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

Tue Feb 15 20:53:43 CET 2011

```Hi,

Thanks for the clarification. Perhaps something along those lines
could be added to the help page.

Regards,

baptiste

On 15 February 2011 19:51, Paul Murrell <paul at stat.auckland.ac.nz> wrote:
> Hi
>
> baptiste auguie wrote:
>>
>> 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)."
>
> That description is referring to a single *grob* object that draws multiple
> shapes, something like ...
>
> g1 <- circleGrob(x=1:3/4, y=1:3/4, r=.1)
>
>
> The behaviour of gTrees is pretty much undefined, but the user is free to
> slap a class on the gTree and write their own xDetails() and yDetails()
> methods to achieve the outcome that they want.  I have wondered about
> supplying a default that makes some sort of union of the boundaries of the
> children of a gTree, but have not yet implemented that.
>
> The gList case has been explicitly coded to produce the result from the last
> object in the gList, but I cannot recall why I ever thought that might be a
> good default.  Again, making the gList the children of a gTree with a
> specific class provides the opportunity to control what grobX() and grobY()
> return for yourself.
>
> Paul
>
>> 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:
>> [1] en_GB.UTF-8/en_GB.UTF-8/C/C/en_GB.UTF-8/en_GB.UTF-8
>>
>> attached base packages:
>> [1] grid      stats     graphics  grDevices utils     datasets
>> methods   base
>>
>> loaded via a namespace (and not attached):
>> [1] tools_2.12.1
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
> --
> Dr Paul Murrell
> Department of Statistics
> The University of Auckland
> Private Bag 92019
> Auckland
> New Zealand
> 64 9 3737599 x85392
> paul at stat.auckland.ac.nz
> http://www.stat.auckland.ac.nz/~paul/
>

```