# [Rd] gList and gTree methods of grid::grobX

baptiste auguie baptiste.auguie at googlemail.com
Tue Feb 15 09:42:26 CET 2011

```Dear all,

In an attempt to draw fill patterns in grid graphics, I have
encountered a behavior of grobX that I cannot understand from the
documentation. Consider this,

library(grid)

## gTree
g1 <- gTree(children=gList(
rectGrob(0.5,0.5, width=unit(0.8,"npc"),
height=unit(2,"cm")),
circleGrob(r=0.3)), vp=viewport(0.5,0.5))

## gList
g1 <- gList(rectGrob(0.5,0.5, width=unit(0.8,"npc"),
height=unit(2,"cm")),
circleGrob(r=0.3))

## loop over angles to map the boundary
gtheta <- function(g, theta){

sapply(theta, function(.t){
gx <- convertX(grobX(g, .t), "npc")
gy <- convertY(grobY(g, .t), "npc")

c(gx,gy)
})

}

angles <- seq(0,360,by=30)
p1 <- gtheta(g1, angles)

grid.newpage()
grid.draw(g1)
grid.points(p1[1,],p1[2,], gp=gpar(cex=0.2),
default.units="npc")

If I'm not mistaken, neither gList nor gTree seem to produce the
documented behavior,

"If the grob describes multiple shapes, the boundary value will either
correspond to the edge of a bounding box around all of the shapes
described by the grob (for multiple rectangles, circles, xsplines, or
text), or to a convex hull around all vertices of all shapes described
by the grob (for multiple polygons, points, lines, polylines, and
segments)."

with gList, I observe that the boundary is only considered for the
first shape, whilst gTree ignores all children altogether.

It works fine for single shapes (e.g. g1 = circleGrob(r=0.3)).

The same behavior is observed with quartz(), pdf() and png().

Sincerely,

baptiste

sessionInfo()
R version 2.12.1 Patched (2010-12-30 r53895)
Platform: i386-apple-darwin9.8.0 (32-bit)

locale:
 en_GB.UTF-8/en_GB.UTF-8/C/C/en_GB.UTF-8/en_GB.UTF-8

attached base packages:
 grid      stats     graphics  grDevices utils     datasets
methods   base

loaded via a namespace (and not attached):
 tools_2.12.1

```