[R] using alpha transparency for lines in levelplot

[Michael Sumner]

Hello, I'm having trouble with using the alpha channel for transparency
with lines with lattice levelplots.

If I use transparency via the alpha argument to rgb to overplot lines on
levelplot the transparent colour affects all of the region colours in the
plot.

Can anyone explain why the difference in region colours?

#### Warning: this code attempts to create PDF files in working 
directory - they may be downloaded, see below

library(lattice)

## prepare data
data(volcano)
xy <- expand.grid(x = 1:nrow(volcano), y = 1:ncol(volcano))
xy$z <- as.vector(volcano)

## panel function to add lines with grey(0.3)
my.panel <- function(...) {
    panel.contourplot(...);
    panel.xyplot(1:60, runif(60, 1, 60), type = "l", lwd = 3, col = 
grey(0.3))
}

## this works fine
pdf("plot.pdf", version = "1.4")
levelplot(z~x+y, xy, panel = my.panel)
dev.off()

## panel function to add lines with grey transparency
my.paneltransp <- function(...) {
    panel.contourplot(...);
    panel.xyplot(1:60, runif(60, 1, 60), type = "l", lwd = 3, col = 
rgb(0.3, 0.3, 0.3, 0.5))
}

## this results in grey bleeding on the coloured regions of the plot
pdf("alpha.pdf", version = "1.4")
levelplot(z~x+y, xy, panel = my.paneltransp)
dev.off()

I've put the pdfs created here for reference:

http://staff.acecrc.org.au/~mdsumner/R/

Cheers, Mike.

I'm using Windows XP, SP2:

sessionInfo()
R version 2.4.1 (2006-12-18)
i386-pc-mingw32

locale:
LC_COLLATE=English_Australia.1252;LC_CTYPE=English_Australia.1252;LC_MONETARY=English_Australia.1252;LC_NUMERIC=C;LC_TIME=English_Australia.1252

attached base packages:
[1] "stats"     "graphics"  "grDevices" "utils"     "datasets"  "methods"  
[7] "base"     

other attached packages:
  lattice
"0.14-16"