Class: Pura::Tiff::Encoder

Inherits:
Object
  • Object
show all
Defined in:
lib/pura/tiff/encoder.rb

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(image) ⇒ Encoder

Returns a new instance of Encoder.



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# File 'lib/pura/tiff/encoder.rb', line 12

def initialize(image)
  @image = image
  @width = image.width
  @height = image.height
  @pixels = image.pixels
end

Class Method Details

.encode(image, output_path) ⇒ Object 



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# File 'lib/pura/tiff/encoder.rb', line 6

def self.encode(image, output_path)
  data = new(image).encode
  File.binwrite(output_path, data)
  data.bytesize
end

Instance Method Details

#encode ⇒ Object 



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# File 'lib/pura/tiff/encoder.rb', line 19

def encode
  out = String.new(encoding: Encoding::BINARY)

  # We'll build: header (8) + IFD + tag data + pixel data
  # Layout:
  #   0..7:     header
  #   8..N:     IFD (2 + num_tags*12 + 4 bytes for next IFD pointer)
  #   N+1..M:   tag value data that doesn't fit in 4 bytes
  #   M+1..end: pixel data (strips)

  pixel_data = @pixels
  strip_byte_count = pixel_data.bytesize

  # Tags we'll write (sorted by tag ID as required by TIFF spec)
  tags = []
  # ImageWidth (256)
  tags << make_tag(256, TYPE_LONG, 1, [@width])
  # ImageLength (257)
  tags << make_tag(257, TYPE_LONG, 1, [@height])
  # BitsPerSample (258) - 3 values: 8, 8, 8
  tags << make_tag(258, TYPE_SHORT, 3, [8, 8, 8])
  # Compression (259) - None
  tags << make_tag(259, TYPE_SHORT, 1, [1])
  # PhotometricInterpretation (262) - RGB
  tags << make_tag(262, TYPE_SHORT, 1, [2])
  # StripOffsets (273) - will be patched
  tags << make_tag(273, TYPE_LONG, 1, [0])
  # SamplesPerPixel (277)
  tags << make_tag(277, TYPE_SHORT, 1, [3])
  # RowsPerStrip (278) - all rows in one strip
  tags << make_tag(278, TYPE_LONG, 1, [@height])
  # StripByteCounts (279)
  tags << make_tag(279, TYPE_LONG, 1, [strip_byte_count])
  # XResolution (282)
  tags << make_tag(282, TYPE_RATIONAL, 1, [72, 1])
  # YResolution (283)
  tags << make_tag(283, TYPE_RATIONAL, 1, [72, 1])
  # ResolutionUnit (296) - inches
  tags << make_tag(296, TYPE_SHORT, 1, [2])

  tags.sort_by! { |t| t[:id] }

  num_tags = tags.size
  ifd_offset = 8
  ifd_size = 2 + (num_tags * 12) + 4 # count + entries + next IFD pointer

  # Calculate overflow data offset (for values > 4 bytes)
  overflow_offset = ifd_offset + ifd_size
  overflow_data = String.new(encoding: Encoding::BINARY)

  # Assign offsets for overflow values
  tags.each do |tag|
    value_bytes = tag_value_bytes(tag)
    if value_bytes.bytesize > 4
      tag[:value_offset] = overflow_offset + overflow_data.bytesize
      overflow_data << value_bytes
    else
      tag[:value_offset] = nil # fits inline
    end
  end

  # Pixel data offset
  pixel_offset = overflow_offset + overflow_data.bytesize

  # Patch StripOffsets to point to pixel data
  strip_tag = tags.find { |t| t[:id] == 273 }
  strip_tag[:values] = [pixel_offset]

  # Write header (little-endian)
  out << "II" # Little-endian
  out << [42].pack("v")               # Magic
  out << [ifd_offset].pack("V")       # IFD offset

  # Write IFD
  out << [num_tags].pack("v")

  tags.each do |tag|
    out << [tag[:id]].pack("v")
    out << [tag[:type]].pack("v")
    out << [tag[:count]].pack("V")

    value_bytes = tag_value_bytes(tag)
    if value_bytes.bytesize > 4
      out << [tag[:value_offset]].pack("V")
    else
      # Pad to 4 bytes
      padded = value_bytes + ("\x00".b * (4 - value_bytes.bytesize))
      out << padded
    end
  end

  # Next IFD offset (0 = no more IFDs)
  out << [0].pack("V")

  # Write overflow data
  out << overflow_data

  # Write pixel data
  out << pixel_data

  out
end