Class: PureJPEG::Decoder

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

Overview

Baseline JPEG decoder.

Decodes baseline DCT (SOF0) JPEGs with 1 or 3 components, any chroma subsampling factor, and restart markers.

Use read for a convenient entry point.

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(data) ⇒ Decoder

Returns a new instance of Decoder.



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# File 'lib/pure_jpeg/decoder.rb', line 24

def initialize(data)
  @data = data
end

Class Method Details

.decode(path_or_data) ⇒ Image

Decode a JPEG from a file path or binary string.

Parameters:

  • path_or_data (String) —

    a file path or raw JPEG bytes

Returns:

  • (Image) —

    decoded image with pixel access



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# File 'lib/pure_jpeg/decoder.rb', line 15

def self.decode(path_or_data)
  data = if path_or_data.is_a?(String) && !path_or_data.start_with?("\xFF\xD8".b) && File.exist?(path_or_data)
           File.binread(path_or_data)
         else
           path_or_data.b
         end
  new(data).decode
end

Instance Method Details

#decode ⇒ Object 



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# File 'lib/pure_jpeg/decoder.rb', line 28

def decode
  jfif = JFIFReader.new(@data)
  @icc_profile = jfif.icc_profile
  validate_dimensions!(jfif.width, jfif.height)
  return decode_progressive(jfif) if jfif.progressive

  width = jfif.width
  height = jfif.height

  # Build Huffman decode tables
  dc_tables = {}
  ac_tables = {}
  jfif.huffman_tables.each do |(table_class, table_id), info|
    table = Huffman::DecodeTable.new(info[:bits], info[:values])
    if table_class == 0
      dc_tables[table_id] = table
    else
      ac_tables[table_id] = table
    end
  end

  # Map component IDs to their info
  comp_info = {}
  jfif.components.each { |c| comp_info[c.id] = c }

  # Determine max sampling factors
  max_h = jfif.components.map(&:h_sampling).max
  max_v = jfif.components.map(&:v_sampling).max

  # MCU dimensions in pixels
  mcu_px_w = max_h * 8
  mcu_px_h = max_v * 8
  mcus_x = (width + mcu_px_w - 1) / mcu_px_w
  mcus_y = (height + mcu_px_h - 1) / mcu_px_h

  # Allocate channel buffers (full padded size)
  padded_w = mcus_x * mcu_px_w
  padded_h = mcus_y * mcu_px_h
  channels = {}
  jfif.components.each do |c|
    ch_w = (padded_w * c.h_sampling) / max_h
    ch_h = (padded_h * c.v_sampling) / max_v
    channels[c.id] = { data: Array.new(ch_w * ch_h, 0), width: ch_w, height: ch_h }
  end

  # Decode scan data
  reader = BitReader.new(jfif.scan_data)
  prev_dc = Hash.new(0)
  restart_interval = jfif.restart_interval
  mcu_count = 0

  # Reusable buffers
  zigzag = Array.new(64, 0)
  raster = Array.new(64, 0)
  dequant = Array.new(64, 0)

  mcus_y.times do |mcu_row|
    mcus_x.times do |mcu_col|
      # Handle restart interval
      if restart_interval > 0 && mcu_count > 0 && (mcu_count % restart_interval) == 0
        reader.reset
        prev_dc.clear
      end

      jfif.scan_components.each do |sc|
        comp, dc_tab, ac_tab = resolve_scan_references!(sc, comp_info, dc_tables, ac_tables)
        qt = fetch_quant_table!(jfif, comp)
        ch = channels[comp.id]

        comp.v_sampling.times do |bv|
          comp.h_sampling.times do |bh|
            # Decode one 8x8 block
            decode_block(reader, dc_tab, ac_tab, prev_dc, sc.id, zigzag)

            # Inverse pipeline: unzigzag -> dequantize -> IDCT -> level shift
            Zigzag.unreorder!(zigzag, raster)
            Quantization.dequantize!(raster, qt, dequant)
            DCT.inverse!(dequant)

            # Write block into channel buffer
            bx = (mcu_col * comp.h_sampling + bh) * 8
            by = (mcu_row * comp.v_sampling + bv) * 8
            write_block(dequant, ch[:data], ch[:width], bx, by)
          end
        end
      end

      mcu_count += 1
    end
  end

  # Assemble pixels
  num_components = jfif.components.length
  if num_components == 1
    assemble_grayscale(width, height, channels, jfif.components[0])
  elsif num_components == 3
    assemble_color(width, height, channels, jfif.components, max_h, max_v)
  else
    raise DecodeError, "Unsupported number of components: #{num_components}"
  end
end