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

#build_pdf_object(document) ⇒ Object

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



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# File 'lib/prawn/images/png.rb', line 135

def build_pdf_object(document)
  if compression_method != 0
    raise Errors::UnsupportedImageType,
      'PNG uses an unsupported compression method'
  end

  if filter_method != 0
    raise Errors::UnsupportedImageType,
      'PNG uses an unsupported filter method'
  end

  if interlace_method != 0
    raise Errors::UnsupportedImageType,
      'PNG uses unsupported interlace method'
  end

  # some PNG types store the colour and alpha channel data together,
  # which the PDF spec doesn't like, so split it out.
  split_alpha_channel!

  case colors
  when 1
    color = :DeviceGray
  when 3
    color = :DeviceRGB
  else
    raise Errors::UnsupportedImageType,
      "PNG uses an unsupported number of colors (#{png.colors})"
  end

  # build the image dict
  obj = document.ref!(
    :Type             => :XObject,
    :Subtype          => :Image,
    :Height           => height,
    :Width            => width,
    :BitsPerComponent => bits,
    :Length           => img_data.size,
    :Filter           => :FlateDecode
  )

  unless alpha_channel
    obj.data[:DecodeParms] = {:Predictor => 15,
                              :Colors    => colors,
                              :BitsPerComponent => bits,
                              :Columns   => width}
  end

  # append the actual image data to the object as a stream
  obj << img_data
  
  # sort out the colours of the image
  if palette.empty?
    obj.data[:ColorSpace] = color
  else
    # embed the colour palette in the PDF as a object stream
    palette_obj = document.ref!(:Length => palette.size)
    palette_obj << palette

    # build the color space array for the image
    obj.data[:ColorSpace] = [:Indexed, 
                             :DeviceRGB,
                             (palette.size / 3) -1,
                             palette_obj]
  end

  # *************************************
  # add transparency data if necessary
  # *************************************

  # For PNG color types 0, 2 and 3, the transparency data is stored in
  # a dedicated PNG chunk, and is exposed via the transparency attribute
  # of the PNG class.
  if transparency[:grayscale]
    # Use Color Key Masking (spec section 4.8.5)
    # - An array with N elements, where N is two times the number of color
    #   components.
    val = transparency[:grayscale]
    obj.data[:Mask] = [val, val]
  elsif transparency[:rgb]
    # Use Color Key Masking (spec section 4.8.5)
    # - An array with N elements, where N is two times the number of color
    #   components.
    rgb = transparency[:rgb]
    obj.data[:Mask] = rgb.collect { |x| [x,x] }.flatten
  elsif transparency[:indexed]
    # TODO: broken. I was attempting to us Color Key Masking, but I think
    #       we need to construct an SMask i think. Maybe do it inside
    #       the PNG class, and store it in alpha_channel
    #obj.data[:Mask] = transparency[:indexed]
  end

  # For PNG color types 4 and 6, the transparency data is stored as a alpha
  # channel mixed in with the main image data. The PNG class seperates
  # it out for us and makes it available via the alpha_channel attribute
  if alpha_channel?
    smask_obj = document.ref!(
      :Type             => :XObject,
      :Subtype          => :Image,
      :Height           => height,
      :Width            => width,
      :BitsPerComponent => alpha_channel_bits,
      :Length           => alpha_channel.size,
      :Filter           => :FlateDecode,
      :ColorSpace       => :DeviceGray,
      :Decode           => [0, 1]
    )
    smask_obj << alpha_channel
    obj.data[:SMask] = smask_obj
  end

  obj
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

#min_pdf_version ⇒ Object

Returns the minimum PDF version required to support this image.



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# File 'lib/prawn/images/png.rb', line 250

def min_pdf_version
  if bits > 8
    # 16-bit color only supported in 1.5+ (ISO 32000-1:2008 8.9.5.1)
    1.5
  elsif alpha_channel?
    # Need transparency for SMask
    1.4
  else
    1.0
  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