Plot raster in grey scale

[aloboa]

Given
r <- rast(system.file("ex/logo.tif", package="terra"))

I'm trying to get the equivalent to
plotRGB(r,1,1,1)

I have tried:

ggplot() + geom_spatraster(data = r[[1]]) + scale_fill_gradientn(colors=grey.colors(256), guide = FALSE) #why the lower contrast?
ggplot() + geom_spatraster(data = r[[1]]) + scale_color_grey() # why blueish?

[dieghernan]

Why not just use geom_spatraster_rgb(), the equivalent of plotRGB()?

library(terra)
#> terra 1.7.71
library(tidyterra)
library(ggplot2)

r <- rast(system.file("ex/logo.tif", package="terra"))
crs(r) <- NULL

plotRGB(r,1,1,1)

ggplot() +
  geom_spatraster_rgb(data = r, r =1, g=1, b=1) +
  coord_sf()

^{Created on 2024-03-05 with reprex v2.1.0}

[dieghernan]

I think there is nothing wrong in your code, but the issue is that the default grey.colors(245) maybe is not what you are looking for. See alternatives, in my opinion is better to use scale_fill_gradient() for finer control:

scales::show_col(gray.colors(256), labels = FALSE)

scales::show_col(grey.colors(256, start = 0, end = 1), labels = FALSE)

scales::show_col(grey.colors(256, start = 0, end = 0.5), labels = FALSE)


library(terra)
#> terra 1.7.71
library(tidyterra)
#> 
#> Attaching package: 'tidyterra'
#> The following object is masked from 'package:stats':
#> 
#>     filter

library(ggplot2)

r <- rast(system.file("ex/elev.tif", package="terra"))

p <- ggplot() +
  geom_spatraster(data=r, na.rm = TRUE)

p +
  scale_fill_gradientn(
    colors = grey.colors(n = 256),
    na.value = "transparent") +
  labs(title = "default grey colors")

p +
  scale_fill_gradientn(
    colors = grey.colors(n = 256, start = 0, end = 1),
    na.value = "transparent") +
  labs(title = "grey colors start 0 end 1")

# Another option
p +
  scale_fill_gradient(low = "grey10", high = "grey95",
                      na.value = "transparent") +
  labs(title = "scale_fill_gradient")

^{Created on 2024-03-05 with reprex v2.1.0}

[aloboa]

Thanks again.

This is good:

require(terra)
require(tidyterra)
require(ggplot2)

r <- rast(system.file("ex/logo.tif", package="terra"))
crs(r) <- NULL

ggplot() +
  geom_spatraster(data=r) +
  coord_sf() +
  scale_fill_gradient(low = "grey10", high = "grey95",na.value = "transparent", guide=FALSE) +
  facet_wrap(~lyr)

but still do not get the same than with geom_spatraster_rgb() and wish to understand why. I suspect there is
some linear stretching in geom_spatraster_rgb() (perhaps some smoothing as well?):

ggplot() +
  geom_spatraster_rgb(data = r[[1]], r =1, g=1, b=1) +
  coord_sf()
ggplot() +
  geom_spatraster(data=r[[1]]) +
  coord_sf() + 
  scale_fill_gradient(low = "grey10", high = "grey95",na.value = "transparent",guide=FALSE)

[dieghernan]

I am afraid I don't have an answer on this. I think this is coming from some transformation done in scale_fill_gradient(), since tidyterra just does internally this:

tidyterra/R/geom_spatraster_rgb.R

Line 270 in 92b6031

make_hexcol <- function(data, max_col_value = 255) {

See a reprex to better understand this:

require(terra)
#> Loading required package: terra
#> terra 1.7.71
require(tidyterra)
#> Loading required package: tidyterra
#> 
#> Attaching package: 'tidyterra'
#> The following object is masked from 'package:stats':
#> 
#>     filter
require(ggplot2)
#> Loading required package: ggplot2


r <- rast(system.file("ex/logo.tif", package="terra"))
crs(r) <- NULL

p <- ggplot() +
  geom_spatraster_rgb(data=r, r=1, g=1, b=1) +
  coord_sf()

p

p_data <- ggplot2::layer_data(p)
tibble::as_tibble(p_data)
#> # A tibble: 7,777 × 11
#>        x     y hexcol  PANEL group spatraster  xmin  xmax  ymin  ymax alpha
#>    <dbl> <dbl> <chr>   <fct> <int> <lgl>      <dbl> <dbl> <dbl> <dbl> <dbl>
#>  1   0.5  76.5 #FFFFFF 1        -1 NA             0     1    76    77     1
#>  2   1.5  76.5 #FFFFFF 1        -1 NA             1     2    76    77     1
#>  3   2.5  76.5 #FFFFFF 1        -1 NA             2     3    76    77     1
#>  4   3.5  76.5 #FFFFFF 1        -1 NA             3     4    76    77     1
#>  5   4.5  76.5 #FFFFFF 1        -1 NA             4     5    76    77     1
#>  6   5.5  76.5 #FFFFFF 1        -1 NA             5     6    76    77     1
#>  7   6.5  76.5 #FFFFFF 1        -1 NA             6     7    76    77     1
#>  8   7.5  76.5 #FFFFFF 1        -1 NA             7     8    76    77     1
#>  9   8.5  76.5 #FFFFFF 1        -1 NA             8     9    76    77     1
#> 10   9.5  76.5 #FFFFFF 1        -1 NA             9    10    76    77     1
#> # ℹ 7,767 more rows

# This is similar to
colg <- values(r$red)
grey_cols <- rgb(colg, colg, colg, 
                 maxColorValue = 255)

head(grey_cols)
#> [1] "#FFFFFF" "#FFFFFF" "#FFFFFF" "#FFFFFF" "#FFFFFF" "#FFFFFF"

# Just use the created colors as fills, not via scales

p2 <- ggplot() +
  geom_spatraster(data=r %>% select(1), fill=grey_cols, interpolate = TRUE) +
  coord_sf()

p2

p2_data <- ggplot2::layer_data(p2)
tibble::as_tibble(p2_data)
#> # A tibble: 7,777 × 13
#>        x     y lyr   value spatraster group PANEL  xmin  xmax  ymin  ymax fill  
#>    <dbl> <dbl> <chr> <int> <lgl>      <int> <fct> <dbl> <dbl> <dbl> <dbl> <chr> 
#>  1   0.5  76.5 red     255 NA             1 1         0     1    76    77 #FFFF…
#>  2   1.5  76.5 red     255 NA             1 1         1     2    76    77 #FFFF…
#>  3   2.5  76.5 red     255 NA             1 1         2     3    76    77 #FFFF…
#>  4   3.5  76.5 red     255 NA             1 1         3     4    76    77 #FFFF…
#>  5   4.5  76.5 red     255 NA             1 1         4     5    76    77 #FFFF…
#>  6   5.5  76.5 red     255 NA             1 1         5     6    76    77 #FFFF…
#>  7   6.5  76.5 red     255 NA             1 1         6     7    76    77 #FFFF…
#>  8   7.5  76.5 red     255 NA             1 1         7     8    76    77 #FFFF…
#>  9   8.5  76.5 red     255 NA             1 1         8     9    76    77 #FFFF…
#> 10   9.5  76.5 red     255 NA             1 1         9    10    76    77 #FFFF…
#> # ℹ 7,767 more rows
#> # ℹ 1 more variable: alpha <lgl>


identical(p_data$hexcol, p2_data$fill)
#> [1] TRUE

^{Created on 2024-03-06 with reprex v2.1.0}

[dieghernan]

I just realized that geom_spatraster_rgb() interpolates by default (interpolate = TRUE) while geom_spatraster() don't (interpolate = FALSE). Adjusting also the start and end color to black and white it is almost there:

require(terra)
#> Loading required package: terra
#> terra 1.7.71
require(tidyterra)
#> Loading required package: tidyterra
#> 
#> Attaching package: 'tidyterra'
#> The following object is masked from 'package:stats':
#> 
#>     filter
require(ggplot2)
#> Loading required package: ggplot2

r <- rast(system.file("ex/logo.tif", package="terra"))
crs(r) <- NULL

ggplot() +
  geom_spatraster_rgb(data = r[[1]], r =1, g=1, b=1) +
  coord_sf()

# Not quite but...
ggplot() +
  geom_spatraster(data=r[[1]], interpolate = TRUE) +
  coord_sf() + 
  scale_fill_gradient(low = "black", high = "white",na.value = "transparent",
                       guide=FALSE)
#> Warning: The `guide` argument in `scale_*()` cannot be `FALSE`. This was deprecated in
#> ggplot2 3.3.4.
#> ℹ Please use "none" instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.

^{Created on 2024-03-06 with reprex v2.1.0}

[aloboa]

Gracias!