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> suppressPackageStartupMessages(library(stars))
> #suppressPackageStartupMessages(library(lwgeom))
> geomatrix = system.file("tif/geomatrix.tif", package = "stars")
> (x = read_stars(geomatrix))
stars object with 2 dimensions and 1 attribute
attribute(s):
               Min. 1st Qu. Median    Mean 3rd Qu. Max.
geomatrix.tif    74     107    123 126.765     132  255
dimension(s):
  from to  offset delta                refsys point x/y
x    1 20 1841002   1.5 WGS 84 / UTM zone 11N  TRUE [x]
y    1 20 1144003  -1.5 WGS 84 / UTM zone 11N  TRUE [y]
sheared raster with parameters: -5 -5 
> new = st_crs(4326)
> y = st_transform(x, new)
> plot(st_transform(st_as_sfc(st_bbox(x)), new), col = NA, border = 'red')
> plot(st_as_sfc(y, as_points=FALSE), col = NA, border = 'green', axes = TRUE, add = TRUE)
> image(y, add = TRUE)
> plot(st_as_sfc(y, as_points=TRUE), pch=3, cex=.5, col = 'blue', add = TRUE)
> plot(st_transform(st_as_sfc(x, as_points=FALSE), new), add = TRUE)
> 
> tif = system.file("tif/L7_ETMs.tif", package = "stars")
> x = read_stars(tif)[,1:10,1:10,1:3]
> x_ = st_transform(x, st_crs(4326))
> if (require(lwgeom, quietly = TRUE)) {
+   x__ = st_transform_proj(x, st_crs(4326)$proj4string)
+   st_crs(x_) = st_crs(x__)
+   print(all.equal(x_,x__))
+ 
+ # x__ = st_transform(x, x_) #now in st_warp 
+ # all.equal(x_, x__)
+ 
+   x = st_xy2sfc(x, as_points = FALSE)
+   (x_ = st_transform(x, st_crs(4326)))
+   (x__ = st_transform_proj(x, st_crs(4326)))
+   print(all.equal(x_,x__,check.attributes = FALSE))
+ }
Linking to liblwgeom 3.0.0beta1 r16016, GEOS 3.12.1, PROJ 9.3.1

Attaching package: 'lwgeom'

The following object is masked from 'package:sf':

    st_perimeter

[1] TRUE
[1] TRUE
> 
> # nothing to transform: now errs:
> try(st_transform(st_as_stars(list(m = matrix(1,10,10))), st_crs(4326)))
Error in st_transform.stars(st_as_stars(list(m = matrix(1, 10, 10))),  : 
  !is.na(st_crs(x)) is not TRUE
> try(st_transform_proj(st_as_stars(list(m = matrix(1,10,10))), st_crs(4326)))
Error in st_transform_proj.stars(st_as_stars(list(m = matrix(1, 10, 10))),  : 
  !is.na(st_crs(x)) is not TRUE
> 
> proc.time()
   user  system elapsed 
   0.84    0.06    0.92