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Accuracy questions

How Accurate / Precise are your systems?

The answer to this question depends significantly on the definition of the terms ‘Accuracy’ and ‘Precision’ (see ‘How Are the Terms ‘Accuracy’ & ‘Precision’ defined?), and how it is measured (see ‘How Do You Test Accuracy / Precision?’). Differing methods produce widely varying results (image below) and care must be taken when making comparisons to take account of all the variables involved.

The figures provided below are obtained following ASTM E3064 standard (‘Standard Test Method for Evaluating the Performance of Optical Tracking Systems that Measure Six Degrees of Freedom (6DOF) Pose‘) and represent the ability to track the relative position of a rigid object while the object is moving, with no filtering or post-processing.

High-End Volume (Vantage V16) Mid-Range Volume (Vantage V5)
Total Number of Capture Frames 41993 62525
Reference Value (Measured Bar Length, r, mm) 320.880 320.880
Mean Observed Bar Length (?̅, mm) 320.863 320.897
Accuracy of Mean to Reference Value (or ‘Trueness’, |?̅−?|, mm) 0.017 0.017
Standard Deviation of Observations (?, mm) 0.198 0.321
Root Mean Squared Error (RMSE, mm) 0.201 0.324

* Mean of all trials

Example trial data for a single capture from the larger dataset
Example trial data for a single capture from the larger dataset. In total, there were 20 trials across 4 calibrations for each configuration, for a volume of 192m3 with 24 cameras.

Please contact Vicon ([email protected]) for further details or to request a white paper detailing the experimental methods used.


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How Are the Terms ‘Accuracy’ & ‘Precision’ defined?

The following terms are used and referenced as defined by the International Standards Organization:

  • Accuracy – As defined by ISO 3534:2014: “The closeness of agreement between a test result and the accepted reference value”
  • Precision – As defined by ISO 3534:2014 “The closeness of agreement between independent test results obtained under stipulated conditions. Note 1: Precision depends only on the distribution of random errors and does not relate to the true value or the specified value”
  • Accuracy of the Mean (‘Trueness’ or ‘Bias’) – As described in ISO 5725-1:1994-0.1 there are ‘two terms “trueness” and “precision” to describe the accuracy of a measurement method. “Trueness” refers to the closeness of agreement between the arithmetic mean of a large number of test results and the true or accepted reference value. “Precision” refers to the closeness of agreement between test results’.

 

Targets

Our experimental method measures the precision as the level of agreement of each frame within capture. When the term ‘Accuracy of the mean’ is used, it equates to ‘Trueness’ in the above ISO definition.

Please contact Vicon ([email protected]) for further details or to request a white paper detailing the experimental methods used.


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How Do You Test Dynamic Accuracy / Precision? Why Is It More Important Than Static Measurements?

Due to the complexity and many variables involved in motion capture systems, direct comparisons between different experimental methods are close to impossible, and results will vary with different test conditions.

It is important any measurement used reflects the typical use for an optical motion capture system, and in particular measures a moving (dynamic) object. Our standardized test results were obtained using the method described in ASTM E3064 (‘Standard Test Method for Evaluating the Performance of Optical Tracking Systems that Measure Six Degrees of Freedom (6DOF) Pose‘) which is highly reflective of a typical use case.

Below in an example, a separate test shows the difference simply between a measurement taken in a single capture, for an object that starts stationary, and then begins moving part way through the capture, with no other changes being introduced.

Measured Bar Length - Statis vs Dynamic Test
Single capture showing difference between measurement of a static and dynamic object

Mean (mm) Max Error (mm) Root Mean Squared Error (mm) Standard Deviation (mm)
Static Object 320.875 0.035 0.007 0.005
Dynamic Object 320.979 0.397 0.142 0.102

The introduction of movement to on object significantly increases the variability of the measurements being taken in the raw data.

This involves the measurement of an object comprising two clusters of passive markers (forming a rigid body at each end). The distance between them was measured to produce a reference measurement. This object was moved around an optical capture volume in a defined way, according to the standard ASTM E3064, and the two rigid bodies were tracked independently by the capture system and compared to an external fixed reference measurement.

Please contact Vicon ([email protected]) for further details or to request a white paper detailing the experimental methods used.


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What Variables Can Affect A System’s Performance, and How Can I Maximize the Effectiveness of my System?

Optical motion capture systems are complex. The resolution and quality of the cameras and lenses themselves, the calibration process, the object and the software tracking algorithms all play a important part in the quality of the data.

In addition to these, environmental factors can influence the measurements, such as the size and shape of the capture volume, the physical shape and movement of the object being measured, or the mounting of cameras and the physical structure they are mounted to.

For an example, this paper [P4, Figure 4] demonstrates the movement of a camera mount over a 24 hour period as measured by a laser measurement device, showing the difference in movement between an internal and external wall mounting.

This method is focused on the positional tracking of rigid bodies. Other variables may be involved in other applications, such as biomechanics, such as marker movement when attached to the skin, which may introduce additional errors.

In addition to world class equipment and software, Vicon has a highly trained expert team of support engineers who will install your system for you and make sure that our experience of 35 years in motion capture systems ensures it translates to the best possible performance for your system.

Please contact Vicon ([email protected]) for further details or to request a white paper detailing the experimental methods used.


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