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 binary string of PNG data



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# 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_channel ⇒ Object (readonly)

Returns the value of attribute alpha_channel.



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

def alpha_channel
  @alpha_channel
end

#bits ⇒ Object (readonly)

Returns the value of attribute bits.



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

def bits
  @bits
end

#color_type ⇒ Object (readonly)

Returns the value of attribute color_type.



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

def color_type
  @color_type
end

#compression_method ⇒ Object (readonly)

Returns the value of attribute compression_method.



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

def compression_method
  @compression_method
end

#filter_method ⇒ Object (readonly)

Returns the value of attribute filter_method.



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

def filter_method
  @filter_method
end

#height ⇒ Object (readonly)

Returns the value of attribute height.



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

def height
  @height
end

#img_data ⇒ Object (readonly)

Returns the value of attribute img_data.



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

def img_data
  @img_data
end

#interlace_method ⇒ Object (readonly)

Returns the value of attribute interlace_method.



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

def interlace_method
  @interlace_method
end

#palette ⇒ Object (readonly)

Returns the value of attribute palette.



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

def palette
  @palette
end

#scaled_height ⇒ Object

Returns the value of attribute scaled_height.



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

def scaled_height
  @scaled_height
end

#scaled_width ⇒ Object

Returns the value of attribute scaled_width.



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

def scaled_width
  @scaled_width
end

#transparency ⇒ Object (readonly)

Returns the value of attribute transparency.



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

def transparency
  @transparency
end

#width ⇒ Object (readonly)

Returns the value of attribute width.



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

def width
  @width
end

Instance Method Details

#alpha_channel? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/prawn/images/png.rb', line 121

def alpha_channel?
  @color_type == 4 || @color_type == 6
end

#alpha_channel_bits ⇒ Object

Adobe Reader can’t handle 16-bit png channels – chop off the second byte (least significant)



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# File 'lib/prawn/images/png.rb', line 128

def alpha_channel_bits
  8
end

#colors ⇒ Object

number of color components to each pixel



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# 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_bitlength ⇒ Object

number of bits used per pixel



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# 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.



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

def split_alpha_channel!
  unfilter_image_data if alpha_channel?
end