Class: Prawn::Images::PNG

Inherits:
Image
  • 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

Extension API collapse

Extension API collapse

Methods inherited from Image

#calc_image_dimensions

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 36

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

  data.read(8) # Skip the default header

  @palette = +''
  @img_data = +''
  @transparency = {}

  loop do
    chunk_size = data.read(4).unpack1('N')
    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
        @transparency[:palette] = data.read(chunk_size).unpack('C*')
      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).unpack1('n')
        @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

  @img_data = Zlib::Inflate.inflate(@img_data)
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 25

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 23

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 24

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 24

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 24

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 23

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 22

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 25

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 22

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 26

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 26

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 22

def transparency
  @transparency
end

#width ⇒ Object (readonly)

Returns the value of attribute width.



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

def width
  @width
end

Class Method Details

.can_render?(image_blob) ⇒ Boolean

Returns:

  • (Boolean) 


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

def self.can_render?(image_blob)
  image_blob[0, 8].unpack('C*') == [137, 80, 78, 71, 13, 10, 26, 10]
end

Instance Method Details

#alpha_channel? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/prawn/images/png.rb', line 120

def alpha_channel?
  return true if color_type == 4 || color_type == 6
  return @transparency.any? if color_type == 3

  false
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 130

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
  )

  # append the actual image data to the object as a stream
  obj << img_data

  obj.stream.filters << {
    FlateDecode: {
      Predictor: 15,
      Colors: colors,
      BitsPerComponent: bits,
      Columns: width
    }
  }

  # 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!({})
    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.map { |x| [x, x] }.flatten
  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: bits,
      ColorSpace: :DeviceGray,
      Decode: [0, 1]
    )
    smask_obj.stream << alpha_channel

    smask_obj.stream.filters << {
      FlateDecode: {
        Predictor: 15,
        Colors: 1,
        BitsPerComponent: bits,
        Columns: width
      }
    }
    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 98

def colors
  case color_type
  when 0, 3, 4
    1
  when 2, 6
    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

#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 110

def split_alpha_channel!
  if alpha_channel?
    if color_type == 3
      generate_alpha_channel
    else
      split_image_data
    end
  end
end