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#' Calculate signal envelope.
#'
#' The function calculates envelopes of the input signals as
#' cosine-tapered envelope of the Hilbert-transformed signal. The signal
#' should be detrended and/or the mean should be removed before processing.
#'
#' @param data \code{eseis} object, \code{numeric} vector or list of
#' objects, data set to be processed.
#'
#' @param p \code{Numeric} value, proportion of the signal to be tapered,
#' default is \code{0}.
#'
#' @return \code{Numeric} vector or list of vectors, signal envelope.
#' @author Michael Dietze
#' @keywords eseis
#' @examples
#'
#' ## load example data set
#' data(rockfall)
#'
#' ## detrend data set
#' rockfall_detrend <- signal_detrend(data = rockfall_eseis)
#'
#' ## calculate envelope
#' rockfall_envelope <- signal_envelope(data = rockfall_detrend)
#'
#' ## plot envelope
#' plot_signal(data = rockfall_envelope)
#'
#' @export signal_envelope
signal_envelope <- function(
data,
p = 0
) {
## check data structure
if(class(data)[1] == "list") {
## apply function to list
data_out <- lapply(X = data,
FUN = eseis::signal_envelope,
p = p)
## return output
return(data_out)
} else {
## get start time
eseis_t_0 <- Sys.time()
## collect function arguments
eseis_arguments <- list(data = "")
## check if input object is of class eseis
if(class(data)[1] == "eseis") {
## set eseis flag
eseis_class <- TRUE
## store initial object
eseis_data <- data
## extract signal vector
data <- eseis_data$signal
} else {
## set eseis flag
eseis_class <- FALSE
}
## calculate Hilbert transform
data_hilbert <- signal_hilbert(data = data)
## calculate absolute values
data_out <- abs(data_hilbert)
## apply taper
if(p > 0) {
data_out <- signal_taper(data = data_out,
p = p)
}
## optionally rebuild eseis object
if(eseis_class == TRUE) {
## assign aggregated signal vector
eseis_data$signal <- data_out
## calculate function call duration
eseis_duration <- as.numeric(difftime(time1 = Sys.time(),
time2 = eseis_t_0,
units = "secs"))
## update object history
eseis_data$history[[length(eseis_data$history) + 1]] <-
list(time = Sys.time(),
call = "signal_envelope()",
arguments = eseis_arguments,
duration = eseis_duration)
names(eseis_data$history)[length(eseis_data$history)] <-
as.character(length(eseis_data$history))
## update data type
eseis_data$meta$type = "envelope"
## assign eseis object to output data set
data_out <- eseis_data
}
## return output
return(data_out)
}
}
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