image edges. Complete the distortion-correction procedure before calculating the planar calibration.
A 2-D workflow
- Create the project and add the video clips. Give clips names that preserve camera and recording identity, select the intended master clip, and save the project. See Managing video clips in VidSync.
- Enable the full playback controls for a single clip. Open File → VidSync Preferences → Misc and select “Show advanced playback controls with only one video clip loaded.”
- Correct lens distortion. Use footage recorded with the same optical state as the study video and inspect the corrected result before continuing.
- Enter the grid coordinates and identify the nodes. Use one grid plane. Click visible nodes in their listed order and distribute the selected nodes across the working area. Delete an invisible node from the list before proceeding; do not infer its position. The point-entry principles described on the Calibration page also apply to the planar grid.
- Calculate the 2-D calibration for each clip or camera. Cameras covering different planar regions can have separate calibrations. Use the same coordinate definitions, or record the transformation between them, if their measurements will be combined.
- Validate the calibration. Measure known positions and lengths distributed across the working area. Include targets near the image edges and at representative heights or depths if subjects will depart from the plane. If the results do not meet the study’s requirements, inspect coordinate order, screen-click placement, distortion correction, grid flatness and position, and any camera movement before recalculating.
- Define and measure Objects and Events. Use written landmark definitions, point order, inclusion rules, and Notes codes as described in Measuring Objects and Events in VidSync.
- Review and export a sample. Check identities, point locations, units, and known measurements before processing the full dataset. See Exporting Data from VidSync.
Interpret the output
Only the two coordinates in the calibrated plane are determined geometrically. VidSync may display zeros for the camera position or unused dimensions after a 2-D calibration. These values are placeholders; the camera’s 3-D position cannot be recovered from a single plane. Use the two planar coordinates and the distances derived from them.
Lengths, trajectories, and speeds are projected into the calibrated plane. If two landmarks lie at different heights or depths, their planar separation differs from their true 3-D separation. Name the axes for the study system, such as tank x and y or downstream and cross-stream, and record the origin, positive directions, and units.
Using more than one camera
One camera is sufficient for a planar view. Additional cameras can extend coverage or record different planar regions. Each view requires its own distortion correction and planar calibration. Check overlapping regions with common known points before combining measurements. A top view and a side view may each support a separate 2-D analysis; recovering depth from the paired views requires a 3-D calibration.
Reporting a 2-D analysis
Describe the location and orientation of the calibrated plane, the grid and coordinate system, the camera and optical state, distortion correction, validation targets and results, and the expected departure of subjects from the plane. See How to Cite and Report a VidSync Study for the broader reporting framework.