# File lib/graster/image.rb, line 14
    def self.from_file pathname
      raise "file not found #{pathname}" unless File.exist? pathname
      img = Magick::Image.read(pathname)
      raise "bad image data in #{pathname}" unless img = img[0]
      new :filename => File.basename(pathname),
          :size => [img.columns,img.rows],
          :pixels => img.export_pixels(0,0,img.columns,img.rows,"I")
    end