Reading a TIFF file

Thanks Uwe. I totally understood the problem. BTW, in your code you forgot to define "reduce". Can you define it in the function?

If the file has different minimum as well as maximum levels from this one, will it work?

Regards.

Julio


--- 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, 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