Class: Prawn::Images::PNG

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

Overview

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

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(data) ⇒ PNG

Process a new PNG image

data

A string containing a full PNG file


30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
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
# File 'lib/prawn/images/png.rb', line 30

def initialize(data)
  data = StringIO.new(data.dup)

  data.read(8)  # Skip the default header

  @palette  = ""
  @img_data = ""
  @transparency = {}

  loop do
    chunk_size  = data.read(4).unpack("N")[0]
    section     = data.read(4)
    case section
    when 'IHDR'
      # we can grab other interesting values from here (like width,
      # height, etc)
      values = data.read(chunk_size).unpack("NNCCCCC")

      @width              = values[0]
      @height             = values[1]
      @bits               = values[2]
      @color_type         = values[3]
      @compression_method = values[4]
      @filter_method      = values[5]
      @interlace_method   = values[6]
    when 'PLTE'
      @palette << data.read(chunk_size)
    when 'IDAT'
      @img_data << data.read(chunk_size)
    when 'tRNS'
      # This chunk can only occur once and it must occur after the
      # PLTE chunk and before the IDAT chunk
      @transparency = {}
      case @color_type
      when 3
        # Indexed colour, RGB. Each byte in this chunk is an alpha for
        # the palette index in the PLTE ("palette") chunk up until the
        # last non-opaque entry. Set up an array, stretching over all
        # palette entries which will be 0 (opaque) or 1 (transparent).
        @transparency[:indexed]  = data.read(chunk_size).unpack("C*")
        short = 255 - @transparency[:indexed].size
        @transparency[:indexed] += ([255] * short) if short > 0
      when 0
        # Greyscale. Corresponding to entries in the PLTE chunk.
        # Grey is two bytes, range 0 .. (2 ^ bit-depth) - 1
        grayval = data.read(chunk_size).unpack("n").first
        @transparency[:grayscale] = grayval
      when 2
        # True colour with proper alpha channel.
        @transparency[:rgb] = data.read(chunk_size).unpack("nnn")
      end
    when 'IEND'
      # we've got everything we need, exit the loop
      break
    else
      # unknown (or un-important) section, skip over it
      data.seek(data.pos + chunk_size)
    end

    data.read(4)  # Skip the CRC
  end
end

Instance Attribute Details

#alpha_channelObject (readonly)

Returns the value of attribute alpha_channel


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

def alpha_channel
  @alpha_channel
end

#bitsObject (readonly)

Returns the value of attribute bits


21
22
23
# File 'lib/prawn/images/png.rb', line 21

def bits
  @bits
end

#color_typeObject (readonly)

Returns the value of attribute color_type


22
23
24
# File 'lib/prawn/images/png.rb', line 22

def color_type
  @color_type
end

#compression_methodObject (readonly)

Returns the value of attribute compression_method


22
23
24
# File 'lib/prawn/images/png.rb', line 22

def compression_method
  @compression_method
end

#filter_methodObject (readonly)

Returns the value of attribute filter_method


22
23
24
# File 'lib/prawn/images/png.rb', line 22

def filter_method
  @filter_method
end

#heightObject (readonly)

Returns the value of attribute height


21
22
23
# File 'lib/prawn/images/png.rb', line 21

def height
  @height
end

#img_dataObject (readonly)

Returns the value of attribute img_data


20
21
22
# File 'lib/prawn/images/png.rb', line 20

def img_data
  @img_data
end

#interlace_methodObject (readonly)

Returns the value of attribute interlace_method


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

def interlace_method
  @interlace_method
end

#paletteObject (readonly)

Returns the value of attribute palette


20
21
22
# File 'lib/prawn/images/png.rb', line 20

def palette
  @palette
end

#scaled_heightObject

Returns the value of attribute scaled_height


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

def scaled_height
  @scaled_height
end

#scaled_widthObject

Returns the value of attribute scaled_width


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

def scaled_width
  @scaled_width
end

#transparencyObject (readonly)

Returns the value of attribute transparency


20
21
22
# File 'lib/prawn/images/png.rb', line 20

def transparency
  @transparency
end

#widthObject (readonly)

Returns the value of attribute width


21
22
23
# File 'lib/prawn/images/png.rb', line 21

def width
  @width
end

Instance Method Details

#colorsObject

number of color components to each pixel


95
96
97
98
99
100
101
102
# File 'lib/prawn/images/png.rb', line 95

def colors
  case self.color_type
  when 0, 3, 4
    return 1
  when 2, 6
    return 3
  end
end

#pixel_bitlengthObject

number of bits used per pixel


106
107
108
109
110
111
112
# File 'lib/prawn/images/png.rb', line 106

def pixel_bitlength
  if alpha_channel?
    self.bits * (self.colors + 1)
  else
    self.bits * self.colors
  end
end

#split_alpha_channel!Object

split the alpha channel data from the raw image data in images where it's required.


117
118
119
# File 'lib/prawn/images/png.rb', line 117

def split_alpha_channel!
  unfilter_image_data if alpha_channel?
end