Module: PureJPEG::Quantization

Defined in:
lib/pure_jpeg/quantization.rb

Constant Summary collapse

LUMINANCE_BASE =

Standard luminance quantization table (JPEG Annex K, Table K.1)

[
  16, 11, 10, 16,  24,  40,  51,  61,
  12, 12, 14, 19,  26,  58,  60,  55,
  14, 13, 16, 24,  40,  57,  69,  56,
  14, 17, 22, 29,  51,  87,  80,  62,
  18, 22, 37, 56,  68, 109, 103,  77,
  24, 35, 55, 64,  81, 104, 113,  92,
  49, 64, 78, 87, 103, 121, 120, 101,
  72, 92, 95, 98, 112, 100, 103,  99
].freeze
CHROMINANCE_BASE =

Standard chrominance quantization table (JPEG Annex K, Table K.2)

[
  17, 18, 24, 47, 99, 99, 99, 99,
  18, 21, 26, 66, 99, 99, 99, 99,
  24, 26, 56, 99, 99, 99, 99, 99,
  47, 66, 99, 99, 99, 99, 99, 99,
  99, 99, 99, 99, 99, 99, 99, 99,
  99, 99, 99, 99, 99, 99, 99, 99,
  99, 99, 99, 99, 99, 99, 99, 99,
  99, 99, 99, 99, 99, 99, 99, 99
].freeze

Class Method Summary collapse

Class Method Details

.dequantize!(block, table, out) ⇒ Object

Dequantize: multiply each coefficient by its quantization table entry.



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# File 'lib/pure_jpeg/quantization.rb', line 57

def self.dequantize!(block, table, out)
  64.times { |i| out[i] = block[i] * table[i] }
  out
end

.quantize!(block, table, out) ⇒ Object

Quantize a 64-element DCT block into ‘out`. Uses integer rounding division (round-to-nearest) to match the behavior of Float division + round from the previous float DCT.



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# File 'lib/pure_jpeg/quantization.rb', line 42

def self.quantize!(block, table, out)
  i = 0
  while i < 64
    v = block[i]; t = table[i]
    out[i] = if v >= 0
               (v + (t >> 1)) / t
             else
               -((-v + (t >> 1)) / t)
             end
    i += 1
  end
  out
end

.scale_table(base, quality) ⇒ Object

Scale a base quantization table for a given quality (1-100).



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# File 'lib/pure_jpeg/quantization.rb', line 30

def self.scale_table(base, quality)
  quality = quality.clamp(1, 100)
  scale = quality < 50 ? 5000.0 / quality : 200.0 - 2.0 * quality

  base.map { |v|
    ((v * scale + 50.0) / 100.0).round.clamp(1, 255)
  }
end