https://github.com/cran/fields
Tip revision: c8904131c3352e1d3ff03bb5c07f5434648cc406 authored by Doug Nychka on 17 November 2011, 11:25:44 UTC
version 6.6.2
version 6.6.2
Tip revision: c890413
stationary.image.cov.R
# fields, Tools for spatial data
# Copyright 2004-2011, Institute for Mathematics Applied Geosciences
# University Corporation for Atmospheric Research
# Licensed under the GPL -- www.gpl.org/licenses/gpl.html
stationary.image.cov <- function(ind1, ind2, Y, cov.obj = NULL,
setup = FALSE, grid, M = NULL, N = NULL, Covariance = "Matern",
Distance = "rdist", ...) {
#
# if cov object is missing then create
# basically need to enlarge domain and find the FFT of the
# covariance
#
if (is.null(cov.obj)) {
dx <- grid$x[2] - grid$x[1]
dy <- grid$y[2] - grid$y[1]
m <- length(grid$x)
n <- length(grid$y)
#
# determine size of padding
# default is twice domain and will then yeild exact results
#
if (is.null(M))
M <- (2 * m)
if (is.null(N))
N <- (2 * n)
xg <- make.surface.grid(list((1:M) * dx, (1:N) * dy))
center <- matrix(c((dx * M)/2, (dy * N)/2), nrow = 1,
ncol = 2)
#
# here is where the actual covarinace form is used
# note passed arguments from call for parameters etc.
#
out <- stationary.cov(xg, center, Covariance = Covariance,
Distance = Distance, ...)
# coerce to a matrix (image)
out <- matrix(c(out), nrow = M, ncol = N)
temp <- matrix(0, nrow = M, ncol = N)
#
# a simple way to normalize. This could be avoided by
# translating image from the center ...
#
temp[M/2, N/2] <- 1
wght <- fft(out)/(fft(temp) * M * N)
#
# wght is the discrete FFT for the covariance suitable for fast
# multiplication by convolution.
#
cov.obj <- list(m = m, n = n, grid = grid, N = N, M = M,
wght = wght, call = match.call())
if (setup) {
return(cov.obj)
}
}
temp <- matrix(0, nrow = cov.obj$M, ncol = cov.obj$N)
if (missing(ind1)) {
temp[1:cov.obj$m, 1:cov.obj$n] <- Y
Re(fft(fft(temp) * cov.obj$wght, inverse = TRUE)[1:cov.obj$m,
1:cov.obj$n])
}
else {
if (missing(ind2)) {
temp[ind1] <- Y
}
else {
temp[ind2] <- Y
}
#
# as promised this is a single clean step
#
Re(fft(fft(temp) * cov.obj$wght, inverse = TRUE)[ind1])
}
}