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

Extension API collapse

Constructor Details

#initialize(data) ⇒ JPG

Process a new JPG image.

Parameters:

  • data (String) —

    A binary string of JPEG data.

Source Code
lib/prawn/images/jpg.rb, line 58
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def initialize(data)

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  super()

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  @data = data

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  d = StringIO.new(@data)

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  d.binmode

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  c_marker = 0xff # Section marker.

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  d.seek(2) # Skip the first two bytes of JPEG identifier.

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  loop do

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    marker, code, length = d.read(4).unpack('CCn')

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    raise FormatError, 'JPEG marker not found!' if marker != c_marker

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    if JPEG_SOF_BLOCKS.include?(code)

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      @bits, @height, @width, @channels = d.read(6).unpack('CnnC')

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      break

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    end

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

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  end

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end







  



  

    
Instance Attribute Details

    
      
      
      

  

  
    #bits  ⇒ Integer  (readonly)
  

  

  


  

    
Sample Precision in bits.



  





  


Returns:


    
  
    
  • 
      
      
        (Integer)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 27

  
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def bits

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  @bits

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end







    
      
      
      

  

  
    #channels  ⇒ Integer  (readonly)
  

  

  


  

    
Number of image components (channels).



  





  


Returns:


    
  
    
  • 
      
      
        (Integer)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 31

  
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def channels

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  @channels

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end







    
      
      
      

  

  
    #height  ⇒ Integer  (readonly)
  

  

  


  

    
Image height in pixels.



  





  


Returns:


    
  
    
  • 
      
      
        (Integer)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 23

  
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def height

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  @height

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end







    
      
      
      

  

  
    #scaled_height  ⇒ Number 
  

  

  


  

    
Scaled height of the image in PDF points.



  





  


Returns:


    
  
    
  • 
      
      
        (Number)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 39

  
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def scaled_height

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  @scaled_height

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end







    
      
      
      

  

  
    #scaled_width  ⇒ Number 
  

  

  


  

    
Scaled width of the image in PDF points.



  





  


Returns:


    
  
    
  • 
      
      
        (Number)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 35

  
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def scaled_width

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  @scaled_width

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end







    
      
      
      

  

  
    #width  ⇒ Integer  (readonly)
  

  

  


  

    
Image width in pixels.



  





  


Returns:


    
  
    
  • 
      
      
        (Integer)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 19

  
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def width

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  @width

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end







    
  



  

    
Class Method Details


    
      

  

  
    .can_render?(image_blob)  ⇒ Boolean 
  

  

  


  

    
Can this image handler process this image?



  





  

Parameters:


    
  
    
  • 
      
        image_blob
      
      
        (String)
      
      
      
    
    • 
  




Returns:


    
  
    
  • 
      
      
        (Boolean)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 51

  
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def self.can_render?(image_blob)

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  image_blob[0, 3].unpack('C*') == [255, 216, 255]

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end







    
  


  

    
Instance Method Details


    
      

  

  
    #build_pdf_object(document)  ⇒ PDF::Core::Reference 
  

  

  


  

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



  





  

Parameters:




Returns:


    
  
    
  • 
      
      
        (PDF::Core::Reference)
      
      
      
    
    • 
  







  Source Code
  
lib/prawn/images/jpg.rb, line 84

  
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def build_pdf_object(document)

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  color_space =

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    case channels

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    when 1

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      :DeviceGray

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    when 3

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      :DeviceRGB

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    when 4

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      :DeviceCMYK

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    else

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      raise ArgumentError, 'JPG uses an unsupported number of channels'

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    end

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  obj = document.ref!(

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    Type: :XObject,

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    Subtype: :Image,

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    ColorSpace: color_space,

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    BitsPerComponent: bits,

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    Width: width,

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    Height: height,

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  )

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  # add extra decode params for CMYK images. By swapping the

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  # min and max values from the default, we invert the colours. See

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  # section 4.8.4 of the spec.

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  if color_space == :DeviceCMYK

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    obj.data[:Decode] = [1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0]

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  end

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  obj.stream << @data

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  obj.stream.filters << :DCTDecode

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  obj

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end