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Adjusting marker tracking

There are minor differences between the infrared sensitivy of the Cells, so it may be necessary to adjust the sensitivity of the tracking algorithm. To adjust the sensitivity, navigate to Calibration => Markers menu and click on the "Advanced" button to get access to all the marker tracking parameters.

In the advanced view there are series of sliders that can be adjusted, related to the tracking. Only a couple of these are useful for end user calibration. The most important parameters are:

  1. Luminosity threshold
  2. Keep alive when lost
  3. Marker motion filtering strength
  4. Marker motion filtering radius

Below is the meaning of these parameters, including the ones which are more seldom used.

  • Luminosity threshold (brightness of the while areas) is the most important value for marker tracking. It controls the expected brightness of the white areas of the markers. Typical range is 30 to 60. Values that are too high or too low will cause poor tracking.
  • Maximum length of the contour relates to the size of the markers. Larger values indicate larger markers, and smaller values indicate smaller markers. For typical applications, the default length (200 pixels) is a good setting.
  • Minimum length of the contour (relative to the maximum length) controls the minimum size of the marker. This is a value between 0.0 and 1.0. Lower values mean that the markers can be very different sizes, while higher values indicate that the markers must have a more constant size. Typical range for this parameter is .5 t o 0.8, although very low values seldom cause problems for the tracking. Values too close to 1.0 may cause poor tracking. There is seldom a need to adjust this parameter.
  • The number of knots between intervals for corner detection is an internal parameter related to the detection of corner points of the markers. Typical values are between 5 and 10. For smaller markers, smaller values need to be used.
  • Select between interior and exterior modes is an internal parameter related to making markers with inverted edge detection colors

  • Marker division indicates the information layout inside the marker, which in turn implies the number of bits that are stored in the marker. This value should match the division value that is used to generate the marker with the MarkerFactory application. The number of information bits is dependent on this parameter, with higher values giving more information. The number of information bits can be calculated with the following formula: 

      division * division - 4 = bits

    For example a 4x4 marker has 4 * 4 - 4 = 12 bits of information. This indicates that there are 4096 unique 4x4 markers.

  • Keep alive when lost: The marker tracker may not be able to detect a given marker on each frame, resulting in flickering of the tracking results. This effect can be cirmcumvented by using the keep-alive feature, that holds the marker in memory even if it is temporarily lost. This slider affects the delay for finally removing the marker from the list of current markers. Lower values may cause flickering tracking, while higher values make the marker tracking more stable. Typical values are between 5 and 20.
  • Minimum difference between dark and bright areas is an internal parameter that controls rejection of marker like noise patterns. Typical value is 10, and it is seldom adjusted.
  • Delay before reporting a new marker: The marker tracking includes a filtering step to stabilize the marker code value. This filtering requires a few frames to work reliably. Using this parameter, one can cause an artificial delay before reporting a new marker that is detected. Increasing this value makes the initial marker code value calculation more accurate.
  • Maximum allowed ratio for sides controls how easily a (slightly) non-square marker is accepted as a square marker. This value is usually between 1.2, and 1.4. There is seldom need to adjust this parameter.
  • Adjust the corner offset is an internal parameter that is used for fine-tuning the read-out of the marker control points and code value. Typical values are between 0.2 and 0.3. There is seldom need to adjust this parameter.
  • Marker motion filtering strength: The marker locations are filtered to reduce jitter in the tracking results. This parameter controls the maximum strength of the filter. Lower values cause less filtering (and faster response), while higher values cause smoother tracking, but with higher latency. This filtering only affects slow movements, when the movement speed is increased the filter is not used any more.
  • Marker motion filtering radius is another parameter that affects the behavior of the motion filter. Lower values cause less filtering (and faster response), while higher values cause smoother tracking with higher latency.