GIF (Graphics Interchange Format) is an image file format, developed in 1987. It became popular in 1990s as one of the main image formats used in World Wide Web.
GIF format allows encoding of palette-based images up to 256 colors (each of the colors can be chosen from a 24-bit RGB colorspace). Image data stream uses LZW (Lempel-Ziv-Welch) lossless compression.
Over the years, several version of the format were published and several extensions to it were made, namely, a popular Netscape extension that allows to store several images in one file, switching between them, which produces crude form of animation.
Structurally, format consists of several mandatory headers and then a stream of blocks follows. Blocks can carry additional metainformation or image data.
This page hosts a formal specification of GIF (Graphics Interchange Format) image file using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.
The Python runtime library can be installed from PyPI:
python3 -m pip install kaitaistructParse a local file and get structure in memory:
data = Gif.from_file("path/to/local/file.gif")
Or parse structure from a bytes:
from kaitaistruct import KaitaiStream, BytesIO
raw = b"\x00\x01\x02..."
data = Gif(KaitaiStream(BytesIO(raw)))
After that, one can get various attributes from the structure by invoking getter methods like:
data.hdr # => get hdr
# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
from enum import Enum
if getattr(kaitaistruct, 'API_VERSION', (0, 9)) < (0, 9):
raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))
class Gif(KaitaiStruct):
"""GIF (Graphics Interchange Format) is an image file format, developed
in 1987. It became popular in 1990s as one of the main image formats
used in World Wide Web.
GIF format allows encoding of palette-based images up to 256 colors
(each of the colors can be chosen from a 24-bit RGB
colorspace). Image data stream uses LZW (Lempel-Ziv-Welch) lossless
compression.
Over the years, several version of the format were published and
several extensions to it were made, namely, a popular Netscape
extension that allows to store several images in one file, switching
between them, which produces crude form of animation.
Structurally, format consists of several mandatory headers and then
a stream of blocks follows. Blocks can carry additional
metainformation or image data.
"""
class BlockType(Enum):
extension = 33
local_image_descriptor = 44
end_of_file = 59
class ExtensionLabel(Enum):
graphic_control = 249
comment = 254
application = 255
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.hdr = Gif.Header(self._io, self, self._root)
self.logical_screen_descriptor = Gif.LogicalScreenDescriptorStruct(self._io, self, self._root)
if self.logical_screen_descriptor.has_color_table:
self._raw_global_color_table = self._io.read_bytes((self.logical_screen_descriptor.color_table_size * 3))
_io__raw_global_color_table = KaitaiStream(BytesIO(self._raw_global_color_table))
self.global_color_table = Gif.ColorTable(_io__raw_global_color_table, self, self._root)
self.blocks = []
i = 0
while True:
_ = Gif.Block(self._io, self, self._root)
self.blocks.append(_)
if ((self._io.is_eof()) or (_.block_type == Gif.BlockType.end_of_file)) :
break
i += 1
class ImageData(KaitaiStruct):
"""
.. seealso::
- section 22 - https://www.w3.org/Graphics/GIF/spec-gif89a.txt
"""
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.lzw_min_code_size = self._io.read_u1()
self.subblocks = Gif.Subblocks(self._io, self, self._root)
class ColorTableEntry(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.red = self._io.read_u1()
self.green = self._io.read_u1()
self.blue = self._io.read_u1()
class LogicalScreenDescriptorStruct(KaitaiStruct):
"""
.. seealso::
- section 18 - https://www.w3.org/Graphics/GIF/spec-gif89a.txt
"""
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.screen_width = self._io.read_u2le()
self.screen_height = self._io.read_u2le()
self.flags = self._io.read_u1()
self.bg_color_index = self._io.read_u1()
self.pixel_aspect_ratio = self._io.read_u1()
@property
def has_color_table(self):
if hasattr(self, '_m_has_color_table'):
return self._m_has_color_table
self._m_has_color_table = (self.flags & 128) != 0
return getattr(self, '_m_has_color_table', None)
@property
def color_table_size(self):
if hasattr(self, '_m_color_table_size'):
return self._m_color_table_size
self._m_color_table_size = (2 << (self.flags & 7))
return getattr(self, '_m_color_table_size', None)
class LocalImageDescriptor(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.left = self._io.read_u2le()
self.top = self._io.read_u2le()
self.width = self._io.read_u2le()
self.height = self._io.read_u2le()
self.flags = self._io.read_u1()
if self.has_color_table:
self._raw_local_color_table = self._io.read_bytes((self.color_table_size * 3))
_io__raw_local_color_table = KaitaiStream(BytesIO(self._raw_local_color_table))
self.local_color_table = Gif.ColorTable(_io__raw_local_color_table, self, self._root)
self.image_data = Gif.ImageData(self._io, self, self._root)
@property
def has_color_table(self):
if hasattr(self, '_m_has_color_table'):
return self._m_has_color_table
self._m_has_color_table = (self.flags & 128) != 0
return getattr(self, '_m_has_color_table', None)
@property
def has_interlace(self):
if hasattr(self, '_m_has_interlace'):
return self._m_has_interlace
self._m_has_interlace = (self.flags & 64) != 0
return getattr(self, '_m_has_interlace', None)
@property
def has_sorted_color_table(self):
if hasattr(self, '_m_has_sorted_color_table'):
return self._m_has_sorted_color_table
self._m_has_sorted_color_table = (self.flags & 32) != 0
return getattr(self, '_m_has_sorted_color_table', None)
@property
def color_table_size(self):
if hasattr(self, '_m_color_table_size'):
return self._m_color_table_size
self._m_color_table_size = (2 << (self.flags & 7))
return getattr(self, '_m_color_table_size', None)
class Block(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.block_type = KaitaiStream.resolve_enum(Gif.BlockType, self._io.read_u1())
_on = self.block_type
if _on == Gif.BlockType.extension:
self.body = Gif.Extension(self._io, self, self._root)
elif _on == Gif.BlockType.local_image_descriptor:
self.body = Gif.LocalImageDescriptor(self._io, self, self._root)
class ColorTable(KaitaiStruct):
"""
.. seealso::
- section 19 - https://www.w3.org/Graphics/GIF/spec-gif89a.txt
"""
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.entries = []
i = 0
while not self._io.is_eof():
self.entries.append(Gif.ColorTableEntry(self._io, self, self._root))
i += 1
class Header(KaitaiStruct):
"""
.. seealso::
- section 17 - https://www.w3.org/Graphics/GIF/spec-gif89a.txt
"""
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.magic = self._io.read_bytes(3)
if not self.magic == b"\x47\x49\x46":
raise kaitaistruct.ValidationNotEqualError(b"\x47\x49\x46", self.magic, self._io, u"/types/header/seq/0")
self.version = (self._io.read_bytes(3)).decode(u"ASCII")
class ExtGraphicControl(KaitaiStruct):
"""
.. seealso::
- section 23 - https://www.w3.org/Graphics/GIF/spec-gif89a.txt
"""
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.block_size = self._io.read_bytes(1)
if not self.block_size == b"\x04":
raise kaitaistruct.ValidationNotEqualError(b"\x04", self.block_size, self._io, u"/types/ext_graphic_control/seq/0")
self.flags = self._io.read_u1()
self.delay_time = self._io.read_u2le()
self.transparent_idx = self._io.read_u1()
self.terminator = self._io.read_bytes(1)
if not self.terminator == b"\x00":
raise kaitaistruct.ValidationNotEqualError(b"\x00", self.terminator, self._io, u"/types/ext_graphic_control/seq/4")
@property
def transparent_color_flag(self):
if hasattr(self, '_m_transparent_color_flag'):
return self._m_transparent_color_flag
self._m_transparent_color_flag = (self.flags & 1) != 0
return getattr(self, '_m_transparent_color_flag', None)
@property
def user_input_flag(self):
if hasattr(self, '_m_user_input_flag'):
return self._m_user_input_flag
self._m_user_input_flag = (self.flags & 2) != 0
return getattr(self, '_m_user_input_flag', None)
class Subblock(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.len_bytes = self._io.read_u1()
self.bytes = self._io.read_bytes(self.len_bytes)
class ApplicationId(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.len_bytes = self._io.read_u1()
if not self.len_bytes == 11:
raise kaitaistruct.ValidationNotEqualError(11, self.len_bytes, self._io, u"/types/application_id/seq/0")
self.application_identifier = (self._io.read_bytes(8)).decode(u"ASCII")
self.application_auth_code = self._io.read_bytes(3)
class ExtApplication(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.application_id = Gif.ApplicationId(self._io, self, self._root)
self.subblocks = []
i = 0
while True:
_ = Gif.Subblock(self._io, self, self._root)
self.subblocks.append(_)
if _.len_bytes == 0:
break
i += 1
class Subblocks(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.entries = []
i = 0
while True:
_ = Gif.Subblock(self._io, self, self._root)
self.entries.append(_)
if _.len_bytes == 0:
break
i += 1
class Extension(KaitaiStruct):
def __init__(self, _io, _parent=None, _root=None):
self._io = _io
self._parent = _parent
self._root = _root if _root else self
self._read()
def _read(self):
self.label = KaitaiStream.resolve_enum(Gif.ExtensionLabel, self._io.read_u1())
_on = self.label
if _on == Gif.ExtensionLabel.application:
self.body = Gif.ExtApplication(self._io, self, self._root)
elif _on == Gif.ExtensionLabel.comment:
self.body = Gif.Subblocks(self._io, self, self._root)
elif _on == Gif.ExtensionLabel.graphic_control:
self.body = Gif.ExtGraphicControl(self._io, self, self._root)
else:
self.body = Gif.Subblocks(self._io, self, self._root)