focalCpp function

Compute focal values with an iterating C++ function

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.

## S4 method for signature 'SpatRaster'
focalCpp(x, w=3, fun, ..., fillvalue=NA, 
                silent=TRUE, 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 Rcpp::NumericVector, Rcpp::IntegerVector or a std::vector<double>. This is the container that receives 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
• silent: logical. If TRUE error messages are printed that may occur when trying fun to determine the length of the returned value. This can be useful in debugging a fun that does not work
• filename: character. Output filename
• overwrite: logical. If TRUE, filename is overwritten
• wopt: additional arguments for writing files as in writeRaster

Returns

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
                size_t start = 0;
                for (size_t i=0; i<ni; i++) {
                        size_t end = start + nw;
                        // compute something for a window
                        double v = 0;
                        // loop over the values of a window
                        for (size_t j=start; j<end; j++) {
                                v += x[j];
                        }
                        out[i] = v * m;
                        start = end;
                }
                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)
## End(Not run)