3. Data Collection¶
3.1. Data Collection Principles¶
The following guidelines describe data collection best practices, but are not exhaustive. You may obtain high quality markerless motion capture data under different conditions from those described below.
Camera Setup
Camera system setups differ from location to location, and may be subject to challenging data collection environment constraints. General recommendations for setting up your camera system include:
Cameras as close as possible to the capture volume, while ensuring the entire capture volume remains within view for all cameras.
Cameras 4 to 8 feet above the ground.
Avoid partial views of subjects, such as lower body or upper body only.
Avoid unusual camera views, especially very high or very low.
Aim for a symmetrical camera setup that surrounds the entire capture volume, such as a circle, oval, or rectangle.
Camera Settings
The camera settings you choose will depend on the movements being collected. Selection of appropriate camera settings is crucial for collecting clear, crisp video images. The most important camera settings to consider include:
Frame rate: fast movements require high frame rates.
Shutter speed / Exposure: should be set based on the frame rate. General rule: 1/(2x frame rate, or higher) seconds. E.g. for 60 FPS frame rate, an appropriate shutter speed would be 1/125 seconds or 8000 microseconds.
If you are collecting high-speed movements, you may need to consider introducing additional light into your capture volume in order to capture videos that are adequately bright. In general, faster movements require higher frame rates, faster shutter speeds / shorter exposures, and more light.
Subject Attire
Theia3D provides 3D pose estimates that are robust to changes in attire. General recommendations for subject attire include:
Body-fitting clothing. Each limb should be discernible from the rest of the body.
Clothing should provide rich visual features, such as visible creasing, shadows, or other textures.
Lighting is often more important than attire color, as it impacts the visual richness of the attire. Under adequate lighting conditions black attire is acceptable, but lighter colors generally provide more visual features.
3.2. How-To: Record Intrinsic Lens Calibrations¶
Intrinsic lens calibration trials are used to determine parameters associated with the camera lenses and are used to correct for distortion and other visual effects. Lens calibration trials are required for OptiTrack Prime Color camera users, and may be required for Qualisys and Vicon camera users who are not using those third parties’ wand calibration procedures.
Lens calibrations must be performed at least once per video resolution that will be used to record movement data, and any time the lenses or focal lengths change. If you intend on collecting data at 1080p, 720p, and 540p, you will need to perform separate lens calibration trials using each of those resolutions.
Note: Adjusting the aperture and focus of OptiTrack Prime Color cameras using the dials on their lenses does not necessitate new lens calibrations.
Recommendations:
Record lens calibration trials at a low frame rate (20 Hz) to reduce file size and processing time.
Use a computer monitor facing the person performing the calibration to provide visual feedback during the calibration.
Move slowly and deliberately to prevent chessboard blur.
Keep the chessboard as flat as possible throughout the calibration trial.
Recording Lens Calibrations:
Place all cameras side by side on a desk or table, facing the same direction and capturing identical views.
Stand at a distance where the chessboard occupies ~1/4 of the camera views.
Begin the recording.
Slowly move the chessboard in a systematic grid pattern, covering the entire field of view for every camera. Ensure the chessboard goes right to every edge of every camera field of view, and every corner.
Take a step back, and repeat Step 4., covering the camera fields of view again.
While covering the field of view, angle the chessboard slightly in random multiple directions, varying its orientation throughout.
When you are confident that the entire field of view has been covered for all cameras, end the recording.
3.3. How-To: Record Extrinsic Chessboard Calibrations¶
Extrinsic chessboard calibration trials are used to determine the position and orientation of every camera in your camera system relative to your desired global coordinate system. A minimum of one chessboard calibration trial should be collected every time you set up your camera system, but collecting multiple is recommended. A new calibration is required any time a camera is moved, so having multiple calibration trials collected throughout a long data collection session can help prevent data loss due to accidental or unnoticed camera movements.
Chessboard calibrations must be performed at the resolution that will be used to record your movement data. If you will be using more than one resolution during your data collection, you must record chessboard calibration trials at each resolution.
Recommendations:
Record chessboard calibration trials at a low frame rate (20 Hz) to reduce file size and processing time.
Avoid bending the chessboard during the calibration trial.
Focus on achieving groupings of 3+ cameras that can see the chessboard at all times.
Align the chessboard with marks on the ground so you can confirm that the global coordinate system is positioned correctly when processing movement trials.
Recording Chessboard Calibrations:
Set up your camera system as desired for your data collection, following the recommendations in Data Collection Principles.
Choose or place visible marks on the ground which will be aligned with the global coordinate system, for confirmation when processing data.
Check that the chessboard is fully visible in at least 3 camera views when it is placed at the desired global coordinate system origin position.
Begin the recording.
Slowly ‘show’ the chessboard to groupings of 3 or more cameras while varying the position and orientation of the chessboard slightly. Ensure the chessboard is visible to cameras that overlap between groupings, ideally so that no fewer than 3 cameras can see the chessboard at all times. Focus on achieving groupings of 3+ cameras at all times.
When you are confident that all cameras have had sufficient views of the chessboard, slowly place the chessboard on the ground, aligned with your preselected marks.
Ensure you are not obstructing the view of the chessboard in any cameras during this localization phase.
End the recording.
3.4. How-To: Record Extrinsic Object Calibrations¶
Note: Chessboard calibration is the preferred extrinsic calibration method (rather than object calibration).
Extrinsic object calibration trials are used to determine the position and orientation of every camera in your system relative to the desired global coordinate system. Extrinsic object calibration uses the static position of a calibration object with known dimensions or known positions of specific key points on the object. These 3D dimensions or positions must be measured with high precision. The calibration object should be sufficiently large or the calibration key points should be spaced far apart within the capture volume and most points should be visible in every camera view. The key points can be coplanar or can vary in all three global dimensions.
A calibration object file must be created for your calibration object, containing 3D coordinates of the object key points. Each line of the file must contain three comma-separated values representing the global x-, y-, and z-coordinates of the key point, in millimeters. Each key point should be on its own line.
Object calibrations must be performed at the resolution that will be used to record your movement data. If you will be using more than one resolution during your data collection, you must record object calibration trials at each resolution.
Recommendations:
Record object calibrations trials at a low frame rate (20 Hz) to reduce file size and processing time.
Object calibration trials only need to be a few seconds long.
Recording Object Calibrations:
Place your calibration object within the capture volume, at the desired position and orientation to define the global coordinate system.
Ensure the key points on your calibration object are visible in every camera view, and there are no obstructions.
Begin the recording.
After a few seconds, end the recording.