On Fri, 14 Aug 2009, Barry Rowlingson wrote:
I'm wondering if there's an easy way to write PNG files from a matrix?
I've got an NxM matrix of values, z, and I want to produce an NxM
pixel PNG file. I have a function pfunc(.) that converts values to
colours.
So currently I do this:
library(rtiff)
library(rgdal)
z = matrix(runif(100),10,10)
pfunc=colorRamp(c("blue","yellow"))
tiffPath = "foo.tif"
pngPath = "foo.png"
# get the rgb colours:
rgb = pfunc(t(z)[ncol(z):1,])
# as a pixmapRGB
rgbpixmap = pixmapRGB(c(rgb[,1],rgb[,2],rgb[,3]),ncol(z),nrow(z))
# save as a tiff (require(rtiff) package)
writeTiff(rgbpixmap,tiffPath)
# now read in using rgdal and copy, since GDAL PNG driver doesn't have
a 'create' call:
xp=GDAL.open(tiffPath)
xx=copyDataset(xp,driver="PNG")
saveDataset(xx,pngPath)
GDAL.close(xx)
GDAL.close(xp)
# we don't need this:
file.remove(tiffPath)
But it seems a bit kludgy. I know its not really a 'geo' question, but
it seems that rgdal might have a better way to do this that I've not
hit on yet. Any ideas (short of system("convert foo.tif foo.png")). Or
is my solution as elegant as it can be?
rgdal 0.6-14 (now on sourceforge as source, on its way to CRAN) tries to
detect whether the driver chosen by the user is one of those flagged as
not supporting direct creation but supporting GDAL dataset copying,
including the "PNG" driver. When writeGDAL() sees such a driver, it will
now write to a temporary dataset (defualt driver "GTiff"), and copy from
that to the desired file. Using:
GT <- GridTopology(c(0.5, 0.5), c(1, 1), c(10, 10))
set.seed(1)
SGDF <- SpatialGridDataFrame(GT, data=data.frame(z=runif(100)))
opar <- par(mfrow=c(2,2), mar=c(1,1,4,1))
image(SGDF, "z", col=colorRampPalette(c("blue", "yellow"))(20))
title(main="input values")
pfunc <- colorRamp(c("blue","yellow"))
RGB <- pfunc(SGDF$z)
SGDF$red <- RGB[,1]
SGDF$green <- RGB[,2]
SGDF$blue <- RGB[,3]
image(SGDF, red="red", green="green", blue="blue")
title(main="input RGB")
#
# to set up the data for export,
#
tf <- tempfile()
writeGDAL(SGDF[c("red", "green", "blue")], tf, type="Byte",
drivername="PNG")
#
# will now write the PNG file. Reading in to check gives:
#
t1 <- readGDAL(tf)
image(t1, red=1, green=2, blue=3)
title(main="output PNG RGB")
par(opar)
I know that this is a "geo" answer, but getting the NxM matrix into an
object that writeGDAL() can export shouldn't be too hard, by first
defining the GridTopology object, then reshaping the matrix to suit.
Looking at ?as.image.SpatialGridDataFrame and the code for this function
may help, as well as image2Grid, to get the right incantation.
Roger
Barry
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--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: roger.biv...@nhh.no
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