Centre of Pressure Per Region

Computes one centre-of-pressure trajectory for each region mask, weighted only by that region's sensors. Returns a list of pr_cop objects, so the existing trajectory vocabulary — path length, mean and maximum velocity, the x/y ranges pr_calc_cop_excursion() reports, and the 95% ellipse sway area — is available per region with no further work.

Usage

pr_calc_cop_masked(trial, masks = NULL, threshold = 0, units = c("grid", "mm"))

Arguments

trial

A pr_trial object.

masks

Named list of pr_mask objects or logical matrices. NULL (default) uses the layout's own regions.

threshold

Numeric. Cells at or below this value are given zero weight. Default 0.

units

Character. "grid" (default) or "mm"; see Units.

Value

A named list of pr_cop objects, one per mask, in the order the masks were given.

Details

A whole-mat COP is a single point that can sit in an unloaded gap between two loaded zones, and it moves whenever load shifts between zones. A per-region COP cannot: it stays inside its own region and reports how load migrated within it, which is what separates a saddle that rocks fore-and-aft from one that slides sideways.

Frames in which a region carries no load at all get NA for both axes, exactly as pr_calc_cop() does for the whole mat. pr_cop() excludes those frames from the trajectory metrics, so a region that is loaded only intermittently still reports a meaningful path length — though one measured across the gaps, not through them.

Units

units = "grid" (the default) returns grid index units: x is the fractional sensor column, y the fractional sensor row, both 1-based, in the convention of pr_calc_cop_grid(). This is always available.

units = "mm" returns the layout's physical x_mm / y_mm coordinates, as pr_calc_cop() does. It is refused for a layout whose coordinates are grid indices because no sensor pitch was ever published (see the Missing physical scale section of pr_layout_from_index_map()) — millimetres would be fabricated, and a path length in fabricated millimetres is worse than none.

Note that a pr_cop object labels its fields mm regardless; under units = "grid" they are grid indices, and the derived path length and velocity are in index units and index units per second.

See also

pr_calc_cop() and pr_calc_cop_grid() for the whole mat, pr_calc_cop_excursion() for the range summary.

Other regional symmetry functions: pr_calc_regional_map(), pr_calc_symmetry_map(), pr_mask_mirror_balance(), pr_mask_rowbands(), pr_symmetry_sensitivity()

Examples

trial <- pr_example_trial("saddle_horse")
cops <- pr_calc_cop_masked(trial)
names(cops)
#> [1] "cranial_left"  "cranial_right" "middle_left"   "middle_right" 
#> [5] "caudal_left"   "caudal_right" 

# Each region's COP stays inside its own rows:
round(vapply(cops, function(c) mean(c$y, na.rm = TRUE), numeric(1)), 2)
#>  cranial_left cranial_right   middle_left  middle_right   caudal_left 
#>          3.80          3.79          8.50          8.49         13.25 
#>  caudal_right 
#>         13.26 

# The full pr_cop vocabulary works unchanged:
round(vapply(cops, function(c) c$path_length, numeric(1)), 3)
#>  cranial_left cranial_right   middle_left  middle_right   caudal_left 
#>        72.419        84.384       104.211       119.473        75.375 
#>  caudal_right 
#>        90.145 

# This layout has a real sensor pitch, so millimetres are honest:
round(pr_calc_cop_masked(trial, units = "mm")$cranial_left$range_x, 1)
#> [1] 26.8