This function determines a common spatial resolution and spatial domain
  for several spatial objects.
The arguments … may be pixel images,
  binary masks, or other spatial objects acceptable to as.mask.
  
The common pixel grid is determined by inspecting all the pixel images
  and binary masks in the argument list, finding the pixel grid with the
  highest spatial resolution, and extending this pixel grid to cover the
  bounding box of all the spatial objects.
The return value is a binary mask M, representing the bounding box
  at the chosen pixel resolution.
  Use as.im(X, W=M) to convert a pixel image X to this new
  pixel resolution.
  Use as.mask(W, xy=M) to convert a window W
  to a binary mask at this new pixel resolution.
  See the Examples.