Fine, looking at the code shows that readTiff read the image with using
libtiff which should be fine. Afterwards, a pixmap obnject is generated
via pixmapRGB() which includes the following lines:
datamax <- max(data)
datamin <- min(data)
data <- as.numeric(data)
if (datamax > 1 || datamin < 0)
data <- (data - datamin)/(datamax - datamin)
That means the data is perfectly fitted into the [0,1] interval.
That means we have not only rescaled but also another 0 now.
What I did is:
pic <- readTiff("test1_layer1.tif")
pic <- pic@red
ARC <- read.csv2("test1_arcgis.csv", header=TRUE)
ARC <- matrix(ARC[,2], nrow=nrow(pic), byrow=TRUE)
plot(pic)
plot(ARC)# looks *very* similar
plot(as.vector(pic) ~ as.vector(ARC))
# all on one line
summary(lm(as.vector(pic) ~ as.vector(ARC)))
# resuiduals < 10^(-14)
So the formula use to get from the ARC to the pixmap data is
pixmapdata = -0.82278 * 0.01266 ARCdata
if you want to get the original data, you can adapt the readTiff
function for your own use for greyscales (and save memory that way) as in:
myReadTiff <- function (fn, page = 0)
{
w <- .C("TiffGetWidth", as.character(fn), w = as.integer(0),
PACKAGE = "rtiff")$w
h <- .C("TiffGetHeight", as.character(fn), h = as.integer(0),
PACKAGE = "rtiff")$h
nw <- ceiling((1 - reduce) * w)
nh <- ceiling((1 - reduce) * h)
if (w > 0 && h > 0) {
tiff <- .C("TiffReadTIFFRGBA", as.character(fn),
page = as.integer(page),
r = integer(w * h), g = integer(w * h), b = integer(w * h),
PACKAGE = "rtiff")
tiff <- tiff$r / 255
tiff <- pixmapGrey(data = tiff, nrow = nh, ncol = nw)
return(tiff)
}
stop("Could not open", fn, ". File corrupted or missing.\n")
}
pic2 <- myReadTiff("test1_layer1.tif")
Now pic2@grey will be exactly the same as ARC/255 from above.
Uwe Ligges
On 22.04.2011 15:10, Julio Rojas wrote:
Dear Uwe, find attached an small portion of the file I'm working with. ArcGIS
values for this file are in the CSV file. They are a vector of all rows put
together.
Thanks and regards.
--- El vie, 4/22/11, Uwe Ligges<lig...@statistik.tu-dortmund.de> escribió:
De: Uwe Ligges<lig...@statistik.tu-dortmund.de>
Asunto: Re: [R] Reading a TIFF file
A: "Julio Rojas"<jcredbe...@ymail.com>
Cc: r-help@r-project.org
Fecha: viernes, 22 de abril de 2011, 12:37 pm
Unless you can provide a small
reproducible example (say a very small
tiff including the values you got from the non-R software)
it will be
hard to tell what is going on.
Uwe Ligges
On 22.04.2011 11:23, Julio Rojas wrote:
Dear all, I have been trying to speed up a process we
have been done in ArcGIS. We have to read a single layer
TIFF (monochrome image) in . For this, I have used the
"rtiff" package. After reading the TIFF file, I compared the
raw values for each pixel that I have in ArcGIS to the ones
obtained in R. In ArcGIS I have discrete values in the range
0..255, while in R I have continuous values between 0..1.
This, in itself might not be a problem if the values
obtained in R, times 255 would show the values obtained in
ArcGIS, but this is not the case. The images are very
different. I tried to settle matters using Photoshop, and
the values there are completely different from the other two
(using RGB, the K value (in CMYK) or the B value (in
HSB))!!!
Can somebody help me with this problem? Can I trust
"rtiff"? Should I stick to a very slow process in ArcGIS?
Why PS, which should be the perfect measuring stick, is
showing another set of values?
Thanks in advance. Regards.
Julio
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