# focal: Focal values In terra: Spatial Data Analysis

## Description

Calculate focal ("moving window") values for the neighborhood of focal cells using a matrix of weights, perhaps in combination with a function.

## Usage

 ```1 2 3``` ```## S4 method for signature 'SpatRaster' focal(x, w=3, fun="sum", na.rm=TRUE, na.only=FALSE, fillvalue=NA, expand=FALSE, filename="", ...) ```

## 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 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` argument, either as actual argument or through use of `...` `na.rm` logical. Should missing values be removed? `na.only` logical. Should only missing values in `x` be changed? `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 `...` additional arguments for writing files as in `writeRaster`

## Details

`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 filters (for edge detection): `fx=matrix(c(-1,-2,-1,0,0,0,1,2,1), nrow=3)` `fy=matrix(c(1,0,-1,2,0,-2,1,0,-1), nrow=3)`

## Value

SpatRaster

`focalMat`, `focalValues`
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13``` ```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) ```