Calibrating a BTSensor

BTSensor Calibration Introduction

If BTSensors that were initially placed in the Space were moved after setup or bumped accidentally, the BTSensors must be calibrated. BTSensor calibration enables the system to determine where the BTSensors are in relation to each other using the calibration wand, and where they are in relation to the origin using the ground plane.

Proper calibration is very important because incorrect calibrations will send incorrect position data and erroneously affect the BTSystem's accuracy.

Attention: Anytime a BTSensor is moved or bumped, BTSensor calibration must be performed and saved to the Motive project file. See Working with Recordings for details saving and playing back Motive recordings.

Differences Between Full Calibration and Refine Calibration

There are three ways to calibrate BTSensors: a Full calibration, a Refine calibration, and a Partial Refine calibration.

  • Full: This method calibrate BTSensors using no previous information. This calibration is always used when using BlackTrax in a new Space.
  • Refine: This calibration method uses existing information from a previously saved calibration as a starting point of the calibration. This calibration method is primarily used when BTSensors are moved slightly after an initial calibration and needs calibration again.
  • Partial: This calibration method uses existing information from a previously saved calibration as a foundation. The user selects either a Full or a Refine calibration type as above, and is able to select a number of BTSensors to perform the partial calibration upon. BTSensor calibration builds upon the previous calibration and corrects any errors. This calibration method is primarily used when some BTSensors are moved slightly after an initial calibration and need correction, but not the entire BTSensor rig.

Wand Calibration

Wand calibration enables BlackTrax to determine the relative position of BTSensors to each other in the Space. This is followed by ground plane calibration, which enables BlackTrax to determine the BTSensor’s relative position to the ground. See Ground Plane calibration below for more information.

Note: When wanding starts, Motive records the wanding and autosaves it to the session folder.

To calibrate BTSensors with the calibration wand

  1. In Motive, change the layout to Calibration preset. From the Layout menu, choose Calibration.
    1. Go to the Camera Calibration window.
    2. Choose either a Full or Refine calibration based on the guidelines above.
    3. In the Calibration tab, under the Wand Option heading, set the OptiWand sub-heading to Custom.
    4. Set the Wand Length (mm) field to 1000.
    5. Set the Center Distance (mm) field to 350.
  2. Click Start Wanding.
    • Result: BTSensors record calibration wand data within the Space. The Calibration Engine section records the samples of each BTSensor and the spread of the samples.
      1. In the Physical Space, plug the power source into the calibration wand, and connect the HPSU.
        • Result: The calibration wand’s LEDs should be visible in Motive as an infrared source.
      2. Wand the Physical Space by moving the powered calibration wand across the Space. Move the wand up and down, back and forth, and rotate the head while you move, to create different data points (think of a figure eight motion). Ensure the calibration wand LEDs are not covered or being blocked from BTSensor view.
        • Tip:
          • When in use, the front facing LED on the Slim 13E BTSensor turns blue to indicate that the wand is visible.
          • You must fill as much of a BTSensor's view with as much wanding data as possible to improve calibration accuracy.
  3. In Motive, look at the Camera Preview and Perspective View window. Perspective View shows all BTSensors on the ground when they do the very first wanding. Examine each BTSensor’s recording of the calibration wand. Wand samples are represented as colored lines. Each BTSensor is represented by a unique color. Each BTSensor should have the same number of samples recorded.
    • Attention:
      • If a BTSensor will not record samples while wanding, consult the Camera Preview window. See if other BTSensors are detecting the wand. If the wand does not appear on any other BTSensors, the area will not calibrate due to lack of coverage. If the wand shows up on at least two other BTSensors, continue wanding, but adjust the path and angle of the wand to give the BTSensors different views. If a section of the intended Space will not record samples after all of the steps above, cancel the calibration. Reposition the BTSensors to obtain better overlapping field of views of the tracking area.
      • If a BTSensor will not record samples while wanding, you may deselect the problem BTSensor in the Devices window and recalibrate with the remaining BTSensors. The area of the Space the problem BTSensor was covering may have less accurate tracking or even no tracking available, but the rest of the Space should be accurate.
    • Note: The Calibration Engine section gives feedback about the BTSensor calibration, including the number of times the calibration wand is viewed by each BTSensor. An estimate on the quality of the calibration is displayed based on the number of samples which were recorded. Calibration quality can be: Low, Medium, High, or Very High.
  4. Click Calculate.
    • Result: Motive starts to calibrate BTSensors based on the collected data. The current state of calibration is displayed at the top of the Calibration Engine followed by the calibration result rating and the overall quality.
    • Note:
      • The calibration data captured in Perspective View is colored to indicate data accuracy.
        • Blue: Less than 6mm accurate in that area.
        • Red: Greater than 88mm accurate in that area.
      • The calibration engine displays a quality rating for all BTSensors as well as the global average and the wand average based on the Desired Quality. The quality ratings are (in order from worst to best): Poor < Fair < Good < Great < Excellent < Exceptional. A full bar in the quality column is the best ranking.
      • A relatively high Error for a BTSensor indicates that the given BTSensor received poor samples during calibration. Restart the wand calibration process. At least two BTSensors must have vision of the calibration wand for data to be captured for calibration. There is a progress bar in the Results column to highlight the quality of each BTSensor.
      • While calibrating, BTSensors initially attempt to calibrate for the lowest level of quality. Once BTSensors have met the criteria of that quality level, the system will advance to the next level quality and attempt to calibrate. This process repeats until the highest quality calibration based off the data has been achieved. Once exceptional quality had been achieved, the system will apply the calibration automatically. 
      • In the Perspective View window you can see a visualization of the wand sample collected for calibration. The position of BTSensors will move to match their position in the physical Space as the system determines their location.
  5. Once the rating for each BTSensor reaches “Excellent” or greater, and the Overall Results quality rating reaches “Excellent” or greater, the calibration results will automatically be applied to the system.
    To apply the calibration manually
    1. In the Calibration Engine section click Apply Results.
      • Result: The Calibration Results Reports window will open. Calibration results are available
    2. In the Calibration Results Reports window click Apply.
      • Result: The calibration data will be applied to the BTSensors and they will know their position in the Space in relation to each other. BTSensors positions can be viewed in the Perspective View window in Motive. Ground plane calibration is required next to determine the BTSensor’s position relative to the origin (if using a Refine calibration, you may skip ground plane calibration).
  6. In Motive 2, click the File menu and select Save Current Take or click File menu and select Save Profile. In Motive 1.10 or lower , to save the changes to the project file, click the File menu and select Save Project.

Camera Calibration  

Partial BTSensor Calibrations

As mentioned above, it is possible for the user to calibrate sections of their BTSensor setup after the initial calibration, as opposed to having to recalibrate the entire BTSensor setup (as of Motive 1.10).

The partial BTSensor calibrations are based off either a Full or Refine calibration as you would do above. To be able to complete a partial calibration, Motive needs the calibrated BTSensors as reference. These are known as Anchor Cameras. You need at least 2 of these anchor BTSensors, in addition to the BTSensors you wish to calibrate in order to complete the calibration successfully. 

To complete a partial BTSensor calibration

  1. In Motive, change the layout to Calibration preset. From the Layout menu, choose Calibration.
    1. Go to the Camera Calibration window.
    2. Choose either a Full or Refine calibration based on the guidelines above.
    3. In the Calibration tab, under the Wand Option heading, set the OptiWand sub-heading to Custom.
    4. Set the Wand Length (mm) field to 1000.
    5. Set the Center Distance (mm) field to 350.
  2. Select the BTSensors you would like to calibrate from the list in the Devices window.
  3. Calibrate the system as you would above, confirming you would like to only calibrate the selected BTSensors.
  4. Once complete, you do not need to set the ground plane.

Note: You will see all BTSensors collecting data. Only the selected BTSensors will use this data to calibrate, and all other BTSensors will ignore this data. When calculating, Motive may return a poor result overall, but this is not indicative of the actual result of just the updated BTSensors, it is reporting as if you wanded the entire Space. It is important to verify the tracking data with a Beacon after a partial calibration has been completed to verify the calibration results. 

Known Wand Calibration Issues

  • Issue: When Calibration Engine is calculating, calibration might return a very high error (100+), a very poor rating for the BTSensors, and you may see the distortion map in the Camera View warp outside the camera square.
    Solution: Either the wand data you provided Motive does not match the real world wand (check the wand length and center distance from above) or Motive displays unsuccessful results. Reboot Motive and run the calibration recording again through the solver and if the wanding data itself is good, this normally fixes the issue.
  • Issue: After calibrating multiple times the Calibration Engine will not give a Global “Excellent” rating.
    Solution: The data samples taken from wanding may be of poor quality and could be an issue. Ensure the Space is properly masked from infrared interference, and BTSensors are aimed correctly. Start calibration again and rewand the Space.

Note: Not all the BTSensors will always receive an 'Excellent' or greater rating. Acceptable tracking can still be achieved with what appears to be poor results. The calibration report will let you know how accurate each BTSensor is relative to each other. If one BTSensor is poor, it might be okay if there are several other BTSensors seeing that same area.

Tip: The best way to verify calibration is to lay out the ground plane and ensure that each point is only shown as 1 marker (not two or more points or the points are moving when the ground plane is sitting on the ground and not moving).

Ground Plane Calibration

After the BTSensors are calibrated with the calibration wand, you must set the ground plane. This step will inform the BTSystem where they are in relation to the origin. It will also inform the BTSystem the direction of the axis.

Attention: The origin of the Space must be visible to a minimum of 2 correctly calibrated BTSensors for ground plane calibration to be accurate. 

Note: As of Motive 1.10, when performing Refine, Full Refine, or Partial Refine calibration types, the user does not have to reset the ground plane. 

To calibrate BTSensors with the ground plane

  1. In the Physical Space, unfold and lock the ground plane into an L shape, ensuring the arms are at a 90° angle.
  2. Plug in the power source to an outlet and connect the ground plane to a power source or battery pack.
  3. Move the center marker or the ground plane over the Origin of the Space. Adjust the feet of the ground plane to align it with the origin as defined in BTWYSIWYG. You can see if the ground plane is level with the built-in level on the ground plane arms.
    • Attention: The origin of the coordinate plane of the tracking area must be the same as the origin of the virtual representation of the tracking Space as made in BTWYSIWYG. If the virtual representation and actual tracking area do not share the same origin and axes orientation, positional data and fixture tracking will be incorrect. The origin also needs to match any Third Parties origin connected to BlackTrax.
    • Tip: Mark the origin of the Space and the orientation of the axes be marked for future reference.
  4. Aim the long arm of the ground plane to the venue +Y axis.
  5. Aim the short arm of the ground plane to the venue +X axis.
  6. In Motive, in the Camera Calibration window, in the Ground Plane tab, set the Vertical Offset (mm) field to 90mm.
    • Attention:
      • If the ground plane is not placed at the height of the Origin, such as on a tripod or stage, measure the distance between the X-Y plane and the center of the ground plane markers. Enter in this value in the Vertical Offset (mm) plus 90mm.
      • You can offset the ground plane position if the ground plane cannot be placed on the origin due to space restrictions, such as obstructions blocking the Origin from BTSensor view.
      • To offset the ground plane
        1. Measure the distance (mm) on each axis from the Origin to the  ground plane's position.
        2. Record the difference in the Capture Volume Translation table.
        3. Click Apply Translation.
          • Note: Each time you Apply Translation, the origin will offset by 1 by each unit you specified. You should clear the offsets back to 0 after making them in case the button is hit accidentally after you finish setting it.
          • Attention: It is possible to rotate the ground plane’s position in Motive without physically moving the ground plane
      • To rotate the ground plane
        • Click Apply Rotation.
        • Enter in the desired rotation values in the Capture Volume Rotation table.
  7. To apply the ground plane calibration, click Set Ground Plane.
    • Result: The BTSensors will know their position in relation to the ground and adjust accordingly in Camera View window in Motive.
    • AttentionIf Motive is having a hard time determining the ground plane, or the markers are not stable, you may try to increase the Maximum Residual in Application Settings > Live Reconstruction. This will help average out BTSensors that may not have a great calibration and provide stable tracking points for the Ground Plane.
  8. In Motive 2, from the File menu, choose Save Current Take or from the File menu, choose Save Profile. In Motive 1.10 or lower, to save the changes to the project file, from the File menu, choose Save Project.
    • Note: The axis in Motive is different than the axis in BlackTrax. Please keep that in mind when offsetting the ground plane in Motive. For reference:
      • BlackTrax +X = Motive -X
      • BlackTrax +Y = Motive +Z
      • BlackTrax +Z = Motive +Y