Lattice, ggplot, and pointsize

Wed, Jun 5, 2013 7:07 AM

On Sun, May 26, 2013 at 12:47 AM, Milan Bouchet-Valat <nalimilan at club.fr> wrote:

Le mardi 21 mai 2013 ? 21:39 +0100, Prof Brian Ripley a ?crit :

On 21/05/2013 21:24, Bert Gunter wrote:

At the risk of misunderstanding... (inline)

On Tue, May 21, 2013 at 12:17 PM, Milan Bouchet-Valat <nalimilan at club.fr> wrote:

Le mardi 21 mai 2013 ? 08:17 -0700, Jeff Newmiller a ?crit :

That is like complaining that your hammer does not fit these
newfangled Philips screws.

These are different tools. Do not expect them to interoperate.

I understand that Lattice and ggplot2 do not use settings from par().
I'm fine with this, as these packages are different from base graphics
and have they own equivalent to tweak settings.

What I do not understand is that one argument passed to output devices,
which are _not_ provided by package graphics, is ignored by these two
packages. Lattice and ggplot2 do not provide an alternative output
system,

False, I believe, depending on what you mean by "output system". They
both use grid graphics, not base graphics and with lattice, anyway,

Indeed.  The issue is a design difference between the base and grid
graphics subsystems.  See the 'R Internals' manual for more details.

Thanks for the pointers. Indeed there is some interesting documentation
there. I've also had a deeper look at the code, and I've traced
pointsize (called ps) back to the R_GE_gcontext struct in
src/include/R_ext/GraphicsEngine.h.

If I understand correctly, base graphics draw text using GText() in
src/library/graphics/src/graphics.c, which in turn calls GEText() in
src/main/engine.c. GEText() does take into account the pointsize.

On the other hand, grid graphics draw text using gridText() from
src/library/grid/src/grid.c. This function uses gcontextFromgpar() to
get its R_GE_gcontext object. gcontextFromgpar() (defined in gpar.c)
computes the pointsize from the fontsize gpar setting and a general
scaling of the output:
    /*
     * Scale by GSS_SCALE (a "zoom" factor)
     */
    gc->ps = gpFontSize(gp, i) * REAL(gridStateElement(dd, GSS_SCALE))[0];

What is interesting is that when a new device gets initialized by grid
(initGPar() atin gpar.c), the fontsize gpar settings is set to the
device starting pointsize:
    REAL(gpfs)[0] = dev->startps;

And indeed this works with svg():

svg("test.svg", pointsize=5)
get.gpar("fontsize")

$fontsize
[1] 5

...but not with Lattice:

trellis.par.get("fontsize")

$text
[1] 12

$points
[1] 8


So the problem does not appear to be a base vs. grid graphics issue, but
rather something specific to Lattice and ggplot2. And indeed, when
looking at Lattice's sources, canonical.theme() in settings.R does:
             fontsize         = list(text = 12, points = 8),

As simple as that! I didn't need to look so deep into the code... :-/


?trellis.par.set says:
     The initial settings for each device defaults to values
     appropriate for that device. In practice, this boils down to three
     distinct settings, one for screen devices like ?x11? and
     ?windows?, one for black and white plots (mostly useful for
     ?postscript?) and one for color printers (color ?postcript?,
     ?pdf?). [This may not be up-to-date, though...]

So it does not appear completely absurd to try to adjust to the device
settings where appropriate. Pointsize seems such a case to me.
canonical.theme() could set the text font size to get.gpar("fontsize").
Since the default value is 12 for most devices, this would not change
anything by default. (ggplot2 could probably benefit from a similar
change.)

If I remember correctly (it was a long time ago), I had nothing in
particular against the default text fontsize. The default symbol size
in grid was larger than I liked, and so fontsize$points was set to
something more reasonable, and fontsize$text just seemed natural to
add. It is only ever used in a single call to gpar() inside
print.trellis. So it seems perfectly reasonable to take the default of
fontsize$text from grid instead.

I don't like the idea of making the default
'get.gpar("fontsize")$fontsize' though, because that requires a device
to be active, which canonical.theme() doesn't by design. Instead I
have changed the default to NULL, and the fontsize is now set from (in
order of priority)

1. trellis.par.get("fontsize")$text  # NULL by default
2. trellis.par.get("grid.pars")$fontsize # NULL by default
3. get.gpars()$fontsize # approximately device pointsize by default

Hopefully this works for you. You can test using the r-forge version.

-Deepayan

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Lattice, ggplot, and pointsize

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