R help - Jan 2008 - png() and pdf() alignment problems

Hi all!

My aim is to print a thematic map containing
vector output and an image backdrop (six orthophoto tiles).
The size of one image tile is 2500 rows and 2500 columns.
The are six tiles to be printed.

Of course this is far to much for sending the job to a pdf() device.
The raster image would consist of 37500000 squares,
each representing a raster cell.

So my a attempt was to separate vector and image output,
and overlay it afterwards.

Doing so I found that png() and pdf() do not register corectly.

The following script shows that a custom device size (not square)
can be responsible for the missing preccesion.

require(grDevices)
x <- y <- seq(-4*pi, 4*pi, len=27)
r <- sqrt(outer(x^2, y^2, "+"))
z <- cos(r^2)*exp(-r/6), col=gray((0:32)/32)

# example1
# standard device size
pdf("pdf_fit.pdf")
image(z)
dev.off()
png("png_fit.png", res = 96)
image(z)
dev.off()

# both files overlay with high precision
# as known from R grtaphics output
# see attached example example1.pdf

# example2
# custom device size
pdf("pdf.pdf", width = 8, height = 4)
image(z)
dev.off()
png("png.png", width = 8, height = 4, units = "in", res =
96)
image(z)
dev.off()

# png output is somewhat larger
# and aligning them is a bit nasty
# see attached example example2.pdf

I attached the two files
showing the overlay of the examples above.

Is there anything I'm missing?

Thanks for any advice,

Roland
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Dear Roland Kaiser,

I think there may different pixel-to-point scaling in different  
devices. This scaling seems not only to vary with the device but even  
with your physical screen. This is what happens with my current box  
(MacBook):

 > quartz()
 > par("mar")/par("mai")
[1] 6 6 6 6
 > par("cra")
[1]  6 12
 > png()
 > par("mar")/par("mai")
[1] 4.686347 4.686347 4.686347 4.686347
 > par("cra")
[1] 13 16
 > dev.off()
 > pdf()
 > par("mar")/par("mai")
[1] 5 5 5 5
 > par("cra")
[1] 10.8 14.4
 > x11()
 > par("mar")/par("mai")
[1] 4.686347 4.686347 4.686347 4.686347
 > par("cra")
[1] 13 16

The quotient gives the par margin line height in inches, par("cra")  
the dimensions of the characters in "rasters" (which may be close to  
size in points, but may differ a lot in some screens). For your  
alignment, I think you want to make margin proportions equal in two  
devices, and you must scale so figs so instead of using physically  
equal dimensions. The mar/mai ratio or cra does not change with  
device width & height, so you must change those so that for fig looks  
like it should.

I have no ideas if this works, but I think this still is your problem...

Greetings to the Moderator,

Jari Oksanen

Cheers, Jari Oksanen