Class: Emfsvg::DibDecoder

Inherits:
Object
  • Object
show all
Defined in:
lib/emfsvg/dib_decoder.rb

Overview

Decodes Windows DIB (Device-Independent Bitmap) data to an intermediate representation suitable for re-encoding as PNG.

DIB layout (per MS-WMF 2.2.2.3 / MS-EMF 2.2.2):

BITMAPINFOHEADER (40+ bytes)
[optional colour table]
pixel array

Supported biCompression values:

BI_RGB (0)        — uncompressed
BI_RLE8 (1)       — run-length encoded, 8-bit
BI_RLE4 (2)       — run-length encoded, 4-bit
BI_BITFIELDS (3)  — uncompressed with custom masks
BI_JPEG (4)       — embedded JPEG (pass-through)
BI_PNG (5)        — embedded PNG (pass-through)

Output: a Result with width, height, colour_type (:rgb / :rgba / :gray), and a pixel array (row-major, RGBA).

Defined Under Namespace

Classes: Result

Class Method Summary collapse

Class Method Details

.decode(dib_bytes, bits_offset: nil, mono_palette: nil) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 36

def decode(dib_bytes, bits_offset: nil, mono_palette: nil)
  return nil if dib_bytes.nil? || dib_bytes.bytesize < 40

  header = parse_header(dib_bytes)
  return nil unless header

  case header[:compression]
  when 0, 3 then decode_bi_rgb(dib_bytes, header, bits_offset, mono_palette)
  when 4 then Result.new(width: header[:width], height: header[:height].abs,
                         color_type: :jpeg, pixels: dib_bytes[header[:header_size]..])
  when 5 then Result.new(width: header[:width], height: header[:height].abs,
                         color_type: :png, pixels: dib_bytes[header[:header_size]..])
  end
end

.decode_bi_bitfields(bytes, header, bits_offset) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 242

def decode_bi_bitfields(bytes, header, bits_offset)
  # Bit-fields uses 3 (or 4) uint32 masks after the header for the
  # R/G/B (and optionally A) channel positions.
  width = header[:width]
  height = header[:height].abs
  bit_count = header[:bit_count]
  return nil unless [16, 32].include?(bit_count)

  mask_offset = header[:header_size]
  r_mask = read_uint32(bytes, mask_offset)
  g_mask = read_uint32(bytes, mask_offset + 4)
  b_mask = read_uint32(bytes, mask_offset + 8)
  a_mask = read_uint32(bytes, mask_offset + 12) if header[:header_size] >= 56

  mask_bytes = header[:header_size] >= 56 ? 16 : 12
  pixel_offset = bits_offset || (mask_offset + mask_bytes)
  row_stride = (((bit_count * width) + 31) / 32) * 4
  pixels = Array.new(width * height * 4, 0)

  (0...height).each do |row|
    src_row = header[:height].negative? ? row : (height - 1 - row)
    row_start = pixel_offset + (src_row * row_stride)
    (0...width).each do |x|
      value = read_uint32(bytes, row_start + (x * (bit_count / 8)))
      r = scale_masked(value, r_mask)
      g = scale_masked(value, g_mask)
      b = scale_masked(value, b_mask)
      a = a_mask ? scale_masked(value, a_mask) : 255
      out_idx = ((row * width) + x) * 4
      pixels[out_idx] = r
      pixels[out_idx + 1] = g
      pixels[out_idx + 2] = b
      pixels[out_idx + 3] = a
    end
  end

  Result.new(width: width, height: height, color_type: :rgba,
             pixels: pixels.pack("C*"))
end

.decode_bi_rgb(bytes, header, bits_offset, mono_palette = nil) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 94

def decode_bi_rgb(bytes, header, bits_offset, mono_palette = nil)
  width = header[:width]
  height = header[:height].abs
  bit_count = header[:bit_count]
  top_down = header[:height].negative?

  pal_start = header[:header_size]
  pal_entries = real_palette_size(header)
  # For 1-bpp MONOCHROME brushes libemf2svg overrides the palette with
  # the device context's text/bk colors. Caller can supply that via
  # mono_palette; otherwise read the palette from the BMI.
  palette = mono_palette || (0...pal_entries).map do |i|
    b = safe_getbyte(bytes, pal_start + (i * 4))
    g = safe_getbyte(bytes, pal_start + (i * 4) + 1)
    r = safe_getbyte(bytes, pal_start + (i * 4) + 2)
    a = safe_getbyte(bytes, pal_start + (i * 4) + 3)
    [r, g, b, a]
  end

  # When the caller supplies bits_offset (== cb_bmi from the EMF
  # record), use it — the BMI may contain trailing padding between
  # the palette and the bitmap bits. Otherwise assume the compact
  # layout: pixels start immediately after the palette.
  pixel_offset = bits_offset || (pal_start + (pal_entries * 4))
  row_stride = (((bit_count * width) + 31) / 32) * 4
  pixels = Array.new(width * height * 4, 0)

  (0...height).each do |row|
    src_row = top_down ? row : (height - 1 - row)
    row_start = pixel_offset + (src_row * row_stride)
    case bit_count
    when 24
      decode_rgb24_row(bytes, row_start, width, pixels, row * width * 4)
    when 32
      decode_rgb32_row(bytes, row_start, width, pixels, row * width * 4)
    when 16
      decode_rgb16_row(bytes, row_start, width, pixels, row * width * 4)
    when 8
      decode_indexed_row(bytes, row_start, width, 1, palette, pixels, row * width * 4)
    when 4
      decode_indexed_row(bytes, row_start, width, 2, palette, pixels, row * width * 4)
    when 1
      decode_indexed_row(bytes, row_start, width, 8, palette, pixels, row * width * 4)
    else
      return nil
    end
  end

  # libemf2svg's rgb2png checks whether ALL alpha bytes are zero
  # across the entire bitmap; if so, it forces every alpha to 0xFF
  # (treating the source as opaque). If any alpha is non-zero, it
  # uses the actual alpha values as-is. We must replicate this
  # global check (NOT a per-pixel "alpha==0 ? 255 : alpha" swap)
  # to produce byte-identical PNG output. Applies to all bit depths
  # — 16-bpp always has alpha=0 so this forces it opaque.
  alpha_all_zero = pixels.each_slice(4).all? { |_, _, _, a| a.zero? }
  (3...pixels.size).step(4).each { |i| pixels[i] = 255 } if alpha_all_zero

  Result.new(width: width, height: height, color_type: :rgba,
             pixels: pixels.pack("C*"))
end

.decode_indexed_row(bytes, row_start, width, pixels_per_byte, palette, pixels, out_start) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 224

def decode_indexed_row(bytes, row_start, width, pixels_per_byte, palette, pixels, out_start)
  (0...width).each do |x|
    if pixels_per_byte == 8
      byte = safe_getbyte(bytes, row_start + (x / 8).floor)
      shift = 7 - (x % 8)
      idx = (byte >> shift) & 0x01
    elsif pixels_per_byte == 2
      byte = safe_getbyte(bytes, row_start + (x / 2).floor)
      shift = (x % 2).zero? ? 4 : 0
      idx = (byte >> shift) & 0x0F
    else
      idx = safe_getbyte(bytes, row_start + x)
    end
    entry = palette[idx] || [0, 0, 0, 255]
    pixels[out_start + (x * 4), 4] = entry
  end
end

.decode_rgb16_row(bytes, row_start, width, pixels, out_start) ⇒ Object

16-bpp BI_RGB is always 5:5:5 (top bit unused) per GDI semantics. Mirrors libuemf's DIB_to_RGBA U_BCBM_COLOR16 case: each pixel is little-endian 2 bytes; b = low5<<3, g = (high3 of low + low2 of high)<<3, r = high5<<1 (libuemf shifts to top 5 then <<1 instead of <<3, but both produce 8-bit values that round to the same quantised levels).



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# File 'lib/emfsvg/dib_decoder.rb', line 209

def decode_rgb16_row(bytes, row_start, width, pixels, out_start)
  (0...width).each do |x|
    lo = safe_getbyte(bytes, row_start + (x * 2))
    hi = safe_getbyte(bytes, row_start + (x * 2) + 1)
    b = (lo & 0x1F) << 3
    g = (lo >> 5) | ((hi & 0x03) << 3)
    g <<= 3
    r = (hi & 0x7C) << 1
    pixels[out_start + (x * 4)] = r
    pixels[out_start + (x * 4) + 1] = g
    pixels[out_start + (x * 4) + 2] = b
    pixels[out_start + (x * 4) + 3] = 0
  end
end

.decode_rgb24_row(bytes, row_start, width, pixels, out_start) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 179

def decode_rgb24_row(bytes, row_start, width, pixels, out_start)
  (0...width).each do |x|
    b = safe_getbyte(bytes, row_start + (x * 3))
    g = safe_getbyte(bytes, row_start + (x * 3) + 1)
    r = safe_getbyte(bytes, row_start + (x * 3) + 2)
    pixels[out_start + (x * 4)] = r
    pixels[out_start + (x * 4) + 1] = g
    pixels[out_start + (x * 4) + 2] = b
    pixels[out_start + (x * 4) + 3] = 255
  end
end

.decode_rgb32_row(bytes, row_start, width, pixels, out_start) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 191

def decode_rgb32_row(bytes, row_start, width, pixels, out_start)
  (0...width).each do |x|
    b = safe_getbyte(bytes, row_start + (x * 4))
    g = safe_getbyte(bytes, row_start + (x * 4) + 1)
    r = safe_getbyte(bytes, row_start + (x * 4) + 2)
    a = safe_getbyte(bytes, row_start + (x * 4) + 3)
    pixels[out_start + (x * 4)] = r
    pixels[out_start + (x * 4) + 1] = g
    pixels[out_start + (x * 4) + 2] = b
    pixels[out_start + (x * 4) + 3] = a
  end
end

.palette_size(header) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 156

def palette_size(header)
  case header[:bit_count]
  when 1, 4, 8
    count = header[:clr_used]
    count.positive? ? count : (1 << header[:bit_count])
  else
    0
  end
end

.parse_header(bytes) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 51

def parse_header(bytes)
  {
    header_size: read_uint32(bytes, 0),
    width: read_int32(bytes, 4),
    height: read_int32(bytes, 8),
    planes: read_uint16(bytes, 12),
    bit_count: read_uint16(bytes, 14),
    compression: read_uint32(bytes, 16),
    image_size: read_uint32(bytes, 20),
    x_ppm: read_int32(bytes, 24),
    y_ppm: read_int32(bytes, 28),
    clr_used: read_uint32(bytes, 32),
    clr_important: read_uint32(bytes, 36)
  }
end

.read_int32(bytes, offset) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 83

def read_int32(bytes, offset)
  v = read_uint32(bytes, offset)
  v >= 0x8000_0000 ? v - 0x1_0000_0000 : v
end

.read_uint16(bytes, offset) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 88

def read_uint16(bytes, offset)
  return 0 if bytes.nil? || offset + 2 > bytes.bytesize

  safe_getbyte(bytes, offset) | (bytes.getbyte(offset + 1) << 8)
end

.read_uint32(bytes, offset) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 74

def read_uint32(bytes, offset)
  return 0 if bytes.nil? || offset + 4 > bytes.bytesize

  safe_getbyte(bytes, offset) |
    (bytes.getbyte(offset + 1) << 8) |
    (bytes.getbyte(offset + 2) << 16) |
    (bytes.getbyte(offset + 3) << 24)
end

.real_palette_size(header) ⇒ Object

libuemf's get_real_color_icount: if ClrUsed is 0, use 1<<BitCount, but cap at width*height (so a 10x10 8-bpp image gets a 100-entry palette, not 256). This prevents reading past the actual palette data when the BMI's palette is sized for the image, not the bit depth.



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# File 'lib/emfsvg/dib_decoder.rb', line 171

def real_palette_size(header)
  nominal = palette_size(header)
  return nominal if nominal.zero?

  area = header[:width].abs * header[:height].abs
  nominal > area ? area : nominal
end

.safe_getbyte(bytes, offset) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 67

def safe_getbyte(bytes, offset)
  return 0 if bytes.nil?

  b = bytes.getbyte(offset)
  b.nil? ? 0 : b
end

.scale_masked(value, mask) ⇒ Object 



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# File 'lib/emfsvg/dib_decoder.rb', line 282

def scale_masked(value, mask)
  return 0 if mask.zero?

  # Find lowest set bit and width of mask
  shift = 0
  tmp = mask
  while tmp.nobits?(1) && tmp != 0
    shift += 1
    tmp >>= 1
  end
  width = 0
  while tmp.allbits?(1)
    width += 1
    tmp >>= 1
  end
  scaled = ((value & mask) >> shift).to_f / ((1 << width) - 1) * 255
  scaled.round.clamp(0, 255)
end