Confused by overlay(x = "SpatialPixelsDataFrame", y = "SpatialPoints")

Hi,
trying to extract by mask using sp package.
Was surprised that I couldn't find 'overlay(x =
"SpatialPixelsDataFrame", y = "SpatialPolygon")' to extract pixels
within a polygon.

There is one except that it is y="SpatialPolygons" rather than 
"SpatialPolygon":

 > showMethods(overlay)
Function: overlay (package sp)
x="SpatialGridDataFrame", y="SpatialPoints"
x="SpatialGrid", y="SpatialPoints"
x="SpatialPixelsDataFrame", y="SpatialPoints"
x="SpatialPixelsDataFrame", y="SpatialPolygons"
    (inherited from: x="SpatialPointsDataFrame", y="SpatialPolygons")
x="SpatialPixels", y="SpatialPoints"
x="SpatialPointsDataFrame", y="SpatialPolygons"
x="SpatialPoints", y="SpatialPolygons"
x="SpatialPolygons", y="SpatialPoints"

(note that it's only visible this way only after you've used it)
So instead I'm using
ascPoints <- SpatialPoints(coordinates(asc))
ascPointsHR <- ascPoints[!is.na(overlay(ascPoints,hr))]

Now when I try to use overlay using 'ascPointsHR' on 'asc' I get a
strange return. sp docs says it 'returns grid values x at spatial
point locations
y' which I interpret as me getting the raster values. 
This is wrong; where exactly does it say so? Documentation is a bit 
scattered sometimes.
Instead I seem
to be getting the index values of the pixels or something. The raster
is float, as seen from the summary below. But the return from
overlay() is ordered integer.

Am I missing something?

Do you manage to get the pixels you want from the indexes returned?
--
Edzer
Thank you
David Wahlund

summary(asc)

Object of class SpatialPixelsDataFrame
Coordinates:
      min     max
x 1592482 1622382
y 7121568 7144668
Is projected: NA
proj4string : [NA]
Number of points: 1105104
Grid attributes:
  cellcentre.offset cellsize cells.dim
x           1592494       25      1196
y           7121580       25       924
Data attributes:
     Min.   1st Qu.    Median      Mean   3rd Qu.      Max.
0.000e+00 0.000e+00 0.000e+00 1.448e-09 4.750e-11 4.954e-08

summary(ascPointsHR)

Object of class SpatialPoints
Coordinates:
      min     max
x 1599144 1617094
y 7127130 7138680
Is projected: NA
proj4string : [NA]
Number of points: 198320

ascHR <- overlay(asc,ascPointsHR)
summary(ascHR)

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
 266300  464700  556800  552900  646600  818600

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Edzer Pebesma
Institute for Geoinformatics (ifgi), University of M?nster
Weseler Stra?e 253, 48151 M?nster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de

Confused by overlay(x = "SpatialPixelsDataFrame", y = "SpatialPoints")

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