Class: ZPNG::Adam7Decoder

Inherits:
Object
  • Object
show all
Defined in:
lib/zpng/adam7_decoder.rb

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(width, height, bpp) ⇒ Adam7Decoder

en.wikipedia.org/wiki/Adam7_algorithm#Passes



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# File 'lib/zpng/adam7_decoder.rb', line 7

def initialize width, height, bpp
  @bpp = bpp
  raise "Invalid BPP #{@bpp.inspect}" if @bpp.to_i == 0
  @widths = [
    [(width/8.0).ceil]     * (height/8.0).ceil,     # pass1
    [((width-4)/8.0).ceil] * (height/8.0).ceil,     # pass2
    [(width/4.0).ceil]     * ((height-4)/8.0).ceil, # pass3
    [((width-2)/4.0).ceil] * (height/4.0).ceil,     # pass4
    [(width/2.0).ceil]     * ((height-2)/4.0).ceil, # pass5
    [((width-1)/2.0).ceil] * (height/2.0).ceil,     # pass6
    [width]                * ((height-1)/2.0).ceil  # pass7
  ].map{ |x| x == [0] ? [] : x }
  @scanlines_count = 0
  # two leading zeroes added specially for convert_coords() code readability
  @pass_starts = [0,0] + @widths.map(&:size).map{ |x| @scanlines_count+=x }
  @widths.flatten! # yahoo! :))
end

Instance Attribute Details

#bpp ⇒ Object

Returns the value of attribute bpp.



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# File 'lib/zpng/adam7_decoder.rb', line 3

def bpp
  @bpp
end

#scanlines_count ⇒ Object (readonly)

Returns the value of attribute scanlines_count.



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# File 'lib/zpng/adam7_decoder.rb', line 4

def scanlines_count
  @scanlines_count
end

Instance Method Details

#convert_coords(x, y) ⇒ Object

convert “flat” coords in scanline number & pos in scanline



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# File 'lib/zpng/adam7_decoder.rb', line 42

def convert_coords x,y
  # optimizing this into one switch/case statement gives
  # about 1-2% speed increase (ruby 1.9.3p286)

  if y%2 == 1
    # 7th pass: last height/2 full scanlines
    [x, y/2 + @pass_starts[7]]
  elsif x%2 == 1 && y%2 == 0
    # 6th pass
    [x/2, y/2 + @pass_starts[6]]
  elsif x%8 == 0 && y%8 == 0
    # 1st pass, starts at 0
    [x/8, y/8]
  elsif x%8 == 4 && y%8 == 0
    # 2nd pass
    [x/8, y/8 + @pass_starts[2]]
  elsif x%4 == 0 && y%8 == 4
    # 3rd pass
    [x/4, y/8 + @pass_starts[3]]
  elsif x%4 == 2 && y%4 == 0
    # 4th pass
    [x/4, y/4 + @pass_starts[4]]
  elsif x%2 == 0 && y%4 == 2
    # 5th pass
    [x/2, y/4 + @pass_starts[5]]
  else
    raise "invalid coords"
  end
end

#pass_start?(idx) ⇒ Boolean

is the specified scanline is a first scanline in the pass? When the image is interlaced, each pass of the interlace pattern is treated as an independent image for filtering purposes

Returns:

  • (Boolean) 


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# File 'lib/zpng/adam7_decoder.rb', line 75

def pass_start? idx
  @pass_starts.include?(idx)
end

#scanline_offset(idx) ⇒ Object

scanline offset in imagedata



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# File 'lib/zpng/adam7_decoder.rb', line 36

def scanline_offset idx
  #TODO: optimize
  (0...idx).map{ |x| scanline_size(x) }.inject(&:+) || 0
end

#scanline_size(idx) ⇒ Object

scanline size in bytes, INCLUDING leading filter byte



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# File 'lib/zpng/adam7_decoder.rb', line 31

def scanline_size idx
  (scanline_width(idx) * bpp / 8.0).ceil + 1
end

#scanline_width(idx) ⇒ Object

scanline width in pixels



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# File 'lib/zpng/adam7_decoder.rb', line 26

def scanline_width idx
  @widths[idx]
end