Choice of markers

Motion capture measurements require the use of markers. The markers can either be reflecting or emitting near infrared light. The first type is referred to as passive markers, and the latter is referred to as active markers.

Passive vs Active markers

Passive markers can be made very light-weight and in many different sizes, therefore it is usually the best option. However, in some setups it can be good to have an active marker, for example because of unwanted reflections or at very long distances.

Qualisys offers several types of active markers for different applications. For more information about Qualisys active marker solutions, see chapter Active marker types.

You can select the Untriggered active marker option if you are using generic, constantly lighting active markers. When using this setting, the strobes of the cameras are inactivated to minimize the amount of unwanted reflections.

For more information about how to use active markers, see chapter How to use active markers.

Marker size

It is important to choose the correct marker size, since a larger marker gives a larger area to use to determine the central point of the marker. The central point data is the input to the tracking of the markers movement and is therefore very important.

Rules to determine which marker size to use are:

  • Use as big markers as possible without the markers being in the way or restraining normal movement.

  • If the markers are attached very closely to each other, use as large markers as possible without the markers merging in the 2D view of the cameras. If the markers merge, the central point calculation of these markers will be ambiguous.

  • To achieve a good accuracy the size of the markers in the 2D view should be at least 200 subpixels. Check the size of the markers in the 2D view for each camera in the Data info window. Right-click in the Data info window and click Display 2D data to choose the camera. If the size is smaller the central point calculation will be poor.

Marker placement

The markers should be placed in a way so that they are visible during as much of the measurement time as possible. Check that clothes or rotating objects, such as body parts, do not move in a way that hides the markers from the camera.

There is a wide range of marker sets for tracking humans for biomechanical and animation applications. For detailed information, refer to the following resources:

  • Marker guides for animation, available via the Skeleton menu.

  • Marker guides for analysis modules, available via the Show guide button in the PAF project view.

  • QAcademy tutorials. There are several QAcademy tutorials available on how to apply specific marker sets, see https://www.qualisys.com/qacademy.