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

[Roger D. Peng]

There happens to be a nice article about this by Paul Murrell in the 
most recent R Newsletter at http://cran.r-project.org/doc/Rnews/.

-roger

kwright at eskimo.com wrote:
> The release notes for R 2.0.0 states:
> 
>   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.
> 
> Thanks,
> 
> Kevin Wright
> 
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