# Focal function across two layers

`focalPairs.Rd`

Calculate values such as a correlation coefficient for focal regions in two neighboring layers. A function is applied to the first and second layer, then to the second and third layer, etc.

## Usage

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

## Arguments

- x
SpatRaster with at least two layers

- w
numeric or matrix to define the focal window. The window an 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`

. Note that if a matrix with numbers other than zero or one are used, the values are used as weights. For this to work,`fun`

must have an argument`weights`

- fun
a function with at least two arguments (one for each layer). There is a built-in function "pearson" (for both the weighted and the unweighted Pearson correlation coefficient. This function has an additional argument

`na.rm=FALSE`

- ...
additional arguments for

`fun`

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

- filename
character. Output filename

- overwrite
logical. If

`TRUE`

,`filename`

is overwritten- wopt
additional arguments for writing files as in

`writeRaster`

## Examples

```
r <- rast(system.file("ex/logo.tif", package="terra"))
set.seed(0)
r[[1]] <- flip(r[[1]], "horizontal")
r[[2]] <- flip(r[[2]], "vertical") + init(rast(r,1), runif)
r[[3]] <- init(rast(r,1), runif)
x <- focalPairs(r, w=5, "pearson", na.rm=TRUE)
plot(x)
# suppress warning "the standard deviation is zero"
suppressWarnings(x <- focalPairs(r, w=5, "pearson", use="complete.obs"))
z <- focalPairs(r, w=9, function(x, y) mean(x) + mean(y))
```