How do I correctly use the alpha aesthetic with geom_spatraster?

[jlegewie]

Hi,

I am unable to use the alpha aesthetic with geom_spatraster.

Here is an easy example. How can I correctly use alpha? With this code, I get the error "object 'lyr.1' not found" for the second plot.

r1 <- rast(nrows=50, ncols=50)
values(r1) <- runif(ncell(r1))

ggplot() +
    geom_spatraster(data = r1, aes(fill = lyr.1)) +
    scale_fill_continuous() +
    theme_void()

ggplot() +
    geom_spatraster(data = r1, aes(fill = lyr.1, alpha = after_stat(lyr.1))) +
    scale_fill_continuous() +
    theme_void()

[dieghernan]

Hi,

You can do it using computed variables (i.e. value, lyr). In this case you are using the name of the layer, that does not work. See the docs:

This geom computes internally some variables that are available for use as aesthetics, using (for example) aes(alpha = after_stat(value)) ...:

library(terra)
#> terra 1.7.46
library(tidyterra)
#> 
#> Attaching package: 'tidyterra'
#> The following object is masked from 'package:stats':
#> 
#>     filter
library(ggplot2)

r1 <- rast(nrows = 50, ncols = 50)
values(r1) <- runif(ncell(r1))

ggplot() +
  geom_spatraster(data = r1, aes(fill = lyr.1)) +
  scale_fill_continuous() +
  theme_void()

# With alpha, use computed variable value
ggplot() +
  geom_spatraster(data = r1, aes(fill = lyr.1, alpha = after_stat(value))) +
  scale_fill_continuous() +
  scale_alpha(range = c(0.1, 0.5)) +
  theme_void()

^{Created on 2023-09-22 with reprex v2.0.2}