[Rd] Re: [R] grid - deleting and erasing grobs?

Paul Murrell p.murrell at auckland.ac.nz
Wed May 14 10:31:13 MEST 2003


M.Kondrin wrote:
> Hello!
> Don't quite understand how can I delete grobs and simultaneously erase 
> graphic output they produce. I first change grob's "vp" field to null 
> (grid.edit(gr,vp=NULL)) to erase it and then call rm(gr) (as grobs are 
> external pointers I'm not shure what this method actually frees 
> allocated memory).
> May be there is simpler method?
> Does garbage collector have any effect on grobs?
> Thank you in advance

A grob is an R object containing an external pointer.  For example, see ...

	gr <- grid.rect()

... so simply doing rm(gr) does not necessarily do anything to the 
external pointer (it just removes the R object).  In particular, if 
another grob contains the same external pointer, doing rm(gr) should 
definitely not affect the external pointer.  In the example above, 
another object does contain the external pointer because drawing the 
grid.rect() put a copy on grid's display list.

On top of that, doing rm(gr) is never going to trigger a redraw of 
current image, so there is never going to be any visible change in 
graphical output.

With regard to setting a grob's vp field to NULL:  this has nothing to 
do with whether the grob is drawn or not.  A grob is always drawn within 
the context of the current viewport.  If a grob has a vp field AND that 
field is not NULL, then the viewport in that vp field gets pushed before 
the grob is drawn (and popped again afterward).  So the vp field only 
affects the context within which the grob is drawn.

It should be possible to write something like a grid.delete() function 
which finds the grob on the display list, removes it, and replays the 
display list to redraw the current image, minus the grob that was 
deleted.  (Some extra complications would arise if the grob was drawn in 
more than one output device, and double-buffering on output devices 
would make the redraw a whole lot nicer.)  I will put a basic 
grid.delete() function on my todo list, which I hope to put a dent in 
during the second half of this year.

Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz

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