Class: FormatParser::GIFParser

Inherits:
Object
  • Object
show all
Includes:
IOUtils
Defined in:
lib/parsers/gif_parser.rb

Constant Summary collapse

HEADERS = 
['GIF87a', 'GIF89a'].map(&:b)
NETSCAPE_AND_AUTHENTICATION_CODE = 
'NETSCAPE2.0'

Instance Method Summary collapse

Methods included from IOUtils

#safe_read, #safe_skip

Instance Method Details

#call(io) ⇒ Object 



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# File 'lib/parsers/gif_parser.rb', line 7

def call(io)
  io = FormatParser::IOConstraint.new(io)
  header = safe_read(io, 6)
  return unless HEADERS.include?(header)

  w, h = safe_read(io, 4).unpack('vv')
  gct_byte, _bgcolor_index, _pixel_aspect_ratio = safe_read(io, 5).unpack('Cvv')

  # and actually onwards for this:
  # http://www.matthewflickinger.com/lab/whatsinagif/bits_and_bytes.asp

  # Determine how big our color table is
  has_gct = gct_byte[0] == 1
  bytes_per_color = gct_byte >> 6
  unpacked_radix = gct_byte & 0b00000111
  num_colors = 2**(unpacked_radix + 1)
  gct_table_size = num_colors * bytes_per_color

  # If we have the global color table - skip over it
  safe_read(io, gct_table_size) if has_gct

  # Now it gets interesting - we are at the place where an
  # application extension for the NETSCAPE2.0 block will occur.
  # If it does, it most likely means the application that wrote the
  # GIF needed looping, and if it did, it means that the GIF is
  # very, very likely to be animated. To read the actual animation
  # we need to skip over actual image data frames, which, in case
  # of our paged reads, will incur
  potentially_netscape_app_header = safe_read(io, 64)
  is_animated = potentially_netscape_app_header.include?(NETSCAPE_AND_AUTHENTICATION_CODE)

  FormatParser::Image.new(
    format: :gif,
    width_px: w,
    height_px: h,
    has_multiple_frames: is_animated,
    color_mode: :indexed,
  )
end