R help - Mar 2006 - count pixels of same color in pixmap object?

Dear all,

I try to figure out how to use R to count the number of pixels of the 
same color in some gray-level picture. I managed to read it in either 
tiff or jpeg format, but the returned pixmap object keeps its 
information out of (my) reach. Is there an easy way to tabulate the 
different color/graylevel pixels and their numbers? Or should I use a 
completely different (free) software?

Thanks for any hint, Christian.
-- 
***********************************************************
http://cognition.ups-tlse.fr/vas-y.php?id=chj  jost at cict.fr
Christian Jost                                   (PhD, MdC)
Centre de Recherches sur la Cognition Animale
Universite Paul Sabatier, Bat IV R3
118 route de Narbonne
31062 Toulouse cedex 9, France
Tel: +33 5 61 55 64 37   Fax: +33 5 61 55 61 54

On Thu, 9 Mar 2006, Christian Jost wrote:
> Dear all,
> 
> I try to figure out how to use R to count the number of pixels of the 
> same color in some gray-level picture. I managed to read it in either 
> tiff or jpeg format, but the returned pixmap object keeps its 
> information out of (my) reach. Is there an easy way to tabulate the 
> different color/graylevel pixels and their numbers? Or should I use a 
> completely different (free) software?
The objects are new-style class objects, so they are documented internally 
- query by getSlots() or from ?"pixmap-class":

library(pixmap)
x <- read.pnm(system.file("pictures/logo.ppm",
package="pixmap")[1])
z <- as(x, "pixmapGrey")
class(z)
getSlots(class(z)) # lets you know what the slots are
greydata <- slot(z, "grey") # extract the slot object
summary(c(greydata)) # c() to flatten a matrix
length(unique(c(greydata)))
length((c(greydata))) # rather a lot of different values, let's round:
rgreydata <- round(greydata*10)
length(unique(c(rgreydata))) # looks good
table(c(rgreydata))

I assume that you converted from tiff or jpeg to pnm outside pixmap.
> 
> Thanks for any hint, Christian.
> 
-- 
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