[R-sig-Geo] Speeding up focal

[Kenny Bell]

I am trying to use raster::focal to fill some holes in many RasterLayers. I
call:

focal(raster_layer, fun = mean, w = matrix(1, nrow = 3, ncol = 3), NAonly =
TRUE, na.rm = TRUE)

I am only interested in filling in holes in a small subset of each
RasterLayer (i.e. a country when the raster is global). focal spends a lot
of time trying to fill in the oceans and holes in other countries.

Is there any way to get focal to only change values for some specific
subset of a RasterLayer?

Thanks so much!
Kenny

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