[R] How to use alpha transparency channel for colors?

[Prof Brian Ripley]

On Thu, 7 Oct 2004 kwright at eskimo.com wrote:

> The release notes for R 2.0.0 states:

Would that be the NEWS file?  R does not have `release notes' per se.

>   It is now possible to specify colours with a full alpha
>   transparency channel via the new 'alpha' argument to the
>   rgb() and hsv() functions, or as a string of the form "#RRGGBBAA".
> 
>   NOTE: most devices draw nothing if a colour is not opaque,
>   but PDF and Quartz devices will render semitransparent colours.
> 
>   A new argument 'alpha' to the function col2rgb()
>   provides the ability to return the alpha component of
>   colours (as well as the red, green, and blue components).
> 
> I'm using R 2.0.0 on Windows 2000 and wanted to try this feature.  The
> following simple test works fine:
> 
> pdf("c:/alpha.pdf")
> plot(rnorm(1:100),rnorm(1:100),col="#000055ff",pch=16)
> dev.off()
> 
> But as soon as I change alpha value from "ff" to "fe", the points are no
> longer visible for me.  I've tried viewing the pdf with Acrobat Reader
> 5.1.0 and gsview4.5.
> 
> Do I need a more recent pdf viewer?  Is this feature not working on
> Windows?  Am I doing something wrong?  Any tips would be appreciated.

As so often happens, the `something wrong' is not reading the help file.
As the NEWS file _also_ says

    o	A 'version' argument has been added to pdf() device.  If this is
	set to "1.4", the device will support transparent colours.

pdf("alpha.pdf", version="1.4")  should work for you: it does for me.


-- 
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