[R] how to read individual values from a pixmap object

[Roger Bivand]

On Thu, 4 Aug 2005, Tony Gill wrote:

> 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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> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no