Class: Branding::PNG::Filter

Inherits:
Object
  • Object
show all
Defined in:
lib/branding/png.rb

Overview

private class to handle reconstructing filtered data www.w3.org/TR/PNG/#9Filters

Instance Method Summary collapse

Constructor Details

#initialize(filter_bit, filtered_scanline, previous_scanline, color_channels) ⇒ Filter

Returns a new instance of Filter.



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# File 'lib/branding/png.rb', line 66

def initialize(filter_bit, filtered_scanline, previous_scanline, color_channels)
  @type = filter_bit
  @filtered = filtered_scanline
  @previous = previous_scanline || []
  @pixel_width = color_channels
  @position = 0
end

Instance Method Details

#a ⇒ Object

a: the byte corresponding to x in the pixel immediately before the pixel containing x (or the byte immediately before x, when the bit depth is less than 8)



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# File 'lib/branding/png.rb', line 102

def a
  offset = @position - @pixel_width
  return 0x00 if offset < 0

  @reconstructed_scanline[offset]
end

#average(x) ⇒ Object 



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# File 'lib/branding/png.rb', line 132

def average(x)
  x + ((a + b) / 2.0).floor
end

#b ⇒ Object 



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# File 'lib/branding/png.rb', line 109

def b
  @previous[@position] || 0x00
end

#c ⇒ Object 



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# File 'lib/branding/png.rb', line 113

def c
  offset = @position - @pixel_width
  return 0x00 if offset < 0

  @previous[offset]
end

#none(x) ⇒ Object 



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# File 'lib/branding/png.rb', line 120

def none(x)
  x
end

#paeth(x) ⇒ Object

www.w3.org/TR/PNG/#9Filter-type-4-Paeth



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# File 'lib/branding/png.rb', line 137

def paeth(x)
  x + paeth_predictor(a, b, c)
end

#paeth_predictor(a, b, c) ⇒ Object 



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# File 'lib/branding/png.rb', line 141

def paeth_predictor(a, b, c)
  p = a + b - c
  pa = (p - a).abs
  pb = (p - b).abs
  pc = (p - c).abs

  return a if pa <= pb && pa <= pc

  return b if pb <= pc

  c
end

#reconstructed_scanline ⇒ Object

yields a reconstructed byte



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# File 'lib/branding/png.rb', line 76

def reconstructed_scanline
  @reconstructed_scanline = []
  @filtered.each do |byte|
    recon = case @type
    when 0
      none(byte)
    when 1
      sub(byte)
    when 2
      up(byte)
    when 3
      average(byte)
    when 4
      paeth(byte)
    end

    @reconstructed_scanline << (recon % 256)
    @position += 1
  end

  @reconstructed_scanline
end

#sub(x) ⇒ Object 



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# File 'lib/branding/png.rb', line 124

def sub(x)
  x + a
end

#up(x) ⇒ Object 



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# File 'lib/branding/png.rb', line 128

def up(x)
  x + b
end