class PDF::Reader::Filter

returned untouched. At this stage PDF::Reader has no need to decode images.
Filters that are only used to encode image data are accepted, but the data is

attempts to decode the specified data with the current filter
###############################################################################

def filter (data)
  # leave the data untouched if we don't support the required filter
  return data if @filter.nil?
  # decode the data
  self.send(@filter, data)
end