PngImagePlugin.py

#
# The Python Imaging Library.
# $Id$
#
# PNG support code
#
# See "PNG (Portable Network Graphics) Specification, version 1.0;
# W3C Recommendation", 1996-10-01, Thomas Boutell (ed.).
#
# history:
# 1996-05-06 fl   Created (couldn't resist it)
# 1996-12-14 fl   Upgraded, added read and verify support (0.2)
# 1996-12-15 fl   Separate PNG stream parser
# 1996-12-29 fl   Added write support, added getchunks
# 1996-12-30 fl   Eliminated circular references in decoder (0.3)
# 1998-07-12 fl   Read/write 16-bit images as mode I (0.4)
# 2001-02-08 fl   Added transparency support (from Zircon) (0.5)
# 2001-04-16 fl   Don't close data source in "open" method (0.6)
# 2004-02-24 fl   Don't even pretend to support interlaced files (0.7)
# 2004-08-31 fl   Do basic sanity check on chunk identifiers (0.8)
# 2004-09-20 fl   Added PngInfo chunk container
# 2004-12-18 fl   Added DPI read support (based on code by Niki Spahiev)
# 2008-08-13 fl   Added tRNS support for RGB images
# 2009-03-06 fl   Support for preserving ICC profiles (by Florian Hoech)
# 2009-03-08 fl   Added zTXT support (from Lowell Alleman)
# 2009-03-29 fl   Read interlaced PNG files (from Conrado Porto Lopes Gouvua)
#
# Copyright (c) 1997-2009 by Secret Labs AB
# Copyright (c) 1996 by Fredrik Lundh
#
# See the README file for information on usage and redistribution.
#

import logging
import re
import zlib
import struct

from . import Image, ImageFile, ImagePalette
from ._binary import i8, i16be as i16, i32be as i32, o16be as o16, o32be as o32
from ._util import py3

# __version__ is deprecated and will be removed in a future version. Use
# PIL.__version__ instead.
__version__ = "0.9"

logger = logging.getLogger(__name__)

is_cid = re.compile(br"\w\w\w\w").match


_MAGIC = b"\211PNG\r\n\032\n"


_MODES = {
    # supported bits/color combinations, and corresponding modes/rawmodes
    # Greyscale
    (1, 0): ("1", "1"),
    (2, 0): ("L", "L;2"),
    (4, 0): ("L", "L;4"),
    (8, 0): ("L", "L"),
    (16, 0): ("I", "I;16B"),
    # Truecolour
    (8, 2): ("RGB", "RGB"),
    (16, 2): ("RGB", "RGB;16B"),
    # Indexed-colour
    (1, 3): ("P", "P;1"),
    (2, 3): ("P", "P;2"),
    (4, 3): ("P", "P;4"),
    (8, 3): ("P", "P"),
    # Greyscale with alpha
    (8, 4): ("LA", "LA"),
    (16, 4): ("RGBA", "LA;16B"),  # LA;16B->LA not yet available
    # Truecolour with alpha
    (8, 6): ("RGBA", "RGBA"),
    (16, 6): ("RGBA", "RGBA;16B"),
}


_simple_palette = re.compile(b"^\xff*\x00\xff*$")

# Maximum decompressed size for a iTXt or zTXt chunk.
# Eliminates decompression bombs where compressed chunks can expand 1000x
MAX_TEXT_CHUNK = ImageFile.SAFEBLOCK
# Set the maximum total text chunk size.
MAX_TEXT_MEMORY = 64 * MAX_TEXT_CHUNK


def _safe_zlib_decompress(s):
    dobj = zlib.decompressobj()
    plaintext = dobj.decompress(s, MAX_TEXT_CHUNK)
    if dobj.unconsumed_tail:
        raise ValueError("Decompressed Data Too Large")
    return plaintext


def _crc32(data, seed=0):
    return zlib.crc32(data, seed) & 0xFFFFFFFF


# --------------------------------------------------------------------
# Support classes.  Suitable for PNG and related formats like MNG etc.


class ChunkStream(object):
    def __init__(self, fp):

        self.fp = fp
        self.queue = []

    def read(self):
        """Fetch a new chunk. Returns header information."""
        cid = None

        if self.queue:
            cid, pos, length = self.queue.pop()
            self.fp.seek(pos)
        else:
            s = self.fp.read(8)
            cid = s[4:]
            pos = self.fp.tell()
            length = i32(s)

        if not is_cid(cid):
            if not ImageFile.LOAD_TRUNCATED_IMAGES:
                raise SyntaxError("broken PNG file (chunk %s)" % repr(cid))

        return cid, pos, length

    def __enter__(self):
        return self

    def __exit__(self, *args):
        self.close()

    def close(self):
        self.queue = self.crc = self.fp = None

    def push(self, cid, pos, length):

        self.queue.append((cid, pos, length))

    def call(self, cid, pos, length):
        """Call the appropriate chunk handler"""

        logger.debug("STREAM %r %s %s", cid, pos, length)
        return getattr(self, "chunk_" + cid.decode("ascii"))(pos, length)

    def crc(self, cid, data):
        """Read and verify checksum"""

        # Skip CRC checks for ancillary chunks if allowed to load truncated
        # images
        # 5th byte of first char is 1 [specs, section 5.4]
        if ImageFile.LOAD_TRUNCATED_IMAGES and (i8(cid[0]) >> 5 & 1):
            self.crc_skip(cid, data)
            return

        try:
            crc1 = _crc32(data, _crc32(cid))
            crc2 = i32(self.fp.read(4))
            if crc1 != crc2:
                raise SyntaxError("broken PNG file (bad header checksum in %r)" % cid)
        except struct.error:
            raise SyntaxError("broken PNG file (incomplete checksum in %r)" % cid)

    def crc_skip(self, cid, data):
        """Read checksum.  Used if the C module is not present"""

        self.fp.read(4)

    def verify(self, endchunk=b"IEND"):

        # Simple approach; just calculate checksum for all remaining
        # blocks.  Must be called directly after open.

        cids = []

        while True:
            try:
                cid, pos, length = self.read()
            except struct.error:
                raise IOError("truncated PNG file")

            if cid == endchunk:
                break
            self.crc(cid, ImageFile._safe_read(self.fp, length))
            cids.append(cid)

        return cids


class iTXt(str):
    """
    Subclass of string to allow iTXt chunks to look like strings while
    keeping their extra information

    """

    @staticmethod
    def __new__(cls, text, lang=None, tkey=None):
        """
        :param cls: the class to use when creating the instance
        :param text: value for this key
        :param lang: language code
        :param tkey: UTF-8 version of the key name
        """

        self = str.__new__(cls, text)
        self.lang = lang
        self.tkey = tkey
        return self


class PngInfo(object):
    """
    PNG chunk container (for use with save(pnginfo=))

    """

    def __init__(self):
        self.chunks = []

    def add(self, cid, data):
        """Appends an arbitrary chunk. Use with caution.

        :param cid: a byte string, 4 bytes long.
        :param data: a byte string of the encoded data

        """

        self.chunks.append((cid, data))

    def add_itxt(self, key, value, lang="", tkey="", zip=False):
        """Appends an iTXt chunk.

        :param key: latin-1 encodable text key name
        :param value: value for this key
        :param lang: language code
        :param tkey: UTF-8 version of the key name
        :param zip: compression flag

        """

        if not isinstance(key, bytes):
            key = key.encode("latin-1", "strict")
        if not isinstance(value, bytes):
            value = value.encode("utf-8", "strict")
        if not isinstance(lang, bytes):
            lang = lang.encode("utf-8", "strict")
        if not isinstance(tkey, bytes):
            tkey = tkey.encode("utf-8", "strict")

        if zip:
            self.add(
                b"iTXt",
                key + b"\0\x01\0" + lang + b"\0" + tkey + b"\0" + zlib.compress(value),
            )
        else:
            self.add(b"iTXt", key + b"\0\0\0" + lang + b"\0" + tkey + b"\0" + value)

    def add_text(self, key, value, zip=False):
        """Appends a text chunk.

        :param key: latin-1 encodable text key name
        :param value: value for this key, text or an
           :py:class:`PIL.PngImagePlugin.iTXt` instance
        :param zip: compression flag

        """
        if isinstance(value, iTXt):
            return self.add_itxt(key, value, value.lang, value.tkey, zip=zip)

        # The tEXt chunk stores latin-1 text
        if not isinstance(value, bytes):
            try:
                value = value.encode("latin-1", "strict")
            except UnicodeError:
                return self.add_itxt(key, value, zip=zip)

        if not isinstance(key, bytes):
            key = key.encode("latin-1", "strict")

        if zip:
            self.add(b"zTXt", key + b"\0\0" + zlib.compress(value))
        else:
            self.add(b"tEXt", key + b"\0" + value)


# --------------------------------------------------------------------
# PNG image stream (IHDR/IEND)


class PngStream(ChunkStream):
    def __init__(self, fp):

        ChunkStream.__init__(self, fp)

        # local copies of Image attributes
        self.im_info = {}
        self.im_text = {}
        self.im_size = (0, 0)
        self.im_mode = None
        self.im_tile = None
        self.im_palette = None
        self.im_custom_mimetype = None

        self.text_memory = 0

    def check_text_memory(self, chunklen):
        self.text_memory += chunklen
        if self.text_memory > MAX_TEXT_MEMORY:
            raise ValueError(
                "Too much memory used in text chunks: %s>MAX_TEXT_MEMORY"
                % self.text_memory
            )

    def chunk_iCCP(self, pos, length):

        # ICC profile
        s = ImageFile._safe_read(self.fp, length)
        # according to PNG spec, the iCCP chunk contains:
        # Profile name  1-79 bytes (character string)
        # Null separator        1 byte (null character)
        # Compression method    1 byte (0)
        # Compressed profile    n bytes (zlib with deflate compression)
        i = s.find(b"\0")
        logger.debug("iCCP profile name %r", s[:i])
        logger.debug("Compression method %s", i8(s[i]))
        comp_method = i8(s[i])
        if comp_method != 0:
            raise SyntaxError(
                "Unknown compression method %s in iCCP chunk" % comp_method
            )
        try:
            icc_profile = _safe_zlib_decompress(s[i + 2 :])
        except ValueError:
            if ImageFile.LOAD_TRUNCATED_IMAGES:
                icc_profile = None
            else:
                raise
        except zlib.error:
            icc_profile = None  # FIXME
        self.im_info["icc_profile"] = icc_profile
        return s

    def chunk_IHDR(self, pos, length):

        # image header
        s = ImageFile._safe_read(self.fp, length)
        self.im_size = i32(s), i32(s[4:])
        try:
            self.im_mode, self.im_rawmode = _MODES[(i8(s[8]), i8(s[9]))]
        except Exception:
            pass
        if i8(s[12]):
            self.im_info["interlace"] = 1
        if i8(s[11]):
            raise SyntaxError("unknown filter category")
        return s

    def chunk_IDAT(self, pos, length):

        # image data
        self.im_tile = [("zip", (0, 0) + self.im_size, pos, self.im_rawmode)]
        self.im_idat = length
        raise EOFError

    def chunk_IEND(self, pos, length):

        # end of PNG image
        raise EOFError

    def chunk_PLTE(self, pos, length):

        # palette
        s = ImageFile._safe_read(self.fp, length)
        if self.im_mode == "P":
            self.im_palette = "RGB", s
        return s

    def chunk_tRNS(self, pos, length):

        # transparency
        s = ImageFile._safe_read(self.fp, length)
        if self.im_mode == "P":
            if _simple_palette.match(s):
                # tRNS contains only one full-transparent entry,
                # other entries are full opaque
                i = s.find(b"\0")
                if i >= 0:
                    self.im_info["transparency"] = i
            else:
                # otherwise, we have a byte string with one alpha value
                # for each palette entry
                self.im_info["transparency"] = s
        elif self.im_mode in ("1", "L", "I"):
            self.im_info["transparency"] = i16(s)
        elif self.im_mode == "RGB":
            self.im_info["transparency"] = i16(s), i16(s[2:]), i16(s[4:])
        return s

    def chunk_gAMA(self, pos, length):
        # gamma setting
        s = ImageFile._safe_read(self.fp, length)
        self.im_info["gamma"] = i32(s) / 100000.0
        return s

    def chunk_cHRM(self, pos, length):
        # chromaticity, 8 unsigned ints, actual value is scaled by 100,000
        # WP x,y, Red x,y, Green x,y Blue x,y

        s = ImageFile._safe_read(self.fp, length)
        raw_vals = struct.unpack(">%dI" % (len(s) // 4), s)
        self.im_info["chromaticity"] = tuple(elt / 100000.0 for elt in raw_vals)
        return s

    def chunk_sRGB(self, pos, length):
        # srgb rendering intent, 1 byte
        # 0 perceptual
        # 1 relative colorimetric
        # 2 saturation
        # 3 absolute colorimetric

        s = ImageFile._safe_read(self.fp, length)
        self.im_info["srgb"] = i8(s)
        return s

    def chunk_pHYs(self, pos, length):

        # pixels per unit
        s = ImageFile._safe_read(self.fp, length)
        px, py = i32(s), i32(s[4:])
        unit = i8(s[8])
        if unit == 1:  # meter
            dpi = int(px * 0.0254 + 0.5), int(py * 0.0254 + 0.5)
            self.im_info["dpi"] = dpi
        elif unit == 0:
            self.im_info["aspect"] = px, py
        return s

    def chunk_tEXt(self, pos, length):

        # text
        s = ImageFile._safe_read(self.fp, length)
        try:
            k, v = s.split(b"\0", 1)
        except ValueError:
            # fallback for broken tEXt tags
            k = s
            v = b""
        if k:
            if py3:
                k = k.decode("latin-1", "strict")
                v = v.decode("latin-1", "replace")

            self.im_info[k] = self.im_text[k] = v
            self.check_text_memory(len(v))

        return s

    def chunk_zTXt(self, pos, length):

        # compressed text
        s = ImageFile._safe_read(self.fp, length)
        try:
            k, v = s.split(b"\0", 1)
        except ValueError:
            k = s
            v = b""
        if v:
            comp_method = i8(v[0])
        else:
            comp_method = 0
        if comp_method != 0:
            raise SyntaxError(
                "Unknown compression method %s in zTXt chunk" % comp_method
            )
        try:
            v = _safe_zlib_decompress(v[1:])
        except ValueError:
            if ImageFile.LOAD_TRUNCATED_IMAGES:
                v = b""
            else:
                raise
        except zlib.error:
            v = b""

        if k:
            if py3:
                k = k.decode("latin-1", "strict")
                v = v.decode("latin-1", "replace")

            self.im_info[k] = self.im_text[k] = v
            self.check_text_memory(len(v))

        return s

    def chunk_iTXt(self, pos, length):

        # international text
        r = s = ImageFile._safe_read(self.fp, length)
        try:
            k, r = r.split(b"\0", 1)
        except ValueError:
            return s
        if len(r) < 2:
            return s
        cf, cm, r = i8(r[0]), i8(r[1]), r[2:]
        try:
            lang, tk, v = r.split(b"\0", 2)
        except ValueError:
            return s
        if cf != 0:
            if cm == 0:
                try:
                    v = _safe_zlib_decompress(v)
                except ValueError:
                    if ImageFile.LOAD_TRUNCATED_IMAGES:
                        return s
                    else:
                        raise
                except zlib.error:
                    return s
            else:
                return s
        if py3:
            try:
                k = k.decode("latin-1", "strict")
                lang = lang.decode("utf-8", "strict")
                tk = tk.decode("utf-8", "strict")
                v = v.decode("utf-8", "strict")
            except UnicodeError:
                return s

        self.im_info[k] = self.im_text[k] = iTXt(v, lang, tk)
        self.check_text_memory(len(v))

        return s

    def chunk_eXIf(self, pos, length):
        s = ImageFile._safe_read(self.fp, length)
        self.im_info["exif"] = b"Exif\x00\x00" + s
        return s

    # APNG chunks
    def chunk_acTL(self, pos, length):
        s = ImageFile._safe_read(self.fp, length)
        self.im_custom_mimetype = "image/apng"
        return s


# --------------------------------------------------------------------
# PNG reader


def _accept(prefix):
    return prefix[:8] == _MAGIC


##
# Image plugin for PNG images.


class PngImageFile(ImageFile.ImageFile):

    format = "PNG"
    format_description = "Portable network graphics"

    def _open(self):

        if self.fp.read(8) != _MAGIC:
            raise SyntaxError("not a PNG file")

        #
        # Parse headers up to the first IDAT chunk

        self.png = PngStream(self.fp)

        while True:

            #
            # get next chunk

            cid, pos, length = self.png.read()

            try:
                s = self.png.call(cid, pos, length)
            except EOFError:
                break
            except AttributeError:
                logger.debug("%r %s %s (unknown)", cid, pos, length)
                s = ImageFile._safe_read(self.fp, length)

            self.png.crc(cid, s)

        #
        # Copy relevant attributes from the PngStream.  An alternative
        # would be to let the PngStream class modify these attributes
        # directly, but that introduces circular references which are
        # difficult to break if things go wrong in the decoder...
        # (believe me, I've tried ;-)

        self.mode = self.png.im_mode
        self._size = self.png.im_size
        self.info = self.png.im_info
        self._text = None
        self.tile = self.png.im_tile
        self.custom_mimetype = self.png.im_custom_mimetype

        if self.png.im_palette:
            rawmode, data = self.png.im_palette
            self.palette = ImagePalette.raw(rawmode, data)

        self.__prepare_idat = length  # used by load_prepare()

    @property
    def text(self):
        # experimental
        if self._text is None:
            # iTxt, tEXt and zTXt chunks may appear at the end of the file
            # So load the file to ensure that they are read
            self.load()
        return self._text

    def verify(self):
        """Verify PNG file"""

        if self.fp is None:
            raise RuntimeError("verify must be called directly after open")

        # back up to beginning of IDAT block
        self.fp.seek(self.tile[0][2] - 8)

        self.png.verify()
        self.png.close()

        if self._exclusive_fp:
            self.fp.close()
        self.fp = None

    def load_prepare(self):
        """internal: prepare to read PNG file"""

        if self.info.get("interlace"):
            self.decoderconfig = self.decoderconfig + (1,)

        self.__idat = self.__prepare_idat  # used by load_read()
        ImageFile.ImageFile.load_prepare(self)

    def load_read(self, read_bytes):
        """internal: read more image data"""

        while self.__idat == 0:
            # end of chunk, skip forward to next one

            self.fp.read(4)  # CRC

            cid, pos, length = self.png.read()

            if cid not in [b"IDAT", b"DDAT"]:
                self.png.push(cid, pos, length)
                return b""

            self.__idat = length  # empty chunks are allowed

        # read more data from this chunk
        if read_bytes <= 0:
            read_bytes = self.__idat
        else:
            read_bytes = min(read_bytes, self.__idat)

        self.__idat = self.__idat - read_bytes

        return self.fp.read(read_bytes)

    def load_end(self):
        """internal: finished reading image data"""
        while True:
            self.fp.read(4)  # CRC

            try:
                cid, pos, length = self.png.read()
            except (struct.error, SyntaxError):
                break

            if cid == b"IEND":
                break

            try:
                self.png.call(cid, pos, length)
            except UnicodeDecodeError:
                break
            except EOFError:
                ImageFile._safe_read(self.fp, length)
            except AttributeError:
                logger.debug("%r %s %s (unknown)", cid, pos, length)
                ImageFile._safe_read(self.fp, length)
        self._text = self.png.im_text
        self.png.close()
        self.png = None

    def _getexif(self):
        if "exif" not in self.info:
            self.load()
        if "exif" not in self.info:
            return None
        return dict(self.getexif())

    def getexif(self):
        if "exif" not in self.info:
            self.load()
        return ImageFile.ImageFile.getexif(self)


# --------------------------------------------------------------------
# PNG writer

_OUTMODES = {
    # supported PIL modes, and corresponding rawmodes/bits/color combinations
    "1": ("1", b"\x01\x00"),
    "L;1": ("L;1", b"\x01\x00"),
    "L;2": ("L;2", b"\x02\x00"),
    "L;4": ("L;4", b"\x04\x00"),
    "L": ("L", b"\x08\x00"),
    "LA": ("LA", b"\x08\x04"),
    "I": ("I;16B", b"\x10\x00"),
    "I;16": ("I;16B", b"\x10\x00"),
    "P;1": ("P;1", b"\x01\x03"),
    "P;2": ("P;2", b"\x02\x03"),
    "P;4": ("P;4", b"\x04\x03"),
    "P": ("P", b"\x08\x03"),
    "RGB": ("RGB", b"\x08\x02"),
    "RGBA": ("RGBA", b"\x08\x06"),
}


def putchunk(fp, cid, *data):
    """Write a PNG chunk (including CRC field)"""

    data = b"".join(data)

    fp.write(o32(len(data)) + cid)
    fp.write(data)
    crc = _crc32(data, _crc32(cid))
    fp.write(o32(crc))


class _idat(object):
    # wrap output from the encoder in IDAT chunks

    def __init__(self, fp, chunk):
        self.fp = fp
        self.chunk = chunk

    def write(self, data):
        self.chunk(self.fp, b"IDAT", data)


def _save(im, fp, filename, chunk=putchunk):
    # save an image to disk (called by the save method)

    mode = im.mode

    if mode == "P":

        #
        # attempt to minimize storage requirements for palette images
        if "bits" in im.encoderinfo:
            # number of bits specified by user
            colors = 1 << im.encoderinfo["bits"]
        else:
            # check palette contents
            if im.palette:
                colors = max(min(len(im.palette.getdata()[1]) // 3, 256), 2)
            else:
                colors = 256

        if colors <= 2:
            bits = 1
        elif colors <= 4:
            bits = 2
        elif colors <= 16:
            bits = 4
        else:
            bits = 8
        if bits != 8:
            mode = "%s;%d" % (mode, bits)

    # encoder options
    im.encoderconfig = (
        im.encoderinfo.get("optimize", False),
        im.encoderinfo.get("compress_level", -1),
        im.encoderinfo.get("compress_type", -1),
        im.encoderinfo.get("dictionary", b""),
    )

    # get the corresponding PNG mode
    try:
        rawmode, mode = _OUTMODES[mode]
    except KeyError:
        raise IOError("cannot write mode %s as PNG" % mode)

    #
    # write minimal PNG file

    fp.write(_MAGIC)

    chunk(
        fp,
        b"IHDR",
        o32(im.size[0]),  # 0: size
        o32(im.size[1]),
        mode,  # 8: depth/type
        b"\0",  # 10: compression
        b"\0",  # 11: filter category
        b"\0",  # 12: interlace flag
    )

    chunks = [b"cHRM", b"gAMA", b"sBIT", b"sRGB", b"tIME"]

    icc = im.encoderinfo.get("icc_profile", im.info.get("icc_profile"))
    if icc:
        # ICC profile
        # according to PNG spec, the iCCP chunk contains:
        # Profile name  1-79 bytes (character string)
        # Null separator        1 byte (null character)
        # Compression method    1 byte (0)
        # Compressed profile    n bytes (zlib with deflate compression)
        name = b"ICC Profile"
        data = name + b"\0\0" + zlib.compress(icc)
        chunk(fp, b"iCCP", data)

        # You must either have sRGB or iCCP.
        # Disallow sRGB chunks when an iCCP-chunk has been emitted.
        chunks.remove(b"sRGB")

    info = im.encoderinfo.get("pnginfo")
    if info:
        chunks_multiple_allowed = [b"sPLT", b"iTXt", b"tEXt", b"zTXt"]
        for cid, data in info.chunks:
            if cid in chunks:
                chunks.remove(cid)
                chunk(fp, cid, data)
            elif cid in chunks_multiple_allowed:
                chunk(fp, cid, data)

    if im.mode == "P":
        palette_byte_number = (2 ** bits) * 3
        palette_bytes = im.im.getpalette("RGB")[:palette_byte_number]
        while len(palette_bytes) < palette_byte_number:
            palette_bytes += b"\0"
        chunk(fp, b"PLTE", palette_bytes)

    transparency = im.encoderinfo.get("transparency", im.info.get("transparency", None))

    if transparency or transparency == 0:
        if im.mode == "P":
            # limit to actual palette size
            alpha_bytes = 2 ** bits
            if isinstance(transparency, bytes):
                chunk(fp, b"tRNS", transparency[:alpha_bytes])
            else:
                transparency = max(0, min(255, transparency))
                alpha = b"\xFF" * transparency + b"\0"
                chunk(fp, b"tRNS", alpha[:alpha_bytes])
        elif im.mode in ("1", "L", "I"):
            transparency = max(0, min(65535, transparency))
            chunk(fp, b"tRNS", o16(transparency))
        elif im.mode == "RGB":
            red, green, blue = transparency
            chunk(fp, b"tRNS", o16(red) + o16(green) + o16(blue))
        else:
            if "transparency" in im.encoderinfo:
                # don't bother with transparency if it's an RGBA
                # and it's in the info dict. It's probably just stale.
                raise IOError("cannot use transparency for this mode")
    else:
        if im.mode == "P" and im.im.getpalettemode() == "RGBA":
            alpha = im.im.getpalette("RGBA", "A")
            alpha_bytes = 2 ** bits
            chunk(fp, b"tRNS", alpha[:alpha_bytes])

    dpi = im.encoderinfo.get("dpi")
    if dpi:
        chunk(
            fp,
            b"pHYs",
            o32(int(dpi[0] / 0.0254 + 0.5)),
            o32(int(dpi[1] / 0.0254 + 0.5)),
            b"\x01",
        )

    info = im.encoderinfo.get("pnginfo")
    if info:
        chunks = [b"bKGD", b"hIST"]
        for cid, data in info.chunks:
            if cid in chunks:
                chunks.remove(cid)
                chunk(fp, cid, data)

    exif = im.encoderinfo.get("exif", im.info.get("exif"))
    if exif:
        if isinstance(exif, Image.Exif):
            exif = exif.tobytes(8)
        if exif.startswith(b"Exif\x00\x00"):
            exif = exif[6:]
        chunk(fp, b"eXIf", exif)

    ImageFile._save(im, _idat(fp, chunk), [("zip", (0, 0) + im.size, 0, rawmode)])

    chunk(fp, b"IEND", b"")

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


# --------------------------------------------------------------------
# PNG chunk converter


def getchunks(im, **params):
    """Return a list of PNG chunks representing this image."""

    class collector(object):
        data = []

        def write(self, data):
            pass

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

    def append(fp, cid, *data):
        data = b"".join(data)
        crc = o32(_crc32(data, _crc32(cid)))
        fp.append((cid, data, crc))

    fp = collector()

    try:
        im.encoderinfo = params
        _save(im, fp, None, append)
    finally:
        del im.encoderinfo

    return fp.data


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

Image.register_open(PngImageFile.format, PngImageFile, _accept)
Image.register_save(PngImageFile.format, _save)

Image.register_extensions(PngImageFile.format, [".png", ".apng"])

Image.register_mime(PngImageFile.format, "image/png")