how to read individual values from a pixmap object

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Hi All,

I have a greyscale image that I am reading in through RGDAL and placing in a
pixmap object.

As an example use the logo.jpg file that comes with the RGDAL package:

#Read the file
logo <- system.file("pictures/logo.jpg", package="rgdal")[1]
x <- new("GDALReadOnlyDataset", logo)

#Create the pixmap object
xGrey <- getPixmapGDAL(x)

Now I would like to read individual pixel values and store them in a
separate vector. However I have not found a straight-forward way to do this.

I thought that something like:

x <- xGrey[1,1]

would return the pixel value, but it seems to return a pixmap object with
dimension 1x1 (i.e. only one pixel in size)

xGrey[1,1]
Pixmap image
  Type          : pixmapGrey 
  Size          : 1x1 
  Resolution    : 1x1 
  Bounding box  : 1 0 2 1

Any ideas on how to get the actual pixel value from a pixmap? (Note I could
output to an ascii file and then read the file back in as text - but not
nice!)

There are lots of ideas in the S4 Classes and Methods useR! 04 keynote
talk given by pixmap maintainer Friedrich Leisch at:  

http://www.ci.tuwien.ac.at/Conferences/useR-2004/Keynotes/Leisch.pdf

but the easiest one is:
setAs("pixmapGrey", "matrix", function(from, to) {from at grey})
x
Pixmap image
  Type          : pixmapGrey 
  Size          : 1x1 
  Resolution    : 1x1 
  Bounding box  : 0 0 1 1
as(x, "matrix")
[,1]
[1,]    1

unless you use a hammer and chisel:
str(x)
Formal class 'pixmapGrey' [package "pixmap"] with 6 slots
  ..@ grey    : num [1, 1] 1
  ..@ channels: chr "grey"
  ..@ size    : int [1:2] 1 1
  ..@ cellres : num [1:2] 1 1
  ..@ bbox    : num [1:4] 0 0 1 1
  ..@ bbcent  : logi FALSE
x at grey
[,1]
[1,]    1
Thanks in advance

Tony

******************************************
Tony Gill - PhD Candidate
Centre for Remote Sensing & Spatial Information Science
School of Geography, Planning & Architecture
University of Queensland
Brisbane, Queensland, AUSTRALIA, 4072
Ph: 61-7-3365-7027
email: t.gill1 at uq.edu.au

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On Thu, 4 Aug 2005 18:02:55 +1000,
Tony Gill (TG) wrote:
[...]

  > Now I would like to read individual pixel values and store them in a
  > separate vector. However I have not found a straight-forward way to do this.

  > I thought that something like:

  > x <- xGrey[1,1]

  > would return the pixel value, but it seems to return a pixmap object with
  > dimension 1x1 (i.e. only one pixel in size)

  > xGrey[1,1]
  > Pixmap image
  >   Type          : pixmapGrey 
  >   Size          : 1x1 
  >   Resolution    : 1x1 
  >   Bounding box  : 1 0 2 1

  > Any ideas on how to get the actual pixel value from a pixmap? (Note I could
  > output to an ascii file and then read the file back in as text - but not
  > nice!)

getChannels(xGrey) will get you the matrix of pixel values.

HTH,
-------------------------------------------------------------------
                        Friedrich Leisch 
Institut f??r Statistik                     Tel: (+43 1) 58801 10715
Technische Universit??t Wien                Fax: (+43 1) 58801 10798
Wiedner Hauptstra??e 8-10/1071
A-1040 Wien, Austria             http://www.ci.tuwien.ac.at/~leisch
Tony Gill <t.gill1 <at> uq.edu.au> writes:
Hi All,

I have a greyscale image that I am reading in through RGDAL and placing in a
pixmap object.

library(pixmap)
x <- read.pnm("graypic.pnm")
str(x)
x at grey[1,1]
Ben Bolker <bolker <at> ufl.edu> writes:
Tony Gill <t.gill1 <at> uq.edu.au> writes:

Hi All,

I have a greyscale image ...
library(pixmap)
x <- read.pnm("graypic.pnm")
str(x)
x <at> grey[1,1]

just in case there's any confusion ... that <at> should
be an at-symbol!