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--- visual3d:documentation:pipeline:force_commands:fp_zero [2026/01/18 18:24] – [Example 3] wikisysop
+++ visual3d:documentation:pipeline:force_commands:fp_zero [2026/06/09 20:38] (current) – wikisysop
@@ Line 1: / Line 1: @@
 ====== FP ZERO ======
-This page is about the C3D Parameter FORCE_PLATFORM:ZERO. For Visual3D's ability to track different baseline frames for different force platforms see [[Visual3D:Documentation:Pipeline:Force_Commands:FP_ZEROS|FP_ZEROS]].
+**NOTE**: This page is about the C3D Parameter FORCE_PLATFORM:ZERO. For Visual3D's ability to track different baseline frames for different force platforms see [[Visual3D:Documentation:Pipeline:Force_Commands:FP_ZEROS|FP_ZEROS]].
 The FORCE_PLATFORM:ZERO C3D parameter defines the range of frames used to calculate an average baseline value for each ANALOG signal that is then subtracted from every frame. This does not affect the ANALOG signals (as seen in the data tree), this processing takes place when the Force Platform signals are computed.
-Note that most of the manufacturers arbitrarily set the range to be the first 10 frames of the trial. If the force platform is loaded during these first 10 frames, the ground reaction force signals will be incorrect; in this case the user should either set the frames to 0,0 so that no baseline is subtracted, or should select frames in which the force platform is unloaded.
+Note that most of the manufacturers arbitrarily set the range to be the first 10 frames of the trial. If the force platform is loaded during these first 10 frames, the ground reaction force signals will be incorrect; in this case the user should either:
+  * set the frames to 0,0 so that no baseline is subtracted; or
+  * select frames in which the force platform is unloaded.
-===== Exceptions =====
+===== Modify_Force_Platform_Parameters =====
-In some cases it is not feasible to use the ZERO parameter to define a BASELINE for each signal. All force platform signals have some drift in the amplifiers, so it isn't a good idea to simply set the ZERO parameter to 0,0 so that no baseline is subtracted. The following two examples demonstrate alternatives to using the ZERO parameter to remove a baseline.
+The FP_ZERO parameter can be updated using the [[visual3d:documentation:pipeline:force_commands:modify_force_platform_parameters|Modify_Force_Platform_Parameters]] pipeline command and specifying the desired /FP_ZERO parameter. It can also be updated interactively using the [[visual3d:documentation:pipeline:force_commands:modify_force_platform_parameters#modify_force_platform_parameters_command_dialog|associated dialog]].
-In all 3 examples the ZERO parameter should be set to 0,0 so that no inadvertent baseline is removed.
+<code>
+Modify_Force_Platform_Parameters
+/FP_USED=2
+/FP_ZERO=0+0
+/FP_ZEROS=0+0+0+0
+;
+</code>
+**NOTE**: The value specified for /FP_ZERO can be an expression.
+===== General Examples =====
+The following two examples are general examples for working with the ZERO parameter.
+==== Example 1: Using the last 10 frames for non-treadmill data ====
+For non-treadmill data we recommend that you choose the last 10 frames of the trial as almost every activity ends with the participant off of the force platform.
+<code>
+Modify_Force_Platform_Parameters
+/FP_USED=2
+/FP_ZERO=EOF-10+EOF
+/FP_ZEROS=EOF-10+EOF+EOF-10+EOF
+;
+</code>
+==== Example 2: Using COFP DATA_NOT_FOUND for Treadmill Trials ====
+Working with treadmill data, we suggest finding a period of time for each force platform where the [[visual3d:documentation:visual3d_signal_types:cofp_data_type|COFP]] signal is [[visual3d:documentation:c3d_signal_types:data_not_found|DATA_NOT_FOUND]] since this is a reliable indication that the force platform is unloaded and that these frames can be used as a baseline. This will likely have to be done manually per force platform and per file as your treadmill force platforms will likely have different loading and unloading patterns throughout the trials.
-===== Example 1 =====
+===== Examples Removing Baselines without the ZERO Parameter =====
-Subtracting the mean value from several ranges from each analog signal
+In some cases it is not feasible to use the ZERO parameter to define a BASELINE for each signal. All force platform signals have some drift in the amplifiers, so it isn't a good idea to simply set the ZERO parameter to 0,0 so that no baseline is subtracted. The following three examples demonstrate alternatives to using the ZERO parameter to remove a baseline. In each example the ZERO parameter is set to 0,0 so that no inadvertent baseline is removed:
-Consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.
+==== Example 1: Subtract the mean value for several frame ranges ====
-Given 12 ANALOG signals associated with a type 6 Force Platform:
+This example demonstrates how to subtract the mean value from several ranges from each analog signal. Consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.
-FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4
+Given 12 ANALOG signals associated with a type 6 Force Platform: FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4
 <code>
@@ Line 86: / Line 117: @@
 </code>
-===== Example 2 =====
+==== Example 2: Using Evaluate_Expression to subtract mean values from several frame ranges ====
-Subtracting the mean value from several ranges from each analog signal
+Again we will see how to subtract from each analog signal its mean value computed from several frame ranges; this time using Evaluate_Expression. Again we consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.
-Consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.
+Given 12 ANALOG signals associated with a type 6 Force Platform:FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4
-Given 12 ANALOG signals associated with a type 6 Force Platform:
-FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4
 <code>
@@ Line 137: / Line 164: @@
 ;
-! Now we can evaluate the expression. Note: That in Version 3.69 (the time of writing this topic) the evaluate_expression command was quite slow because of the method used for parsing the expression.
+! Now we can evaluate the expression.
 Evaluate_Expression
@@ Line 150: / Line 177: @@
 ;
 </code>
-===== Example 3 =====
-Computing mean values for the force platform analog signals in one trial, and then use these values in other trials
-For this example assume that:
+==== Example 3: Using mean values from another trial ====
-  - one of the files has TAG=walk.
+This example demonstrates how to compute mean values for the force platform analog signals in one trial, and then use these values in other trials. For this example assume that:
-  - there are two AMTI force platforms
+  * one of the files has TAG=walk.
-  - the force platforms are unloaded for the first 50 frames
+  * there are two AMTI force platforms
+  * the force platforms are unloaded for the first 50 frames
 <code>