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

[Deepayan Sarkar]

On Thursday 07 October 2004 10:01, 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")

You probably just need (see ?pdf)

pdf("c:/alpha.pdf", version = "1.4")

> 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