Focal (stencil) op.

fn receives a LIST of (2r+1)^2 shifted arrays, row-major over (dy, dx) offsets, and returns one array: the whole neighbourhood is processed vectorised across every pixel at once. Write fn with plain arithmetic and the g_* vocabulary (g_ifelse(), g_cast(), ...). Example, a 3x3 sum: function(sh) Reduce("+", sh).

Usage

focal_map(x, fn, radius, boundary = "nodata", bands = NULL)

Arguments

x

LazyRaster, or a LazyDataset.

fn

Function over the list of shifted arrays (see above).

radius

Halo in pixels (mandatory: the footprint cannot be inferred from fn).

boundary

Boundary policy; only "nodata" in v1.

bands

LazyDataset only: bands to apply to (default: all value bands).

Value

A LazyRaster on the same grid as x, or a LazyDataset when given one.

Details

Cells beyond the raster edge are NaN (nodata): v1 supports only this boundary = "nodata" policy; reflect/wrap are not implemented. An integer raster is read as f32 for the stencil, so the edge can be NaN, and the result is f32.

Over a LazyDataset, the stencil is applied to every value band per slice; bands restricts which bands.

See also

focal_kernel(), bilateral_focal(), shrink_footprint()

Examples

f <- system.file("extdata", "garry-example.tif", package = "garry")
red <- lazy_source(f, band = 1L)
# a 3 x 3 window mean; cells at the raster edge are NaN
sm <- focal_map(red, radius = 1L, fn = function(sh) Reduce(`+`, sh) / length(sh))
collect(sm)[1:3, 1:3]
#> Error in confirm_plugin_install(platform, url): The "cpu" PJRT plugin needs to be downloaded for pjrt to work.
#> ℹ Automatic downloads are not performed in non-interactive sessions.
#> ℹ Set `PJRT_INSTALL` to "1" to allow the download, or set
#>   `PJRT_PLUGIN_PATH_CPU` to a local plugin file.