[R] [FORGED] grid.grab(): strange behavior of "wrap" argument when working with lattice output

[Paul Murrell]

Hi

Mostly these are bugs.

One source of confusion:  wrap=FALSE is NOT doing better than wrap=TRUE, 
which you can see if you start a new page before drawing the grabbed 
grob ...

xyplot(1:5 ~ 1:5)
trellis.focus()
panel.abline(h = 3)
panel.abline(h = 5)
myGrob <- grid.grab(wrap = FALSE)
## start new page, otherwise just drawing over the top
grid.newpage()
grid.draw(myGrob)

But you are correct that wrap=TRUE is underperforming.  The problem is 
that your code is producing two grobs with the same name (and 
grid.grab() gets upset because you cannot create a gTree with two grobs 
with the same name).

Here is a workaround (make sure that each grob has a different name) ...

xyplot(1:5 ~ 1:5)
trellis.focus()
panel.abline(h = 3)
panel.abline(h = 5, identifier="abline2")
myGrob <- grid.grab()

grid.grab(wrap=TRUE) does not fix the problem because it does not wrap 
grobs.  For a more permanent solution, I have added a 'wrap.grobs' 
argument to grid.grab() (in r-devel), so that you can now do this 
(wrap.grobs=TRUE implicitly sets wrap=TRUE) ...

xyplot(1:5 ~ 1:5)
trellis.focus()
panel.abline(h = 3)
panel.abline(h = 5)
myGrob <- grid.grab(wrap.grobs = TRUE)

I have also fixed grid.grab() so that it does not complain about grobs 
with the same name if you have set wrap.grobs=TRUE.

Thanks for the report.  I hope this fixes the problems for you.

Paul

On 31/10/18 7:34 AM, John G. Bullock wrote:
> 
> grid.grab() captures all of the current viewports and stores them as a
> grob. In theory, and often in practice, you can use grid.grab() to store
> an image now for drawing later on. But I have noticed some odd behavior
> of grid.grab() when working with lattice output. It is all related to
> the wrap argument:
> 
> 1. When wrap = TRUE, the saved grob should produce a drawing faithful to
> the original viewports. But sometimes it doesn't.
>       2. wrap = FALSE is supposed to produce less faithful output if it
> differs at all, but sometimes its output is more faithful.
>       3. Even when using wrap = TRUE, grid.grab() warns me to use wrap =
> TRUE.
> 
> Here is a minimal example to illustrate all of these behaviors:
> 
> library(grid)
>       library(lattice)
>       xyplot(1:5 ~ 1:5)
> trellis.focus()
>       panel.abline(h = 3)
>       panel.abline(h = 5)
> myGrob <- grid.grab(wrap = TRUE)
> 
> At this point, I receive a warning --
> 
>         In grabDL(warn, wrap, ...) :
>           one of more grobs overwritten (grab WILL not be faithful; try
> 'wrap = TRUE')
> 
> -- which is strange, given that I specified wrap = TRUE.
> 
> I proceed by running grid.draw(myGrob). According to Paul Murrell (R
> Graphics, 2nd ed., page 239), using grid.grab(wrap = TRUE) is
> "guaranteed to replicate the original output." But it doesn't.
> Specifically, the top horizontal line (from panel.abline(h = 5)) isn't
> reproduced.
> 
> Matters get stranger when I consider the following code:
> 
>       xyplot(1:5 ~ 1:5)
>       trellis.focus()
>       panel.abline(h = 3)
> panel.abline(h = 5)
>       myGrob <- grid.grab(wrap = FALSE)
> grid.draw(myGrob)
> 
> These commands are the same as in the previous example, except that this
> time, wrap = FALSE. But this time, grid.draw() does faithfully reproduce
> the captured viewports. So the behavior is the opposite of what I
> expect: when wrap = FALSE, grid.draw() faithfully reproduces the
> captured viewports, but when wrap = TRUE, it doesn't.
> 
> I've run these examples in R 3.3.2 and 3.5.1. (Session info from 3.3.2
> is appended.) The result is the same in both cases. Are these bugs, or
> am I overlooking something?
> 
> Thank you,
> John Bullock
> 
> 
>   > sessionInfo()
> R version 3.3.2 (2016-10-31)
> Platform: x86_64-w64-mingw32/x64 (64-bit)
> Running under: Windows 10 x64 (build 17134)
> 
> locale:
> [1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United
> States.1252    LC_MONETARY=English_United States.1252 LC_NUMERIC=C
> [5] LC_TIME=English_United States.1252
> 
> attached base packages:
> [1] grid      stats     graphics  grDevices utils     datasets methods base
> 
> other attached packages:
> [1] lattice_0.20-34 RevoUtilsMath_10.0.0
> 
> loaded via a namespace (and not attached):
> [1] RevoUtils_10.0.2
> 
> 
> 
> 
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> 
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul using stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/