Sha256: c30e6ef17a9f2e65463dea0f74039cb102a371292f25d7616b115d6842c7c6c6
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Size: 1.73 KB
Versions: 22
Compression:
Stored size: 1.73 KB
Contents
class FormatParser::GIFParser
include FormatParser::IOUtils
HEADERS = ['GIF87a', 'GIF89a'].map(&:b)
NETSCAPE_AND_AUTHENTICATION_CODE = 'NETSCAPE2.0'
def likely_match?(filename)
filename =~ /\.gif$/i
end
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
FormatParser.register_parser new, natures: :image, formats: :gif, priority: 0
end
Version data entries
22 entries across 22 versions & 1 rubygems