Class: PureJPEG::Huffman::Encoder

Inherits:
Object
  • Object
show all
Defined in:
lib/pure_jpeg/huffman/encoder.rb

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(dc_table, ac_table) ⇒ Encoder

Returns a new instance of Encoder.



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# File 'lib/pure_jpeg/huffman/encoder.rb', line 61

def initialize(dc_table, ac_table)
  @dc_table = dc_table
  @ac_table = ac_table
end

Class Method Details

.category(value) ⇒ Object

Return the Huffman category (bit length) for a value. Avoids Array allocation compared to the combined category_and_bits.



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# File 'lib/pure_jpeg/huffman/encoder.rb', line 8

def self.category(value)
  return 0 if value == 0
  v = value.abs
  cat = 0
  while v > 0
    cat += 1
    v >>= 1
  end
  cat
end

.each_ac_item(zigzag) {|0x00, 0| ... } ⇒ Object

Yields:

  • (0x00, 0) 


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# File 'lib/pure_jpeg/huffman/encoder.rb', line 24

def self.each_ac_item(zigzag)
  last_nonzero = 63
  last_nonzero -= 1 while last_nonzero > 0 && zigzag[last_nonzero] == 0

  if last_nonzero == 0
    yield 0x00, 0
    return
  end

  i = 1
  while i <= last_nonzero
    run = 0
    while i <= last_nonzero && zigzag[i] == 0
      run += 1
      i += 1
    end

    while run >= 16
      yield 0xF0, 0
      run -= 16
    end

    value = zigzag[i]
    cat = category(value)
    yield (run << 4) | cat, value
    i += 1
  end

  yield 0x00, 0 if last_nonzero < 63
end

.each_ac_symbol(zigzag) ⇒ Object 



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# File 'lib/pure_jpeg/huffman/encoder.rb', line 55

def self.each_ac_symbol(zigzag)
  each_ac_item(zigzag) do |symbol, _value|
    yield symbol
  end
end

.value_bits(value, cat) ⇒ Object

Return the extra bits to encode for a value with the given category.



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# File 'lib/pure_jpeg/huffman/encoder.rb', line 20

def self.value_bits(value, cat)
  value > 0 ? value : value + (1 << cat) - 1
end

Instance Method Details

#encode_block(zigzag, prev_dc, writer) ⇒ Object

Encode a single 8x8 block (in zigzag order, quantized). ‘prev_dc` is the DC value of the previous block (for DPCM). Writes encoded bits to `writer` (a BitWriter). Returns the current block’s DC value.



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# File 'lib/pure_jpeg/huffman/encoder.rb', line 70

def encode_block(zigzag, prev_dc, writer)
  dc = zigzag[0]
  diff = dc - prev_dc
  encode_dc(diff, writer)
  encode_ac(zigzag, writer)
  dc
end