alpha in geom_raster() overrides na.value = "transparent" in scale_fill_continuous()

[dholstius]

Description

I'm trying to produce figures with geom_raster() that have:

• 0% opacity for NA values; and
• 50% opacity for finite values; with
• fill colors for (continuous) finite values varying by value.

I thought that the code below would achieve that, but supplying alpha = I(0.5) in geom_raster(...) seems to override the intent of supplying na.value = "transparent" in scale_fill_continuous(...).

My expectation is that "50% opaque 'transparent'" should be identical to "transparent", not to "50% opaque white".

Reprex

library(tidyverse)

# use `faithfuld` but substitute `NA` for values less than 0.01
df <- mutate(faithfuld, density = if_else(density < 0.01, NA_real_, density))
p <- ggplot(df, aes(waiting, eruptions)) 

# - as expected: all NAs are 0% opacity
p + geom_raster(aes(fill = density)) + scale_fill_continuous(na.value = "transparent")

# - as expected: finite values are 50% opacity
# - not as expected: NAs are 50% (?) white rather than fully transparent
p + geom_raster(aes(fill = density), alpha = I(0.5)) + scale_fill_continuous(na.value = "transparent")

[thomasp85]

I can see why this can appear baffling. The reason is that you are setting the transparency, not applying it. "transparent" is simply white with an alpha of zero. When you set the alpha of this to 0.5 it becomes semi-transparent white.

You can get what you want by using stage() to modify the calculated colours:

library(tidyverse)

df <- mutate(faithfuld, density = if_else(density < 0.01, NA_real_, density))
ggplot(df, aes(waiting, eruptions))  + 
  geom_raster(
    aes(fill = stage(
      start = density, 
      after_scale = farver::multiply_channel(fill, 'alpha', 0.5)
    ))
  ) + 
  scale_fill_continuous(na.value = "transparent")

^{Created on 2021-10-28 by the reprex package (v2.0.1)}