Class: FileData::ExifStream

Inherits:
Object
  • Object
show all
Extended by:
Forwardable
Includes:
BinaryExtensions
Defined in:
lib/file_data/formats/exif/exif_stream.rb

Overview

Wraps a stream with exif specific logic

Constant Summary collapse

MOTOROLLA_BYTES = 
'MM'.bytes.to_a.freeze
INTEL_BYTES = 
'II'.bytes.to_a.freeze
TYPE_BYTE = 
1
TYPE_ASCII = 
2
TYPE_SHORT = 
3
TYPE_LONG = 
4
TYPE_RATIONAL = 
5
TYPE_UNDEFINED = 
7
TYPE_SLONG = 
9
TYPE_SRATIONAL = 
10
HIGH_BIT_MASK = 
2**31
VALUE_OFFSET_SIZE = 
4

Instance Method Summary collapse

Methods included from BinaryExtensions

#read_ascii, #read_value

Constructor Details

#initialize(stream) ⇒ ExifStream

Returns a new instance of ExifStream.



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 28

def initialize(stream)
  @stream = stream
  @section_offset = stream.pos
end

Instance Method Details

#read_header ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 33

def read_header
  @is_little_endian =
    case @stream.each_byte.take(2)
    when INTEL_BYTES then true
    when MOTOROLLA_BYTES then false
    else raise 'the byte order bytes did not match any expected value'
    end

  raise 'the tiff constant 42 is missing' unless read_value(2) == 42
end

#read_large_val(type) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 77

def read_large_val(type)
  seek_to_large_val
  read_rational(type == TYPE_SRATIONAL)
end

#read_rational(is_srational) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 91

def read_rational(is_srational)
  Array.new(2) do
    piece = read_value(4)
    is_srational ? to_slong(piece) : piece
  end.join('/')
end

#read_raw_val(size) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 72

def read_raw_val(size)
  seek_to_large_val if size > VALUE_OFFSET_SIZE
  @stream.each_byte.take([size, VALUE_OFFSET_SIZE].max)
end

#read_small_val(type) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 82

def read_small_val(type)
  value = read_value(4)
  type == TYPE_SLONG ? to_slong(value) : value
end

#read_tag_value ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 48

def read_tag_value
  type = read_value(2)
  size = read_value(4)

  case type
  when TYPE_RATIONAL, TYPE_SRATIONAL
    read_large_val(type)
  when TYPE_BYTE, TYPE_SHORT, TYPE_LONG, TYPE_SLONG
    read_small_val(type)
  when TYPE_ASCII
    read_text(size)
  when TYPE_UNDEFINED
    read_undefined(size)
  end
end

#read_text(size) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 64

def read_text(size)
  read_raw_val(size).pack('c*').chomp("\x00")
end

#read_undefined(size) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 68

def read_undefined(size)
  [read_raw_val(size), @is_little_endian]
end

#seek_exif(offset) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 44

def seek_exif(offset)
  @stream.seek(@section_offset + offset)
end

#seek_to_large_val ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 87

def seek_to_large_val
  seek_exif(read_value(4))
end

#to_slong(raw_value) ⇒ Object 



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# File 'lib/file_data/formats/exif/exif_stream.rb', line 98

def to_slong(raw_value)
  -(raw_value & HIGH_BIT_MASK) + (raw_value & ~HIGH_BIT_MASK)
end