Class: Prawn::Images::JPG

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

Overview

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

Defined Under Namespace

Classes: FormatError

Extension API collapse

JPEG_SOF_BLOCKS =
[
  0xC0, 0xC1, 0xC2, 0xC3, 0xC5, 0xC6, 0xC7, 0xC9, 0xCA, 0xCB, 0xCD, 0xCE,
  0xCF
].freeze

Extension API collapse

Extension API collapse

Methods inherited from Image

#calc_image_dimensions

Constructor Details

#initialize(data) ⇒ JPG

Process a new JPG image

:data

A binary string of JPEG data


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# File 'lib/prawn/images/jpg.rb', line 39

def initialize(data)
  super()
  @data = data
  d = StringIO.new(@data)
  d.binmode

  c_marker = 0xff # Section marker.
  d.seek(2) # Skip the first two bytes of JPEG identifier.
  loop do
    marker, code, length = d.read(4).unpack('CCn')
    raise FormatError, 'JPEG marker not found!' if marker != c_marker

    if JPEG_SOF_BLOCKS.include?(code)
      @bits, @height, @width, @channels = d.read(6).unpack('CnnC')
      break
    end

    d.seek(length - 2, IO::SEEK_CUR)
  end
end

Instance Attribute Details

#bitsObject (readonly)

Returns the value of attribute bits


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

def bits
  @bits
end

#channelsObject (readonly)

Returns the value of attribute channels


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

def channels
  @channels
end

#heightObject (readonly)

Returns the value of attribute height


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

def height
  @height
end

#scaled_heightObject

Returns the value of attribute scaled_height


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

def scaled_height
  @scaled_height
end

#scaled_widthObject

Returns the value of attribute scaled_width


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

def scaled_width
  @scaled_width
end

#widthObject (readonly)

Returns the value of attribute width


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# File 'lib/prawn/images/jpg.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/jpg.rb', line 31

def self.can_render?(image_blob)
  image_blob[0, 3].unpack('C*') == [255, 216, 255]
end

Instance Method Details

#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/jpg.rb', line 63

def build_pdf_object(document)
  color_space =
    case channels
    when 1
      :DeviceGray
    when 3
      :DeviceRGB
    when 4
      :DeviceCMYK
    else
      raise ArgumentError, 'JPG uses an unsupported number of channels'
    end

  obj = document.ref!(
    Type: :XObject,
    Subtype: :Image,
    ColorSpace: color_space,
    BitsPerComponent: bits,
    Width: width,
    Height: height
  )

  # add extra decode params for CMYK images. By swapping the
  # min and max values from the default, we invert the colours. See
  # section 4.8.4 of the spec.
  if color_space == :DeviceCMYK
    obj.data[:Decode] = [1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0]
  end

  obj.stream << @data
  obj.stream.filters << :DCTDecode
  obj
end