Inspect3D Tutorial: Curve Registration

Overview of Curve Registration

Curve registration has been proven to aid in analysis of biomechanical signals, and can produce more accurate results in time continuous analysis including PCA, due to sensitivity towards time varying events. When looking at a signal in a dataset, we typically see variation in the amplitude and phase. When physical time may not impact or be relevant to real life systems, there is sometimes a need to align the curves on a system time rather than a clock time to correct phase variation. This is done by registering the curves to a specific landmark. In the world of biomechanics and gait analysis, we can register curves by certain events, such as the max or min of a signal to see a better representation of amplitude variation.

Mathematics of Curve Registration

Curves are registered by applying a time warping function h(t), on the original curve x(t).

     x* (t) = x [h(t)]

The time warping function is unique to each curve, that transforms the physical (typically clock) time to the system time. This require a landmark for the whole system to be registered, by taking the mean location of a landmark across all curves. The time warping function than adjusts each individual curve to ensure the curve specific landmark happens at the same time through linear interpolation. The time warping ensures that the following equation is always satisfied.

     h-1 [h(t)] = t

Curve Registration in Inspect3D

In order to complete curve registration a library needs to be loaded, and queries need to be computed. Once that is completed you can select the curve registration tab, at the bottom of the window. You will then be able to see all the queries computed in the bottom left table, labeled Available Traces.

Registering Curves

To register a curve select the wanted query from available trace, and select the Register button. This will prompt a dialog to come up allowing you to chose how you want to register the curve. Expanding the query allows you to choose select workspaces you want to register the data to.

In this example we selected the KneeAngleX query to be registered.

The first line of the dialog says how many curves are available to be registered. The number of traces only includes non excluded data, if some data is excluded the number of traces will reflect that.

The second line prompts the user to select a name. In this example we will register the KneeAngleX query and name it KneeAngleX-Registered.

The third line prompts the user to either register the curve by the maximum or minimum. In this example we will register it by the maximum.

The fourth line of the dialog specifics the interpolation type, at this time the warping function of the registered curve is being computed through linear interpolation.

Once you are happy with your selection you can hit the Register button in the dialog.

Note: Metrics are unable to be registered as they are not time based measures.

Plotting Registered Curves

Once a curve is registered it will appear in the Registered Traces table. Selecting the registered curve and selecting Refresh Plot will make the graph appear. Selecting a single trace will allow you to see the information of the trial.

In this example we can see the knee angle being registered to its maximum, happening around 70% of the gait cycle. Comparing it to the original curve, it is clear the difference, while maintaining the important information. Selecting the trace diverging away from the rest, we can it was part of subjects 5 workspace.

Editing Plots

To edit plots, right click on the plot and a context menu will appear. Selecting the X-axis Edit or Y-axis Edit will allow you to change the axis name and the axis min and max. Selecting the Legend will allow you to choose if you want a legend to appear. To change the line style of the curve (plot by groups, workspaces or subgroup), simply go to the Options tab as you would for the Queried Data.

Registering Additional Curves

Multiple curves can be registered, with each individual registered curve stored in the Registered Traces Table. To register more curves simply follow the steps above. Here we will register the HipAngleX query to its minimum. As you can see it is stored right below the previously registered curve. To view it simply select it and click Refresh Plot.

Exporting Results

Curve registration results can be exported as an image, or as text file containing all the data points. To export an image of the plot, right click on it to show the context menu. In the context menu select Export. A dialog will appear allowing you to choose which type of image you want it exported as, and what size.

To export the data go to the Export Results tab on the tool bar. Change the Export Type to Registered Curves and export the results.