R devel - Jan 2014 - Determining the resolution of a screen device

Dear all,

I am trying to find a way to reliably and programmatically establish the
resolution (i.e. DPI or equivalent) of an on-screen device. It seemed to me
that

  dev.new(width=1, height=1)
  dpi <- dev.size("px")

would do the trick, but the result does not seem to be correct, at least on
OS X 10.9.1 using the "quartz" device. Specifically, the window that
appears is 1 inch square, as expected, but the result from dev.size() is
c(72,72), which isn't correct. My display is 1440x900, but if I call

  dev.new(width=720/72, height=450/72)

the resulting device fills much more than half the screen. But R gets the
size right in inches, so it, or something it calls, must presumably know
the real DPI value.

So, could anyone tell me whether there is a reliable way to determine DPI,
please?

All the best,
Jon

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On 14/01/2014 15:32, Jon Clayden wrote:
> Dear all,
>
> I am trying to find a way to reliably and programmatically establish the
> resolution (i.e. DPI or equivalent) of an on-screen device. It seemed to me
> that
>
>    dev.new(width=1, height=1)
>    dpi <- dev.size("px")
>
> would do the trick, but the result does not seem to be correct, at least on
> OS X 10.9.1 using the "quartz" device.
It is correct: you need to read what it actually claims to do.

    units: the units in which to return the value - inches, cm, or
           pixels (device units).

The device units are nominal pixels: they may or may not be actual ones. 
  PDF (as used by Quartz) has a nominal 72 pixels per inch irrespective 
of display.

 > Specifically, the window that
> appears is 1 inch square, as expected, but the result from dev.size() is
> c(72,72), which isn't correct. My display is 1440x900, but if I call
>
>    dev.new(width=720/72, height=450/72)
>
> the resulting device fills much more than half the screen. But R gets the
> size right in inches, so it, or something it calls, must presumably know
> the real DPI value.
>
> So, could anyone tell me whether there is a reliable way to determine DPI,
> please?
No.  And even when the OS reports this, it is often wrong.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595