Using R 4.1.0's new `grid::pattern` function, how can I rotate a pattern

Question

I am investigating the new pattern functionality of grid. So far, I can cross-hatch a rectangle:

library(grid)

# Note that using absolute coordinates like "cm" for
# x and y does not work
cross <- grobTree(
  linesGrob( x= unit(c(0,1),"npc"), y= unit(c(0,1),"npc")),
  linesGrob( x= unit(c(0,1),"npc"), y= unit(c(1,0),"npc")))

cpat <- pattern(cross,
  width = unit(1,"cm"),
  height = unit(1,"cm"),
  extend = "repeat")

rect <- rectGrob(
  width = unit(5, "cm"),
  height = unit(5, "cm"),
  gp= gpar(fill = cpat))

grid.newpage()
grid.draw(rect)

But I can neither rotate my object without messing up:

grid.newpage()
grid.draw(grobTree(rect,vp=viewport(angle=20)))

And I also cannot rotate the pattern inside the object because each time, it does not rotate the direction in which to repeat the pattern:

vp <- viewport(angle = 30)
cross <- grobTree(
  linesGrob( x= unit(c(0,1),"npc"), y= unit(c(0,1),"npc")),
  linesGrob( x= unit(c(0,1),"npc"), y= unit(c(1,0),"npc")),
  vp = vp)

cpat <- pattern(cross,
  width = unit(1,"cm"),
  height = unit(1,"cm"),
  extend = "repeat")

rect <- rectGrob(
  width = unit(5, "cm"),
  height = unit(5, "cm"),
  gp= gpar(fill = cpat))

grid.newpage()
grid.draw(rect)

How can I rotate the direction in which the pattern is repeated?

EDIT: This is on R 4.3.1

Answer 1

This feels like a bug; the pattern should rotate with the viewport, but it seems that although each instance of the pattern is rotating, their origin relative to the viewport is not. This causes the pattern to misalign.

A workaround is to create a large repeated pattern manually that exceeds the size of the square:

library(grid)

cross <- do.call('grobTree', c(
  lapply(seq(-5, 5, 0.1), function(x) {
    linesGrob(x = c(0, 1) + x, y = c(0, 1))
  }),
  lapply(seq(-5, 5, 0.1), function(x) {
    linesGrob(x = c(0, 1) + x, y = c(1, 0))
  })))

cpat <- pattern(cross,
                width = unit(15, "cm"),
                height = unit(15, "cm"),
                extend = 'none')

rect <- rectGrob(
  width = unit(5, "cm"),
  height = unit(5, "cm"),
  gp = gpar(fill = cpat))

grid.newpage()
grid.draw(rect)

grid.newpage()
grid.draw(grobTree(rect, vp = viewport(angle = 20)))

Of course, this seems to defeat the purpose of having the option of a repeated pattern. This may be a design comprimise rather than a bug per se, but perhaps worth filing a bug report.