extract by attributes from raster-image to polygons

Hi Benny

I once had the same problem and couldn't find a solution, so I coded a workaround. Adjusted to your example, this will look like:

library(rgeos)
library(raster)
library(maptools)
im=brick("P1020393.JPG")
extract=(im[[1]]>50)&(im[[2]]>180)&(im[[3]]>220)
poly_dissolve=rasterToPolygons(extract,fun=function(x){x==1},dissolve=T)

len<-length(poly_dissolve at polygons[[1]]@Polygons)

pol<-poly_dissolve
pol at plotOrder<-1:len
pol at data<-data.frame(1:len)

for(i in 1:len){
   pol at polygons[[i]]<-Polygons(list(poly_dissolve at polygons[[1]]@Polygons[[i]]),i)
}

plot(pol,col=rainbow(len))

May not be the most elegant way, but it solved my problem.

HTH,
Rafi