apply_butter: Applies a Butterworth filter to each event.
In PupilPre: Preprocessing Pupil Size Data

apply_butter applies a Butterworth filter to the pupil size data.

1 2	apply_butter(data = data, n = NULL, W = NULL, type = NULL, plane = "z")

`data`	A data table object.
`n`	A number specifying the filter order (as described in butter).
`W`	The critical frequencies of the filter (as described in butter). W must be a scalar for low-pass or high-pass filters. W must be a two-element vector c(low, high) specifying the lower and upper bands for stop-band or band-pass filters. For digital filters, W must be between 0 and 1 where 1 is the Nyquist frequency.
`type`	The filter type (as described in butter), one of "low" for a low-pass filter, "high" for a high-pass filter, "stop" for a stop-band (band-reject) filter, or "pass" for a pass-band filter.
`plane`	A character string (as described in butter), "z" for a digital filter or "s" for an analog filter.

# Load example data
data("Pupilex5")

dat <- apply_butter(Pupilex5, n = 1, W = 0.1,
                    type = "low", plane = "z")

# Please see the vignettes for detailed example usage.
vignette("PupilPre_Interpolation_and_Filtering", package="PupilPre")