R help - Mar 2011 - transparent grays?

Is there a grayscale equivalent to alpha levels in rgb?

Example:  I have the following to make red transparent circles overlap
with previously plotted blue symbols.

symbols(x=sites$long,y=sites$lat,circles=log(sites$prop.nem
+1),add=T,inches=F,bg=rgb(red=1,green=0,blue=0,
alpha=0.5),fg=rgb(red=1,green=0,blue=0, alpha=0.5))

I'm having a hard time coming up with a grayscale equivalent.

Thanks!

Try this:

with(trees, symbols(Height, Volume, circles=Girth/24, inches=FALSE, fg
= "gray30",
 bg = sapply(apply(rbind(col2rgb(gray(seq(0,1, l = 10)))/255, alpha 0.5), 2,
as.list), do.call, what = rgb)))


On Thu, Mar 31, 2011 at 2:52 PM, Frostygoat <frostygoat at gmail.com>
wrote:
> Is there a grayscale equivalent to alpha levels in rgb?
>
> Example: ?I have the following to make red transparent circles overlap
> with previously plotted blue symbols.
>
> symbols(x=sites$long,y=sites$lat,circles=log(sites$prop.nem
> +1),add=T,inches=F,bg=rgb(red=1,green=0,blue=0,
> alpha=0.5),fg=rgb(red=1,green=0,blue=0, alpha=0.5))
>
> I'm having a hard time coming up with a grayscale equivalent.
>
> Thanks!
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paran?-Brasil
25? 25' 40" S 49? 16' 22" O