Class: Prawn::Images::JPG

Inherits:
Image
  • Object
show all
Defined in:
lib/prawn/images/jpg.rb

Overview

A convenience class that wraps the logic for extracting the parts of a JPG image that we need to embed them in a PDF

Defined Under Namespace

Classes: FormatError

Extension API collapse

JPEG_SOF_BLOCKS = 
[
  0xC0, 0xC1, 0xC2, 0xC3, 0xC5, 0xC6, 0xC7, 0xC9, 0xCA, 0xCB, 0xCD, 0xCE,
  0xCF
].freeze

Extension API collapse

Extension API collapse

Methods inherited from Image

#calc_image_dimensions

Constructor Details

#initialize(data) ⇒ JPG

Process a new JPG image

:data

A binary string of JPEG data



39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
 
# File 'lib/prawn/images/jpg.rb', line 39

def initialize(data)
  super()
  @data = data
  d = StringIO.new(@data)
  d.binmode

  c_marker = 0xff # Section marker.
  d.seek(2) # Skip the first two bytes of JPEG identifier.
  loop do
    marker, code, length = d.read(4).unpack('CCn')
    raise FormatError, 'JPEG marker not found!' if marker != c_marker

    if JPEG_SOF_BLOCKS.include?(code)
      @bits, @height, @width, @channels = d.read(6).unpack('CnnC')
      break
    end

    d.seek(length - 2, IO::SEEK_CUR)
  end
end

Instance Attribute Details

#bits ⇒ Object (readonly)

Returns the value of attribute bits.



23
24
25
 
# File 'lib/prawn/images/jpg.rb', line 23

def bits
  @bits
end

#channels ⇒ Object (readonly)

Returns the value of attribute channels.



23
24
25
 
# File 'lib/prawn/images/jpg.rb', line 23

def channels
  @channels
end

#height ⇒ Object (readonly)

Returns the value of attribute height.



23
24
25
 
# File 'lib/prawn/images/jpg.rb', line 23

def height
  @height
end

#scaled_height ⇒ Object

Returns the value of attribute scaled_height.



24
25
26
 
# File 'lib/prawn/images/jpg.rb', line 24

def scaled_height
  @scaled_height
end

#scaled_width ⇒ Object

Returns the value of attribute scaled_width.



24
25
26
 
# File 'lib/prawn/images/jpg.rb', line 24

def scaled_width
  @scaled_width
end

#width ⇒ Object (readonly)

Returns the value of attribute width.



23
24
25
 
# File 'lib/prawn/images/jpg.rb', line 23

def width
  @width
end

Class Method Details

.can_render?(image_blob) ⇒ Boolean

Returns:

  • (Boolean) 


31
32
33
 
# File 'lib/prawn/images/jpg.rb', line 31

def self.can_render?(image_blob)
  image_blob[0, 3].unpack('C*') == [255, 216, 255]
end

Instance Method Details

#build_pdf_object(document) ⇒ Object

Build a PDF object representing this image in document, and return a Reference to it.



63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
 
# File 'lib/prawn/images/jpg.rb', line 63

def build_pdf_object(document)
  color_space =
    case channels
    when 1
      :DeviceGray
    when 3
      :DeviceRGB
    when 4
      :DeviceCMYK
    else
      raise ArgumentError, 'JPG uses an unsupported number of channels'
    end

  obj = document.ref!(
    Type: :XObject,
    Subtype: :Image,
    ColorSpace: color_space,
    BitsPerComponent: bits,
    Width: width,
    Height: height
  )

  # add extra decode params for CMYK images. By swapping the
  # min and max values from the default, we invert the colours. See
  # section 4.8.4 of the spec.
  if color_space == :DeviceCMYK
    obj.data[:Decode] = [1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0]
  end

  obj.stream << @data
  obj.stream.filters << :DCTDecode
  obj
end