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



26
27
28
29
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
 
# File 'lib/prawn/images/png.rb', line 26

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

  # if our img_data contains alpha channel data, split it out
  unfilter_image_data if alpha_channel?
end

Instance Attribute Details

#alpha_channel ⇒ Object (readonly)

Returns the value of attribute alpha_channel.



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

def alpha_channel
  @alpha_channel
end

#bits ⇒ Object (readonly)

Returns the value of attribute bits.



19
20
21
 
# File 'lib/prawn/images/png.rb', line 19

def bits
  @bits
end

#color_type ⇒ Object (readonly)

Returns the value of attribute color_type.



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

def color_type
  @color_type
end

#compression_method ⇒ Object (readonly)

Returns the value of attribute compression_method.



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

def compression_method
  @compression_method
end

#filter_method ⇒ Object (readonly)

Returns the value of attribute filter_method.



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

def filter_method
  @filter_method
end

#height ⇒ Object (readonly)

Returns the value of attribute height.



19
20
21
 
# File 'lib/prawn/images/png.rb', line 19

def height
  @height
end

#img_data ⇒ Object (readonly)

:nodoc:



18
19
20
 
# File 'lib/prawn/images/png.rb', line 18

def img_data
  @img_data
end

#interlace_method ⇒ Object (readonly)

Returns the value of attribute interlace_method.



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

def interlace_method
  @interlace_method
end

#palette ⇒ Object (readonly)

:nodoc:



18
19
20
 
# File 'lib/prawn/images/png.rb', line 18

def palette
  @palette
end

#transparency ⇒ Object (readonly)

:nodoc:



18
19
20
 
# File 'lib/prawn/images/png.rb', line 18

def transparency
  @transparency
end

#width ⇒ Object (readonly)

Returns the value of attribute width.



19
20
21
 
# File 'lib/prawn/images/png.rb', line 19

def width
  @width
end

Instance Method Details

#pixel_bytes ⇒ Object 



92
93
94
95
96
97
 
# File 'lib/prawn/images/png.rb', line 92

def pixel_bytes
  @pixel_bytes ||= case @color_type
  when 0, 3, 4 then 1
  when 1, 2, 6 then 3
  end
end