Class: Pura::Gif::Encoder

Inherits:
Object
  • Object
show all
Defined in:
lib/pura/gif/encoder.rb

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(image, max_colors: 256) ⇒ Encoder

Returns a new instance of Encoder.



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# File 'lib/pura/gif/encoder.rb', line 12

def initialize(image, max_colors: 256)
  @image = image
  @max_colors = [max_colors, 256].min
  @max_colors = 2 if @max_colors < 2
end

Class Method Details

.encode(image, output_path, max_colors: 256) ⇒ Object 



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# File 'lib/pura/gif/encoder.rb', line 6

def self.encode(image, output_path, max_colors: 256)
  data = new(image, max_colors: max_colors).encode
  File.binwrite(output_path, data)
  data.bytesize
end

Instance Method Details

#encode ⇒ Object 



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# File 'lib/pura/gif/encoder.rb', line 18

def encode
  # Quantize RGB pixels to a palette of at most @max_colors
  palette, indices = quantize(@image.pixels, @image.width, @image.height, @max_colors)

  # Pad palette to next power of 2 (minimum 4 entries for min_code_size >= 2)
  palette_bits = 1
  palette_bits += 1 while (1 << palette_bits) < palette.length
  palette_bits = 2 if palette_bits < 2
  palette_size = 1 << palette_bits

  palette << [0, 0, 0] while palette.length < palette_size

  min_code_size = palette_bits

  # LZW compress the indices
  compressed = lzw_compress(indices, min_code_size)

  # Build the GIF binary
  out = String.new(encoding: Encoding::BINARY)

  # Header
  out << "GIF89a"

  # Logical Screen Descriptor
  out << [@image.width, @image.height].pack("v2")
  packed = 0x80 | ((palette_bits - 1) << 4) | (palette_bits - 1)
  out << packed.chr << "\x00".b << "\x00".b

  # Global Color Table
  palette.each do |r, g, b|
    out << r.chr << g.chr << b.chr
  end

  # Image Descriptor
  out << "\x2C".b
  out << [0, 0, @image.width, @image.height].pack("v4")
  out << "\x00".b # packed: no local color table, not interlaced

  # Image Data
  out << min_code_size.chr

  # Write compressed data as sub-blocks (max 255 bytes each)
  pos = 0
  while pos < compressed.bytesize
    chunk_size = [compressed.bytesize - pos, 255].min
    out << chunk_size.chr
    out << compressed.byteslice(pos, chunk_size)
    pos += chunk_size
  end
  out << "\x00".b # block terminator

  # Trailer
  out << "\x3B".b

  out
end