R help - Aug 2005 - 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!)
 
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@uq.edu.au

	[[alternative HTML version deleted]]

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
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
> 
-- 
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

>>>>> 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]