Module: Tessel::PNG::Encoder

Defined in:

lib/tessel.rb

Class Method Summary

• .encode(image, color_type: :rgba, filter: :none, level: Zlib::DEFAULT_COMPRESSION, metadata: image.metadata) ⇒ Object

Class Method Details

.encode(image, color_type: :rgba, filter: :none, level: Zlib::DEFAULT_COMPRESSION, metadata: image.metadata) ⇒ Object

Raises:

• (TypeError)

# File 'lib/tessel.rb', line 365

def encode(image, color_type: :rgba, filter: :none, level: Zlib::DEFAULT_COMPRESSION, metadata: image.metadata)
  raise TypeError, "image must be a Tessel::Image" unless image.is_a?(Image)
  raise ArgumentError, "PNG dimensions must be positive" unless image.width.positive? && image.height.positive?
  type = { grayscale: 0, gray: 0, rgb: 2, palette: 3, gray_alpha: 4, rgba: 6 }.fetch(color_type.to_sym) { raise UnsupportedError, "unsupported PNG color type" }
  raise UnsupportedError, "palette PNG output needs an explicit palette" if type == 3

  raw = "".b
  previous = nil
  data = image.instance_variable_get(:@data)
  channels = { 0 => 1, 2 => 3, 4 => 2, 6 => 4 }.fetch(type)
  (0...image.height).each do |y|
    rgba = data.byteslice(y * image.width * 4, image.width * 4).bytes.each_slice(4)
    row = case type
    when 0 then rgba.map { |r, g, b, _a| (0.299 * r + 0.587 * g + 0.114 * b).round }.pack("C*")
    when 2 then rgba.flat_map { |r, g, b, _a| [r, g, b] }.pack("C*")
    when 4 then rgba.flat_map { |r, g, b, a| [(0.299 * r + 0.587 * g + 0.114 * b).round, a] }.pack("C*")
    else rgba.to_a.flatten.pack("C*")
    end
    selected = case filter.to_sym
    when :none then 0
    when :sub then 1
    when :up then 2
    when :average then 3
    when :paeth then 4
    when :adaptive
      (0..4).min_by { |kind| Filters.filter(kind, row, previous, channels).bytes.sum { |byte| byte < 128 ? byte : 256 - byte } }
    else raise ArgumentError, "unknown PNG filter: #{filter}"
    end
    raw << selected.chr << Filters.filter(selected, row, previous, channels)
    previous = row
  end

  output = SIGNATURE.dup
  Chunk.write(output, "IHDR", [image.width, image.height, 8, type, 0, 0, 0].pack("N2C5"))
  write_metadata(output, metadata)
  Chunk.write(output, "IDAT", Zlib::Deflate.deflate(raw, level))
  Chunk.write(output, "IEND", "")
  output
end