TIFF.rb

# TIFF.rb
# by Paul Schreiber <paulschreiber at gmail.com>
# 09 June 2010
#
# Based on JPEG.rb by Remco van't Veer and imagesize by Keisuke Minami <keisuke@rccn.com>
# http://snippets.dzone.com/posts/show/805
# http://blog.remvee.net/2005/12/02/hoogte_en_breedte_van_een_JPEG (JPEG
# http://rubyforge.org/projects/imagesize/

class TIFF
  attr_reader :width, :height

  def initialize(file)
    if file.kind_of? IO
      examine(file)
    else
      File.open(file, 'rb') { |io| examine(io) }
    end
  end

private
  def def_read_o(io)
    io.seek(0, 0)
    # define Singleton-method definition to IO (byte, offset)
    def io.read_o(length = 1, offset = nil)
      self.seek(offset, 0) if offset
      ret = self.read(length)
      raise "cannot read!!" unless ret
      ret
    end
    io
  end

  def examine(io)
    img_top = io.read(1024)
    if not (img_top[0, 4] == "MM\x00\x2a" or img_top[0, 4] == "II\x2a\x00")
      raise 'malformed TIFF'
    end
    
    img_io = def_read_o(io)

    endian = if (img_io.read_o(4) =~ /II\x2a\x00/o) then 'v' else 'n' end
# 'v' little-endian   'n' default to big-endian

    packspec = [
      nil,           # nothing (shouldn't happen)
      'C',           # BYTE (8-bit unsigned integer)
      nil,           # ASCII
      endian,        # SHORT (16-bit unsigned integer)
      endian.upcase, # LONG (32-bit unsigned integer)
      nil,           # RATIONAL
      'c',           # SBYTE (8-bit signed integer)
      nil,           # UNDEFINED
      endian,        # SSHORT (16-bit unsigned integer)
      endian.upcase, # SLONG (32-bit unsigned integer)
    ]

    offset = img_io.read_o(4).unpack(endian.upcase)[0] # Get offset to IFD

    ifd = img_io.read_o(2, offset)
    num_dirent = ifd.unpack(endian)[0]                   # Make it useful
    offset += 2
    num_dirent = offset + (num_dirent * 12);             # Calc. maximum offset of IFD

    ifd = width = height = nil
    while(width.nil? || height.nil?)
      ifd = img_io.read_o(12, offset)                 # Get first directory entry
      break if (ifd.nil? || (offset > num_dirent))
      offset += 12
      tag = ifd.unpack(endian)[0]                       # ...and decode its tag
      type = ifd[2, 2].unpack(endian)[0]                # ...and the data type

     # Check the type for sanity.
      next if (type > packspec.size + 0) || (packspec[type].nil?)
      if tag == 0x0100                                  # Decode the value
        @width = ifd[8, 4].unpack(packspec[type])[0]
      elsif tag == 0x0101                               # Decode the value
        @height = ifd[8, 4].unpack(packspec[type])[0]
      end
    end

    raise 'malformed TIFF' if @width.nil? || @height.nil?
  end # examine
end # class