Reading a TIFF file

De: Uwe Ligges <ligges at statistik.tu-dortmund.de>
Asunto: Re: [R] Reading a TIFF file
A: "Julio Rojas" <jcredberry at ymail.com>
Cc: r-help at r-project.org
Fecha: viernes, 22 de abril de 2011, 03:54 pm
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 at 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 at 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<ligges at statistik.tu-dortmund.de>?

escribi?:

De: Uwe Ligges<ligges at statistik.tu-dortmund.de>
Asunto: Re: [R] Reading a TIFF file
A: "Julio Rojas"<jcredberry at ymail.com>
Cc: r-help at 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