r - How does one turn contour lines into filled contours?

Question

Does anyone know of a way to turn the output of contourLines polygons in order to plot as filled contours, as with filled.contours. Is there an order to how the polygons must then be plotted in order to see all available levels? Here is an example snippet of code that doesn't work:

#typical plot
filled.contour(volcano, color.palette = terrain.colors)

#try
cont <- contourLines(volcano)
fun <- function(x) x$level
LEVS <- sort(unique(unlist(lapply(cont, fun))))
COLS <- terrain.colors(length(LEVS))
contour(volcano)
for(i in seq(cont)){
    COLNUM <- match(cont[[i]]$level, LEVS)
    polygon(cont[[i]], col=COLS[COLNUM], border="NA")
}
contour(volcano, add=TRUE)

Answer 1

A solution that uses the raster package (which calls rgeos and sp). The output is a SpatialPolygonsDataFrame that will cover every value in your grid:

library('raster')
rr <- raster(t(volcano))
rc <- cut(rr, breaks= 10)
pols <- rasterToPolygons(rc, dissolve=T)
spplot(pols)

Here's a discussion that will show you how to simplify ('prettify') the resulting polygons.