Incompatibility with tidyterra plotting

[JsLth]

terra rasters stored using geotargets::tar_terra_rast signal an external pointer is not valid when used in a ggplot using tidyterra::geom_spatraster. This does not happen when simply accessing the SpatRaster object or when plotting it through terra::plot. See the minimal example below.

res <- tar_dir({
  tar_script({
    library(geotargets)
    
    tar_option_set(packages = c("terra", "ggplot2", "tidyterra", "magick"))
    
    list(
      tar_terra_rast(
        name = rastex,
        command = rast(data.frame(x = c(1, 2), y = c(3, 4), z = c(100, 200)))
      ),
      
      tar_target(
        name = rastex_plot,
        command = ggplot() +
          geom_spatraster(data = rastex)
      ),
      
      tar_target(
        name = plot_rastex_base,
        command = {
          png(tempf <- tempfile(fileext = ".png"))
          plot(rastex)
          dev.off()
          image_ggplot(image_read(tempf))
        }
      )
    )
  })
  
  tar_make()
  list(
    tar_read(rastex),
    tar_read(rastex_plot),
    tar_read(plot_rastex_base)
  )
})

res[[1]]
#> class       : SpatRaster 
#> dimensions  : 2, 2, 1  (nrow, ncol, nlyr)
#> resolution  : 1, 1  (x, y)
#> extent      : 0.5, 2.5, 2.5, 4.5  (xmin, xmax, ymin, ymax)
#> coord. ref. : lon/lat WGS 84 (CRS84) (OGC:CRS84) 
#> source      : rastex 
#> name        :   z 
#> min value   : 100 
#> max value   : 200

res[[2]]
#> Warning message:
#> Computation failed in `stat_terra_spat_raster()`.
#> Caused by error:
#> ! external pointer is not valid

res[[3]]

Relevant package versions:

pkg <- c("tidyterra", "targets", "geotargets", "ggplot2")
lapply(setNames(pkg, pkg), packageVersion)
#> $tidyterra
#> [1] ‘0.7.0’
#> 
#> $targets
#> [1] ‘1.10.1’
#> 
#> $geotargets
#> [1] ‘0.2.0.9000’
#> 
#> $ggplot2
#> [1] ‘3.5.1’

[Aariq]

Thanks, I'm aware of this. tar_read(rastex) works in your example because the rastex target was created with tar_terra_rast() and there is currently no equivalent target constructor for these special ggplot objects created with geom_spatraster(). I'm fairly certain this is due to the same issue that necessitates tar_terra_rast() for use with SpatRasters—the ggplot object must contain an a pointer to memory that only is valid in the R session it was created in.

We might consider a tar_*() function to handle this, but I suspect marshalling and unmarshalling the ggplot object will be a lot trickier. What I've done in my own workflows is created targets that save plots out to a file and used format = "file". You might also have success with pulling the data out in one target with tidyterra::as_tibble() and then plotting with geom_raster() in a downstream target. You may want to re-project high resolution rasters to a managable resolution before converting to a tibble though!