Transfrom raster extent to a data.frame or a geometry

cast_extent.r

#' Transform extent coordinates to data.frame
#' @param e Raster or Extent object
#' @param loop Should first and last coordinates be the same ? (defautl TRUE, required to create a polygon)
extent_as_data_frame_poly = function (e, loop = TRUE) {
  e = extent(e)
  e_poly = data.frame(x = c(e@xmin, e@xmax, e@xmax, e@xmin), 
                      y = c(e@ymin, e@ymin, e@ymax, e@ymax))
  if (loop) e_poly = rbind(e_poly, e_poly[1, ])
  e_poly
}

#' Transform extent to SpatialPolygon
#' @param r Raster or Extent object
#' @param p4s proj4string data, default is taken from r if it is of class Raster
extent_as_polygon = function(r, p4s = CRS(proj4string(r))) {
  SpatialPolygons(
    list(Polygons(list(Polygon(extent_as_data_frame_poly(r))), 1)), 
    proj4string = p4s)
}