Table of Contents
Metric Compute Volume of a Convex Hull Across Frames
Overview
The metric command Compute Volume of a Convex Hull Across Frames can be used to find the area or volume of the convex hull for a dataset across several frames. A convex hull is defined as the smallest convex set containing all of the points in a Euclidian space. It can be visualized in 2 dimensions as a string or rubber band around the perimeter of a set, or in 3 dimensions as shrink wrap enclosing the entire range of data points.
The volume or area of the convex hull of a set can be used to describe the amount of variance in a set or to describe the geometry of the distribution of points.
Pipeline Command
The pipeline command for Compute Volume of a Convex Hull Across Frames can be found in the Metric folder in the Pipeline Workshop and is as so:
Metric_Compute_Volume_of_a_Convex_Hull_Across_Frames /SIGNAL_TYPES= ! /SIGNAL_FOLDER=ORIGINAL ! /SIGNAL_NAMES= ! /RESULT_FOLDER=PROCESSED /RESULT_NAME= ! /NUMBER_OF_DIMENSIONS= ! /COMPONENT_SEQUENCE= ! /REQUIRE_ALL_SIGNALS=TRUE ! /START_FRAME= ! /END_FRAME= ! /EVENT_SEQUENCE= ! /EXCLUDE_EVENTS= ;
Command Parameters
The following table shows the command parameters seen above and their descriptions:
| SIGNAL_TYPES | Specify the signal type |
| SIGNAL_FOLDER | Specify the origin folder |
| SIGNAL_NAMES | Specify the Signal to be used |
| RESULT_FOLDER | The name of the result signal folder |
| RESULT_NAME | The name of the result signal |
| NUMBER_OF_DIMENSIONS | Number of dimensions the hull will be computed for (2 or 3) |
| COMPONENT_SEQUENCE | Specify the Signal components to be used (e.g. X + Y + Z or 0 + 1 + 2 etc) |
| REQUIRE_ALL_SIGNALS | True or False: require all signals to exist |
| /START_FRAME= | The frame at which to start recording data. |
| END_FRAME= | The frame at which to stop recording data. |
| EVENT_SEQUENCE | A list of events (separated by “+” signs). For example, LHS+RTO |
| EXCLUDE_EVENTS | If this event occurs before the first and last event, do not computed a metric |
Dialog
The command can be edited in a text editor or in a dialog form. To edit in the dialog pop up form either click on the Edit button in the pipeline workshop or double-click on the pipeline command. The dialog is shown below:
The dialog box allows you to assign values to the command parameters outlined above
Example: 2D convex hull
Here the Metric Compute Volume of a Convex Hull Across Frames command is used to compute the convex hull for right and left foot GRF data during a balance trial. By computing a 2 dimensional (X and Y) hull for the GRF at each foot we can make comparisons between sides while balancing and identify potential asymmetry.
The command is as so:
Metric_Compute_Volume_of_a_Convex_Hull_Across_Frames /SIGNAL_TYPES=FORCE /SIGNAL_FOLDER=ORIGINAL /SIGNAL_NAMES=FP3 /RESULT_FOLDER=PROCESSED /RESULT_NAME=GRF_right_hull /NUMBER_OF_DIMENSIONS=2 /COMPONENT_SEQUENCE=X+Y /REQUIRE_ALL_SIGNALS=TRUE ! /START_FRAME= ! /END_FRAME= /EVENT_SEQUENCE=Start+End ! /EXCLUDE_EVENTS= ; Metric_Compute_Volume_of_a_Convex_Hull_Across_Frames /SIGNAL_TYPES=FORCE /SIGNAL_FOLDER=ORIGINAL /SIGNAL_NAMES=FP4 /RESULT_FOLDER=PROCESSED /RESULT_NAME=GRF_left_hull /NUMBER_OF_DIMENSIONS=2 /COMPONENT_SEQUENCE=X+Y /REQUIRE_ALL_SIGNALS=TRUE ! /START_FRAME= ! /END_FRAME= /EVENT_SEQUENCE=Start+End ! /EXCLUDE_EVENTS= ;
The resulting metrics show the area of the 2 dimensional hull created for each side, a larger area points to more variation in force during the trial.
Results:
Example: 3D convex hull
In this example Metric Compute Volume of a Convex Hull Across Frames is used to find the volume occupied by an individual's thorax segment between events as defined in the workspace.
The pipeline command is as follows:
Metric_Compute_Volume_of_a_Convex_Hull_Across_Frames /SIGNAL_TYPES=TARGET ! /SIGNAL_FOLDER=ORIGINAL /SIGNAL_NAMES=CLAV+STRN+CV7+T10+RSHO+LSHO ! /RESULT_FOLDER=PROCESSED /RESULT_NAME=Upper_Thorax_Volume /NUMBER_OF_DIMENSIONS=3 /COMPONENT_SEQUENCE=ALL ! /REQUIRE_ALL_SIGNALS=TRUE ! /START_FRAME= ! /END_FRAME= /EVENT_SEQUENCE=EVENT1+EVENT2 ! /EXCLUDE_EVENTS= ;
Here a 3 dimensional hull is used as the resulting metric will represent a 3 dimensional structure in space. The resulting metric value represents the approximate volume of the space that the upper thorax segment travels through between the events listed:
