how to read individual values from a pixmap object

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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how to read individual values from a pixmap object

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