split exif and other form .jpg image

strip_exif.rb

#! /usr/bin/env ruby
# ruby strip_exif.rb [input.jpg] [output.jpg]

require 'stringio'

io = nil
in_path = ARGV[0]
out_path = ARGV[1]

File.open(in_path, 'rb') do |rf|
  io = StringIO.new(rf.read)
end

if io.read(2) != "\xFF\xD8".b
  fail "not jpeg image"
end

def io.readframe
  field_marker = read(2).unpack("C2")
  fail 'field format error' unless field_marker[0] == 0xFF
  frame_size = read(2).unpack("C2").inject(0){|r,v|
    r = r << 8
    r += v
  }
  [field_marker[1], read(frame_size - 2)]
end

def io.read_to_field(field)
  loop do
    marker, frame = readframe
    case marker
    when field
      return frame
    when 0xDA # sos
      fail "FF#{field.to_s(16)} not found"
    else
      # ignore
    end
  end
end

marker, frame = io.readframe
fail 'not jfif' unless marker == 0xE0

exif = io.read_to_field(0xE1) # app1
after_exif_pos = io.pos
io.rewind

File.open(out_path, 'wb+') do |wf|
  wf.write(io.read(after_exif_pos - (exif.length+4))) # 4 = field_marker(2)+size(2)
  io.pos = after_exif_pos
  wf.write(io.read)
end