Focal values
Calculate focal ("moving window") values for the neighborhood of focal cells using a matrix of weights, perhaps in combination with a function.
## S4 method for signature 'SpatRaster' focal(x, w=3, fun="sum", na.rm=TRUE, na.only=FALSE, fillvalue=NA, expand=FALSE, filename="", ...)
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 Details. |
fun |
function that takes multiple numbers, and returns a single number. For example mean, modal, min or max. It should also accept a |
na.rm |
logical. Should missing values be removed? |
na.only |
logical. Should only missing values in |
fillvalue |
numeric. The value of the cells in the virtual rows and columns outside of the raster |
expand |
logical. If |
filename |
character. Output filename |
... |
additional arguments for writing files as in |
focal
The window used must have odd dimensions. If you need even sides, you can use a matrix and add a column or row with weights of zero.
Window values are typically 0 or 1, or a value between 0 and 1 if you are using a rectangular area and/or the "sum" function. They can also be NA; these are ignored in the computation. That can be useful to compute the mean, min, or max value for a non-rectangular area.
Example weight matrices
Laplacian filter: filter=matrix(c(0,1,0,1,-4,1,0,1,0), nrow=3)
Sobel filter: filter=matrix(c(1,2,1,0,0,0,-1,-2,-1) / 4, nrow=3)
SpatRaster
r <- rast(ncols=10, nrows=10, ext(0, 10, 0, 10)) values(r) <- 1:ncell(r) #f <- focal(r, w=3, fun=function(x, ...) quantile(x, .25, ...), na.rm=TRUE) f <- focal(r, w=3, fun="mean") # the following two statements are equivalent: a <- focal(r, w=matrix(1/9, nc=3, nr=3)) b <- focal(r, w=3, fun=mean, na.rm=FALSE) # but this is different d <- focal(r, w=3, fun=mean, na.rm=TRUE)
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