Module: Pdfrb::ImageLoader::PNG

Defined in:
lib/pdfrb/image_loader/png.rb

Overview

PNG loader. Parses IHDR/IDAT/IEND chunks. Stores the zlib-compressed IDAT bytes verbatim with a PNG-predictor DecodeParms dict so the reader re-applies the PNG unfilter via FlateDecode.

Supported color types: 0 (gray), 2 (RGB), 3 (indexed), 4 (gray+alpha), 6 (RGBA). For RGBA/gray+alpha, alpha is split into a separate /SMask Image XObject (also FlateDecode + PNG predictor).

Class Method Summary collapse

Class Method Details

.build_opaque_image(document, width, height, bit_depth, color_type, decoded, chunks) ⇒ Object 



78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
 
# File 'lib/pdfrb/image_loader/png.rb', line 78

def build_opaque_image(document, width, height, bit_depth, color_type, decoded, chunks)
  predictor_parms = predictor_parms(width, color_type[:channels], bit_depth)
  dict = {
    Type: :XObject,
    Subtype: :Image,
    Width: width,
    Height: height,
    BitsPerComponent: bit_depth,
    Filter: :FlateDecode,
    DecodeParms: predictor_parms
  }
  dict[:ColorSpace] = color_space_for(color_type, bit_depth, chunks)
  image = document.add(dict, type: Pdfrb::Model::Type::XObjectImage)
  image.stream = deflate_with_filter_bytes(decoded, width, color_type[:channels], height)
  image
end

.build_rgba_image(document, width, height, bit_depth, color_type, decoded, _chunks) ⇒ Object 



95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
 
# File 'lib/pdfrb/image_loader/png.rb', line 95

def build_rgba_image(document, width, height, bit_depth, color_type, decoded, _chunks)
  # De-filter PNG rows, split into RGB + alpha channels,
  # re-compress each separately.
  rgb, alpha = split_channels(decoded, width, height,
                              color_type[:channels], bit_depth)
  channels = color_type[:channels] - 1 # alpha removed
  predictor_parms = predictor_parms(width, channels, bit_depth)
  rgb_bytes = with_filter_bytes(rgb, width, channels, height)
  alpha_bytes = with_filter_bytes(alpha, width, 1, height)

  smask = document.add(
    {
      Type: :XObject,
      Subtype: :Image,
      Width: width,
      Height: height,
      BitsPerComponent: bit_depth,
      ColorSpace: :DeviceGray,
      Filter: :FlateDecode,
      DecodeParms: predictor_parms(width, 1, bit_depth)
    },
    type: Pdfrb::Model::Type::XObjectImage
  )
  smask.stream = Zlib::Deflate.deflate(alpha_bytes)

  image = document.add(
    {
      Type: :XObject,
      Subtype: :Image,
      Width: width,
      Height: height,
      BitsPerComponent: bit_depth,
      ColorSpace: color_type[:name] == :rgba ? :DeviceRGB : :DeviceGray,
      Filter: :FlateDecode,
      DecodeParms: predictor_parms,
      SMask: Pdfrb::Model::Reference.new(smask.oid, smask.gen)
    },
    type: Pdfrb::Model::Type::XObjectImage
  )
  image.stream = Zlib::Deflate.deflate(rgb_bytes)
  image
end

.call(document, bytes, **_opts) ⇒ Object 



28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
 
# File 'lib/pdfrb/image_loader/png.rb', line 28

def call(document, bytes, **_opts)
  return nil unless bytes.is_a?(::String) && bytes.start_with?(SIGNATURE)

  chunks = parse_chunks(bytes)
  ihdr = chunks[:IHDR]
  width = unpack_uint(ihdr, 0, 4)
  height = unpack_uint(ihdr, 4, 4)
  bit_depth = ihdr.getbyte(8)
  color_type_id = ihdr.getbyte(9)
  color_type = COLOR_TYPES[color_type_id] ||
               raise(Pdfrb::Error, "PNG: unsupported color type #{color_type_id}")

  idat = chunks[:IDATs].map { |c| c }.join
  decoded = Zlib::Inflate.inflate(idat)

  if color_type[:has_alpha]
    build_rgba_image(document, width, height, bit_depth, color_type, decoded, chunks)
  else
    build_opaque_image(document, width, height, bit_depth, color_type, decoded, chunks)
  end
end