GifImagePlugin.py

#
# The Python Imaging Library.
# $Id$
#
# GIF file handling
#
# History:
# 1995-09-01 fl   Created
# 1996-12-14 fl   Added interlace support
# 1996-12-30 fl   Added animation support
# 1997-01-05 fl   Added write support, fixed local colour map bug
# 1997-02-23 fl   Make sure to load raster data in getdata()
# 1997-07-05 fl   Support external decoder (0.4)
# 1998-07-09 fl   Handle all modes when saving (0.5)
# 1998-07-15 fl   Renamed offset attribute to avoid name clash
# 2001-04-16 fl   Added rewind support (seek to frame 0) (0.6)
# 2001-04-17 fl   Added palette optimization (0.7)
# 2002-06-06 fl   Added transparency support for save (0.8)
# 2004-02-24 fl   Disable interlacing for small images
#
# Copyright (c) 1997-2004 by Secret Labs AB
# Copyright (c) 1995-2004 by Fredrik Lundh
#
# See the README file for information on usage and redistribution.
#

import itertools
import os
import subprocess

from . import Image, ImageChops, ImageFile, ImagePalette, ImageSequence
from ._binary import i8, i16le as i16, o8, o16le as o16

# --------------------------------------------------------------------
# Identify/read GIF files


def _accept(prefix):
    return prefix[:6] in [b"GIF87a", b"GIF89a"]


##
# Image plugin for GIF images.  This plugin supports both GIF87 and
# GIF89 images.


class GifImageFile(ImageFile.ImageFile):

    format = "GIF"
    format_description = "Compuserve GIF"
    _close_exclusive_fp_after_loading = False

    global_palette = None

    def data(self):
        s = self.fp.read(1)
        if s and i8(s):
            return self.fp.read(i8(s))
        return None

    def _open(self):

        # Screen
        s = self.fp.read(13)
        if s[:6] not in [b"GIF87a", b"GIF89a"]:
            raise SyntaxError("not a GIF file")

        self.info["version"] = s[:6]
        self._size = i16(s[6:]), i16(s[8:])
        self.tile = []
        flags = i8(s[10])
        bits = (flags & 7) + 1

        if flags & 128:
            # get global palette
            self.info["background"] = i8(s[11])
            # check if palette contains colour indices
            p = self.fp.read(3 << bits)
            for i in range(0, len(p), 3):
                if not (i // 3 == i8(p[i]) == i8(p[i + 1]) == i8(p[i + 2])):
                    p = ImagePalette.raw("RGB", p)
                    self.global_palette = self.palette = p
                    break

        self.__fp = self.fp  # FIXME: hack
        self.__rewind = self.fp.tell()
        self._n_frames = None
        self._is_animated = None
        self._seek(0)  # get ready to read first frame

    @property
    def n_frames(self):
        if self._n_frames is None:
            current = self.tell()
            try:
                while True:
                    self.seek(self.tell() + 1)
            except EOFError:
                self._n_frames = self.tell() + 1
            self.seek(current)
        return self._n_frames

    @property
    def is_animated(self):
        if self._is_animated is None:
            if self._n_frames is not None:
                self._is_animated = self._n_frames != 1
            else:
                current = self.tell()

                try:
                    self.seek(1)
                    self._is_animated = True
                except EOFError:
                    self._is_animated = False

                self.seek(current)
        return self._is_animated

    def seek(self, frame):
        if not self._seek_check(frame):
            return
        if frame < self.__frame:
            if frame != 0:
                self.im = None
            self._seek(0)

        last_frame = self.__frame
        for f in range(self.__frame + 1, frame + 1):
            try:
                self._seek(f)
            except EOFError:
                self.seek(last_frame)
                raise EOFError("no more images in GIF file")

    def _seek(self, frame):

        if frame == 0:
            # rewind
            self.__offset = 0
            self.dispose = None
            self.dispose_extent = [0, 0, 0, 0]  # x0, y0, x1, y1
            self.__frame = -1
            self.__fp.seek(self.__rewind)
            self._prev_im = None
            self.disposal_method = 0
        else:
            # ensure that the previous frame was loaded
            if not self.im:
                self.load()

        if frame != self.__frame + 1:
            raise ValueError("cannot seek to frame %d" % frame)
        self.__frame = frame

        self.tile = []

        self.fp = self.__fp
        if self.__offset:
            # backup to last frame
            self.fp.seek(self.__offset)
            while self.data():
                pass
            self.__offset = 0

        if self.dispose:
            self.im.paste(self.dispose, self.dispose_extent)

        from copy import copy

        self.palette = copy(self.global_palette)

        info = {}
        while True:

            s = self.fp.read(1)
            if not s or s == b";":
                break

            elif s == b"!":
                #
                # extensions
                #
                s = self.fp.read(1)
                block = self.data()
                if i8(s) == 249:
                    #
                    # graphic control extension
                    #
                    flags = i8(block[0])
                    if flags & 1:
                        info["transparency"] = i8(block[3])
                    info["duration"] = i16(block[1:3]) * 10

                    # disposal method - find the value of bits 4 - 6
                    dispose_bits = 0b00011100 & flags
                    dispose_bits = dispose_bits >> 2
                    if dispose_bits:
                        # only set the dispose if it is not
                        # unspecified. I'm not sure if this is
                        # correct, but it seems to prevent the last
                        # frame from looking odd for some animations
                        self.disposal_method = dispose_bits
                elif i8(s) == 254:
                    #
                    # comment extension
                    #
                    while block:
                        if "comment" in info:
                            info["comment"] += block
                        else:
                            info["comment"] = block
                        block = self.data()
                    continue
                elif i8(s) == 255:
                    #
                    # application extension
                    #
                    info["extension"] = block, self.fp.tell()
                    if block[:11] == b"NETSCAPE2.0":
                        block = self.data()
                        if len(block) >= 3 and i8(block[0]) == 1:
                            info["loop"] = i16(block[1:3])
                while self.data():
                    pass

            elif s == b",":
                #
                # local image
                #
                s = self.fp.read(9)

                # extent
                x0, y0 = i16(s[0:]), i16(s[2:])
                x1, y1 = x0 + i16(s[4:]), y0 + i16(s[6:])
                if x1 > self.size[0] or y1 > self.size[1]:
                    self._size = max(x1, self.size[0]), max(y1, self.size[1])
                self.dispose_extent = x0, y0, x1, y1
                flags = i8(s[8])

                interlace = (flags & 64) != 0

                if flags & 128:
                    bits = (flags & 7) + 1
                    self.palette = ImagePalette.raw("RGB", self.fp.read(3 << bits))

                # image data
                bits = i8(self.fp.read(1))
                self.__offset = self.fp.tell()
                self.tile = [
                    ("gif", (x0, y0, x1, y1), self.__offset, (bits, interlace))
                ]
                break

            else:
                pass
                # raise IOError, "illegal GIF tag `%x`" % i8(s)

        try:
            if self.disposal_method < 2:
                # do not dispose or none specified
                self.dispose = None
            elif self.disposal_method == 2:
                # replace with background colour
                Image._decompression_bomb_check(self.size)
                self.dispose = Image.core.fill("P", self.size, self.info["background"])
            else:
                # replace with previous contents
                if self.im:
                    self.dispose = self.im.copy()

            # only dispose the extent in this frame
            if self.dispose:
                self.dispose = self._crop(self.dispose, self.dispose_extent)
        except (AttributeError, KeyError):
            pass

        if not self.tile:
            # self.__fp = None
            raise EOFError

        for k in ["transparency", "duration", "comment", "extension", "loop"]:
            if k in info:
                self.info[k] = info[k]
            elif k in self.info:
                del self.info[k]

        self.mode = "L"
        if self.palette:
            self.mode = "P"

    def tell(self):
        return self.__frame

    def load_end(self):
        ImageFile.ImageFile.load_end(self)

        # if the disposal method is 'do not dispose', transparent
        # pixels should show the content of the previous frame
        if self._prev_im and self.disposal_method == 1:
            # we do this by pasting the updated area onto the previous
            # frame which we then use as the current image content
            updated = self._crop(self.im, self.dispose_extent)
            self._prev_im.paste(updated, self.dispose_extent, updated.convert("RGBA"))
            self.im = self._prev_im
        self._prev_im = self.im.copy()

    def _close__fp(self):
        try:
            if self.__fp != self.fp:
                self.__fp.close()
        except AttributeError:
            pass
        finally:
            self.__fp = None


# --------------------------------------------------------------------
# Write GIF files


RAWMODE = {"1": "L", "L": "L", "P": "P"}


def _normalize_mode(im, initial_call=False):
    """
    Takes an image (or frame), returns an image in a mode that is appropriate
    for saving in a Gif.

    It may return the original image, or it may return an image converted to
    palette or 'L' mode.

    UNDONE: What is the point of mucking with the initial call palette, for
    an image that shouldn't have a palette, or it would be a mode 'P' and
    get returned in the RAWMODE clause.

    :param im: Image object
    :param initial_call: Default false, set to true for a single frame.
    :returns: Image object
    """
    if im.mode in RAWMODE:
        im.load()
        return im
    if Image.getmodebase(im.mode) == "RGB":
        if initial_call:
            palette_size = 256
            if im.palette:
                palette_size = len(im.palette.getdata()[1]) // 3
            return im.convert("P", palette=Image.ADAPTIVE, colors=palette_size)
        else:
            return im.convert("P")
    return im.convert("L")


def _normalize_palette(im, palette, info):
    """
    Normalizes the palette for image.
      - Sets the palette to the incoming palette, if provided.
      - Ensures that there's a palette for L mode images
      - Optimizes the palette if necessary/desired.

    :param im: Image object
    :param palette: bytes object containing the source palette, or ....
    :param info: encoderinfo
    :returns: Image object
    """
    source_palette = None
    if palette:
        # a bytes palette
        if isinstance(palette, (bytes, bytearray, list)):
            source_palette = bytearray(palette[:768])
        if isinstance(palette, ImagePalette.ImagePalette):
            source_palette = bytearray(
                itertools.chain.from_iterable(
                    zip(
                        palette.palette[:256],
                        palette.palette[256:512],
                        palette.palette[512:768],
                    )
                )
            )

    if im.mode == "P":
        if not source_palette:
            source_palette = im.im.getpalette("RGB")[:768]
    else:  # L-mode
        if not source_palette:
            source_palette = bytearray(i // 3 for i in range(768))
        im.palette = ImagePalette.ImagePalette("RGB", palette=source_palette)

    used_palette_colors = _get_optimize(im, info)
    if used_palette_colors is not None:
        return im.remap_palette(used_palette_colors, source_palette)

    im.palette.palette = source_palette
    return im


def _write_single_frame(im, fp, palette):
    im_out = _normalize_mode(im, True)
    for k, v in im_out.info.items():
        im.encoderinfo.setdefault(k, v)
    im_out = _normalize_palette(im_out, palette, im.encoderinfo)

    for s in _get_global_header(im_out, im.encoderinfo):
        fp.write(s)

    # local image header
    flags = 0
    if get_interlace(im):
        flags = flags | 64
    _write_local_header(fp, im, (0, 0), flags)

    im_out.encoderconfig = (8, get_interlace(im))
    ImageFile._save(im_out, fp, [("gif", (0, 0) + im.size, 0, RAWMODE[im_out.mode])])

    fp.write(b"\0")  # end of image data


def _write_multiple_frames(im, fp, palette):

    duration = im.encoderinfo.get("duration", im.info.get("duration"))
    disposal = im.encoderinfo.get("disposal", im.info.get("disposal"))

    im_frames = []
    frame_count = 0
    background_im = None
    for imSequence in itertools.chain([im], im.encoderinfo.get("append_images", [])):
        for im_frame in ImageSequence.Iterator(imSequence):
            # a copy is required here since seek can still mutate the image
            im_frame = _normalize_mode(im_frame.copy())
            if frame_count == 0:
                for k, v in im_frame.info.items():
                    im.encoderinfo.setdefault(k, v)
            im_frame = _normalize_palette(im_frame, palette, im.encoderinfo)

            encoderinfo = im.encoderinfo.copy()
            if isinstance(duration, (list, tuple)):
                encoderinfo["duration"] = duration[frame_count]
            if isinstance(disposal, (list, tuple)):
                encoderinfo["disposal"] = disposal[frame_count]
            frame_count += 1

            if im_frames:
                # delta frame
                previous = im_frames[-1]
                if encoderinfo.get("disposal") == 2:
                    if background_im is None:
                        background = _get_background(
                            im,
                            im.encoderinfo.get("background", im.info.get("background")),
                        )
                        background_im = Image.new("P", im_frame.size, background)
                        background_im.putpalette(im_frames[0]["im"].palette)
                    base_im = background_im
                else:
                    base_im = previous["im"]
                if _get_palette_bytes(im_frame) == _get_palette_bytes(base_im):
                    delta = ImageChops.subtract_modulo(im_frame, base_im)
                else:
                    delta = ImageChops.subtract_modulo(
                        im_frame.convert("RGB"), base_im.convert("RGB")
                    )
                bbox = delta.getbbox()
                if not bbox:
                    # This frame is identical to the previous frame
                    if duration:
                        previous["encoderinfo"]["duration"] += encoderinfo["duration"]
                    continue
            else:
                bbox = None
            im_frames.append({"im": im_frame, "bbox": bbox, "encoderinfo": encoderinfo})

    if len(im_frames) > 1:
        for frame_data in im_frames:
            im_frame = frame_data["im"]
            if not frame_data["bbox"]:
                # global header
                for s in _get_global_header(im_frame, frame_data["encoderinfo"]):
                    fp.write(s)
                offset = (0, 0)
            else:
                # compress difference
                frame_data["encoderinfo"]["include_color_table"] = True

                im_frame = im_frame.crop(frame_data["bbox"])
                offset = frame_data["bbox"][:2]
            _write_frame_data(fp, im_frame, offset, frame_data["encoderinfo"])
        return True
    elif "duration" in im.encoderinfo and isinstance(
        im.encoderinfo["duration"], (list, tuple)
    ):
        # Since multiple frames will not be written, add together the frame durations
        im.encoderinfo["duration"] = sum(im.encoderinfo["duration"])


def _save_all(im, fp, filename):
    _save(im, fp, filename, save_all=True)


def _save(im, fp, filename, save_all=False):
    # header
    if "palette" in im.encoderinfo or "palette" in im.info:
        palette = im.encoderinfo.get("palette", im.info.get("palette"))
    else:
        palette = None
        im.encoderinfo["optimize"] = im.encoderinfo.get("optimize", True)

    if not save_all or not _write_multiple_frames(im, fp, palette):
        _write_single_frame(im, fp, palette)

    fp.write(b";")  # end of file

    if hasattr(fp, "flush"):
        fp.flush()


def get_interlace(im):
    interlace = im.encoderinfo.get("interlace", 1)

    # workaround for @PIL153
    if min(im.size) < 16:
        interlace = 0

    return interlace


def _write_local_header(fp, im, offset, flags):
    transparent_color_exists = False
    try:
        transparency = im.encoderinfo["transparency"]
    except KeyError:
        pass
    else:
        transparency = int(transparency)
        # optimize the block away if transparent color is not used
        transparent_color_exists = True

        used_palette_colors = _get_optimize(im, im.encoderinfo)
        if used_palette_colors is not None:
            # adjust the transparency index after optimize
            try:
                transparency = used_palette_colors.index(transparency)
            except ValueError:
                transparent_color_exists = False

    if "duration" in im.encoderinfo:
        duration = int(im.encoderinfo["duration"] / 10)
    else:
        duration = 0

    disposal = int(im.encoderinfo.get("disposal", 0))

    if transparent_color_exists or duration != 0 or disposal:
        packed_flag = 1 if transparent_color_exists else 0
        packed_flag |= disposal << 2
        if not transparent_color_exists:
            transparency = 0

        fp.write(
            b"!"
            + o8(249)  # extension intro
            + o8(4)  # length
            + o8(packed_flag)  # packed fields
            + o16(duration)  # duration
            + o8(transparency)  # transparency index
            + o8(0)
        )

    if "comment" in im.encoderinfo and 1 <= len(im.encoderinfo["comment"]):
        fp.write(b"!" + o8(254))  # extension intro
        comment = im.encoderinfo["comment"]
        if isinstance(comment, str):
            comment = comment.encode()
        for i in range(0, len(comment), 255):
            subblock = comment[i : i + 255]
            fp.write(o8(len(subblock)) + subblock)
        fp.write(o8(0))
    if "loop" in im.encoderinfo:
        number_of_loops = im.encoderinfo["loop"]
        fp.write(
            b"!"
            + o8(255)  # extension intro
            + o8(11)
            + b"NETSCAPE2.0"
            + o8(3)
            + o8(1)
            + o16(number_of_loops)  # number of loops
            + o8(0)
        )
    include_color_table = im.encoderinfo.get("include_color_table")
    if include_color_table:
        palette_bytes = _get_palette_bytes(im)
        color_table_size = _get_color_table_size(palette_bytes)
        if color_table_size:
            flags = flags | 128  # local color table flag
            flags = flags | color_table_size

    fp.write(
        b","
        + o16(offset[0])  # offset
        + o16(offset[1])
        + o16(im.size[0])  # size
        + o16(im.size[1])
        + o8(flags)  # flags
    )
    if include_color_table and color_table_size:
        fp.write(_get_header_palette(palette_bytes))
    fp.write(o8(8))  # bits


def _save_netpbm(im, fp, filename):

    # Unused by default.
    # To use, uncomment the register_save call at the end of the file.
    #
    # If you need real GIF compression and/or RGB quantization, you
    # can use the external NETPBM/PBMPLUS utilities.  See comments
    # below for information on how to enable this.
    tempfile = im._dump()

    try:
        with open(filename, "wb") as f:
            if im.mode != "RGB":
                subprocess.check_call(
                    ["ppmtogif", tempfile], stdout=f, stderr=subprocess.DEVNULL
                )
            else:
                # Pipe ppmquant output into ppmtogif
                # "ppmquant 256 %s | ppmtogif > %s" % (tempfile, filename)
                quant_cmd = ["ppmquant", "256", tempfile]
                togif_cmd = ["ppmtogif"]
                quant_proc = subprocess.Popen(
                    quant_cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL
                )
                togif_proc = subprocess.Popen(
                    togif_cmd,
                    stdin=quant_proc.stdout,
                    stdout=f,
                    stderr=subprocess.DEVNULL,
                )

                # Allow ppmquant to receive SIGPIPE if ppmtogif exits
                quant_proc.stdout.close()

                retcode = quant_proc.wait()
                if retcode:
                    raise subprocess.CalledProcessError(retcode, quant_cmd)

                retcode = togif_proc.wait()
                if retcode:
                    raise subprocess.CalledProcessError(retcode, togif_cmd)
    finally:
        try:
            os.unlink(tempfile)
        except OSError:
            pass


# Force optimization so that we can test performance against
# cases where it took lots of memory and time previously.
_FORCE_OPTIMIZE = False


def _get_optimize(im, info):
    """
    Palette optimization is a potentially expensive operation.

    This function determines if the palette should be optimized using
    some heuristics, then returns the list of palette entries in use.

    :param im: Image object
    :param info: encoderinfo
    :returns: list of indexes of palette entries in use, or None
    """
    if im.mode in ("P", "L") and info and info.get("optimize", 0):
        # Potentially expensive operation.

        # The palette saves 3 bytes per color not used, but palette
        # lengths are restricted to 3*(2**N) bytes. Max saving would
        # be 768 -> 6 bytes if we went all the way down to 2 colors.
        # * If we're over 128 colors, we can't save any space.
        # * If there aren't any holes, it's not worth collapsing.
        # * If we have a 'large' image, the palette is in the noise.

        # create the new palette if not every color is used
        optimise = _FORCE_OPTIMIZE or im.mode == "L"
        if optimise or im.width * im.height < 512 * 512:
            # check which colors are used
            used_palette_colors = []
            for i, count in enumerate(im.histogram()):
                if count:
                    used_palette_colors.append(i)

            if optimise or (
                len(used_palette_colors) <= 128
                and max(used_palette_colors) > len(used_palette_colors)
            ):
                return used_palette_colors


def _get_color_table_size(palette_bytes):
    # calculate the palette size for the header
    import math

    if not palette_bytes:
        return 0
    elif len(palette_bytes) < 9:
        return 1
    else:
        return int(math.ceil(math.log(len(palette_bytes) // 3, 2))) - 1


def _get_header_palette(palette_bytes):
    """
    Returns the palette, null padded to the next power of 2 (*3) bytes
    suitable for direct inclusion in the GIF header

    :param palette_bytes: Unpadded palette bytes, in RGBRGB form
    :returns: Null padded palette
    """
    color_table_size = _get_color_table_size(palette_bytes)

    # add the missing amount of bytes
    # the palette has to be 2<<n in size
    actual_target_size_diff = (2 << color_table_size) - len(palette_bytes) // 3
    if actual_target_size_diff > 0:
        palette_bytes += o8(0) * 3 * actual_target_size_diff
    return palette_bytes


def _get_palette_bytes(im):
    """
    Gets the palette for inclusion in the gif header

    :param im: Image object
    :returns: Bytes, len<=768 suitable for inclusion in gif header
    """
    return im.palette.palette


def _get_background(im, infoBackground):
    background = 0
    if infoBackground:
        background = infoBackground
        if isinstance(background, tuple):
            # WebPImagePlugin stores an RGBA value in info["background"]
            # So it must be converted to the same format as GifImagePlugin's
            # info["background"] - a global color table index
            background = im.palette.getcolor(background)
    return background


def _get_global_header(im, info):
    """Return a list of strings representing a GIF header"""

    # Header Block
    # http://www.matthewflickinger.com/lab/whatsinagif/bits_and_bytes.asp

    version = b"87a"
    for extensionKey in ["transparency", "duration", "loop", "comment"]:
        if info and extensionKey in info:
            if (extensionKey == "duration" and info[extensionKey] == 0) or (
                extensionKey == "comment" and not (1 <= len(info[extensionKey]) <= 255)
            ):
                continue
            version = b"89a"
            break
    else:
        if im.info.get("version") == b"89a":
            version = b"89a"

    background = _get_background(im, info.get("background"))

    palette_bytes = _get_palette_bytes(im)
    color_table_size = _get_color_table_size(palette_bytes)

    return [
        b"GIF"  # signature
        + version  # version
        + o16(im.size[0])  # canvas width
        + o16(im.size[1]),  # canvas height
        # Logical Screen Descriptor
        # size of global color table + global color table flag
        o8(color_table_size + 128),  # packed fields
        # background + reserved/aspect
        o8(background) + o8(0),
        # Global Color Table
        _get_header_palette(palette_bytes),
    ]


def _write_frame_data(fp, im_frame, offset, params):
    try:
        im_frame.encoderinfo = params

        # local image header
        _write_local_header(fp, im_frame, offset, 0)

        ImageFile._save(
            im_frame, fp, [("gif", (0, 0) + im_frame.size, 0, RAWMODE[im_frame.mode])]
        )

        fp.write(b"\0")  # end of image data
    finally:
        del im_frame.encoderinfo


# --------------------------------------------------------------------
# Legacy GIF utilities


def getheader(im, palette=None, info=None):
    """
    Legacy Method to get Gif data from image.

    Warning:: May modify image data.

    :param im: Image object
    :param palette: bytes object containing the source palette, or ....
    :param info: encoderinfo
    :returns: tuple of(list of header items, optimized palette)

    """
    used_palette_colors = _get_optimize(im, info)

    if info is None:
        info = {}

    if "background" not in info and "background" in im.info:
        info["background"] = im.info["background"]

    im_mod = _normalize_palette(im, palette, info)
    im.palette = im_mod.palette
    im.im = im_mod.im
    header = _get_global_header(im, info)

    return header, used_palette_colors


# To specify duration, add the time in milliseconds to getdata(),
# e.g. getdata(im_frame, duration=1000)
def getdata(im, offset=(0, 0), **params):
    """
    Legacy Method

    Return a list of strings representing this image.
    The first string is a local image header, the rest contains
    encoded image data.

    :param im: Image object
    :param offset: Tuple of (x, y) pixels. Defaults to (0,0)
    :param \\**params: E.g. duration or other encoder info parameters
    :returns: List of Bytes containing gif encoded frame data

    """

    class Collector:
        data = []

        def write(self, data):
            self.data.append(data)

    im.load()  # make sure raster data is available

    fp = Collector()

    _write_frame_data(fp, im, offset, params)

    return fp.data


# --------------------------------------------------------------------
# Registry

Image.register_open(GifImageFile.format, GifImageFile, _accept)
Image.register_save(GifImageFile.format, _save)
Image.register_save_all(GifImageFile.format, _save_all)
Image.register_extension(GifImageFile.format, ".gif")
Image.register_mime(GifImageFile.format, "image/gif")

#
# Uncomment the following line if you wish to use NETPBM/PBMPLUS
# instead of the built-in "uncompressed" GIF encoder

# Image.register_save(GifImageFile.format, _save_netpbm)