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# This function will reimplement Akimas regular grid splines:
# (TOMS 464) improved version: TOMS 760
# http://www.netlib.org/toms/760
# NOT YET FINISHED!
#
# Currently it simply maps the call to the algorithm for
# irregular gridded data, as this can also be applied to regular grids.
# The results will not be exactly the same!
bicubic <- function(x,y,z,x0,y0){
nx <- length(x)
ny <- length(y)
if(dim(z)[1]!=nx)
stop("dim(z)[1] and length of x differs!")
if(dim(z)[2]!=ny)
stop("dim(z)[2] and length of y differs!")
n0 <- length(x0)
if(length(y0)!=n0)
stop("length of y0 and x0 differs!")
ret <- interp(x,y,z,x0,y0,input="grid", output="points", method="akima")
warning("this output is generated according to Akimas irregular grid splines, not the regular grid one! This is a temporary workaround until Akimas ACM algorithm 760 is reimplmented from scratch!")
list(x=x0,y=y0,z=ret$z)
}
bicubic.grid <- function(x,y,z,xlim=c(min(x),max(x)),ylim=c(min(y),max(y)),
nx=40,ny=40,dx=NULL,dy=NULL){
Nx <- length(x)
Ny <- length(y)
if(dim(z)[1]!=Nx)
stop("dim(z)[1] and length of x differs!")
if(dim(z)[2]!=Ny)
stop("dim(z)[2] and length of y differs!")
if(!is.null(dx)){
xi <- seq(xlim[1],xlim[2],by=dx)
nx <- length(xi)
} else {
xi <- seq(xlim[1],xlim[2],length=nx)
}
if(!is.null(dx)){
yi <- seq(ylim[1],ylim[2],by=dy)
ny <- length(yi)
} else {
yi <- seq(ylim[1],ylim[2],length=ny)
}
xmat <- matrix(rep(xi,ny),nrow=ny,ncol=nx,byrow=TRUE)
ymat <- matrix(rep(yi,nx),nrow=ny,ncol=nx,byrow=FALSE)
xy <- cbind(c(xmat),c(ymat))
n0 <- nx*ny
ret <- bicubic(x,y,z,xy[,1],xy[,2])
# return cell boundaries
list(x=xi,y=yi,z=t(matrix(ret$z,nrow=ny,ncol=nx,byrow=F)))
}
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