Why is the output of buffer in terra not working as a mask?

Question

library(terra)

r <- rast(ncols=36, nrows=18)
r[500] <- 1
b <- buffer(r, width=5000000) 
plot(b)

s <- rast(ncols=36, nrows=18)
values(s) <- runif(ncell(s))

tst <- mask(s, b)


plot(tst)

Does anyone know why is tst here exactly the same as s? can I use the output of buffer as a mask?

Answer 1

What gets masked is configured by maskvalues and by default it's NA.
So either change values outside of b buffer to NA or use mask(s, b, maskvalues = FALSE):

library(terra)
#> terra 1.7.39
par(mfrow = c(2, 1))

r <- rast(ncols=36, nrows=18)
r[500] <- 1
b <- buffer(r, width=5000000) 
plot(b)

s <- rast(ncols=36, nrows=18)
values(s) <- runif(ncell(s))

# set FALSE values to NA
# b[!b] <- NA
# or change maskvalues arg to match the mask raster:
tst <- mask(s, b, maskvalues = FALSE)
plot(tst)

^{Created on 2023-08-03 with reprex v2.0.2}