[R-sig-Geo] using alpha transparency for lines in spplot

[Michael Sumner]

Hello, I'm having trouble with using the alpha channel for transparency 
with lines with spplot and
sp.lines.

If I use transparency via the alpha argument to rgb to overplot lines on 
spplot.grid the transparent colour affects all of the colours in the 
plot.  Example follows.

I've put the pdfs as created below here for reference:

http://staff.acecrc.org.au/~mdsumner/Rutas/misc/

BTW, I know my usage of the panel plots is not right, but I'm learning, 
so any advice would be appreciated.

Cheers, Mike.

I'm using Windows XP, SP2 with

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:
     sp
"0.9-9"


Example:
library(sp)

data(meuse.grid)
coordinates(meuse.grid) <- ~x+y
gridded(meuse.grid) <- TRUE

## create some lines

bb <- bbox(meuse.grid)
xy <- cbind(runif(10, bb[1,1], bb[1,2]), runif(10, bb[2,1], bb[2,2]))
ll <- SpatialLines(list(Lines(list(Line(xy)), "1")))


## this is fine, colour grid with black lines
pdf("test.pdf", version = "1.4")
pg <- function(...) {
    panel.gridplot(...)
    sp.lines(ll)
}
spplot(meuse.grid[1], panel = pg)
dev.off()

## in this the colour grid is "greyed" like the lines
pdf("testAlpha.pdf", version = "1.4")
pg <- function(...) {
    panel.gridplot(...)
    sp.lines(ll , col = rgb(190, 190, 190, 100, max = 255))
}
spplot(meuse.grid[1], panel = pg)
dev.off()

## try with blue lines - colour grid is "blued" . . .
pdf("testBlueAlpha.pdf", version = "1.4")
pg <- function(...) {
    panel.gridplot(...)
    sp.lines(ll , col = rgb(0, 0, 190, 100, max = 255))
}
spplot(meuse.grid[1], panel = pg)
dev.off()