terra (version 1.4-22)

focalCpp: Compute focal values with an iterating C++ function

Description

Calculate focal values with a C++ function that iterates over cells to speed up computations by avoiding an R loop (with apply).

See focal for an easier to use method.

Usage

# S4 method for SpatRaster
focalCpp(x, w=3, fun, ..., fillvalue=NA, 
		expand=FALSE, filename="", overwrite=FALSE, wopt=list())

Arguments

x

SpatRaster

w

window. The window can be defined as one (for a square) or two numbers (row, col); or with an odd-sized weights matrix. See the Details section in focal

fun

cppFunction that iterates over cells. For C++ functions that operate on a single focal window, or for R functions use focal instead. The function must have at least three arguments. The first argument can have any name, but it must be a NumericVector or std::vector<double> and gets the focal values. The other two arguments ni and wi must be of type size_t. ni represents the number of cells and nw represents the size of (number of elements in) the window

...

additional arguments to fun

fillvalue

numeric. The value of the cells in the virtual rows and columns outside of the raster

expand

logical. If TRUE The value of the cells in the virtual rows and columns outside of the raster are set to be the same as the value on the border

filename

character. Output filename

overwrite

logical. If TRUE, filename is overwritten

wopt

additional arguments for writing files as in writeRaster

Value

SpatRaster

See Also

focal, focalValues

Examples

# NOT RUN {
library(Rcpp)
cppFunction( 
	'NumericVector sum_and_multiply(NumericVector x, double m, size_t ni, size_t nw) {
		NumericVector out(ni);
		// loop over cells
		for (size_t i=0; i<ni; i++) {
			// compute something for a window
			size_t start = i * nw;
			double v = 0;
			// loop over the values of a window
			for (size_t j=start; j<(start+nw); j++) {
				v += x[j];
			}
			out[i] = v * m;
		}		
		return out;
	}'
)

nr <- nc <- 10
r <- rast(ncols=nc, nrows=nr, ext= c(0, nc, 0, nr))
values(r) <- 1:ncell(r)

raw <- focalCpp(r, w=3, fun=sum_and_multiply, fillvalue=0, m=10)

# same as
f1 <- focal(r, w=3, fun=sum, fillvalue=0) *10
all(values(f1) == values(raw))

# and as
ffun <- function(x, m) { sum(x) * m }
f2 <- focal(r, w=3, fun=ffun, fillvalue=0, m=10)


# You can also use an R function with focalCpp but this
# is not recommended 

R_sm_iter <- function(x, m, ni, nw) {
	out <- NULL
	for (i in 1:ni) {
		start <- (i-1) * nw + 1
		out[i] <- sum(x[start:(start+nw-1)]) * m
	}		
	out
}

fr <- focalCpp(r, w=3, fun=R_sm_iter, fillvalue=0, m=10)

# }