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304 lines
8.6 KiB
Python
304 lines
8.6 KiB
Python
# encoding: utf-8
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from __future__ import absolute_import, division, print_function
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from .constants import MIME_TYPE, PNG_CHUNK_TYPE
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from .exceptions import InvalidImageStreamError
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from .helpers import BIG_ENDIAN, StreamReader
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from .image import BaseImageHeader
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class Png(BaseImageHeader):
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"""
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Image header parser for PNG images
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"""
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@property
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def content_type(self):
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"""
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MIME content type for this image, unconditionally `image/png` for
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PNG images.
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"""
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return MIME_TYPE.PNG
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@property
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def default_ext(self):
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"""
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Default filename extension, always 'png' for PNG images.
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"""
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return 'png'
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@classmethod
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def from_stream(cls, stream):
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"""
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Return a |Png| instance having header properties parsed from image in
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*stream*.
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"""
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parser = _PngParser.parse(stream)
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px_width = parser.px_width
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px_height = parser.px_height
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horz_dpi = parser.horz_dpi
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vert_dpi = parser.vert_dpi
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return cls(px_width, px_height, horz_dpi, vert_dpi)
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class _PngParser(object):
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"""
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Parses a PNG image stream to extract the image properties found in its
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chunks.
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"""
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def __init__(self, chunks):
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super(_PngParser, self).__init__()
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self._chunks = chunks
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@classmethod
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def parse(cls, stream):
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"""
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Return a |_PngParser| instance containing the header properties
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parsed from the PNG image in *stream*.
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"""
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chunks = _Chunks.from_stream(stream)
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return cls(chunks)
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@property
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def px_width(self):
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"""
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The number of pixels in each row of the image.
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"""
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IHDR = self._chunks.IHDR
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return IHDR.px_width
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@property
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def px_height(self):
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"""
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The number of stacked rows of pixels in the image.
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"""
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IHDR = self._chunks.IHDR
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return IHDR.px_height
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@property
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def horz_dpi(self):
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"""
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Integer dots per inch for the width of this image. Defaults to 72
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when not present in the file, as is often the case.
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"""
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pHYs = self._chunks.pHYs
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if pHYs is None:
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return 72
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return self._dpi(pHYs.units_specifier, pHYs.horz_px_per_unit)
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@property
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def vert_dpi(self):
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"""
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Integer dots per inch for the height of this image. Defaults to 72
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when not present in the file, as is often the case.
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"""
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pHYs = self._chunks.pHYs
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if pHYs is None:
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return 72
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return self._dpi(pHYs.units_specifier, pHYs.vert_px_per_unit)
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@staticmethod
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def _dpi(units_specifier, px_per_unit):
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"""
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Return dots per inch value calculated from *units_specifier* and
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*px_per_unit*.
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"""
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if units_specifier == 1 and px_per_unit:
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return int(round(px_per_unit * 0.0254))
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return 72
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class _Chunks(object):
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"""
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Collection of the chunks parsed from a PNG image stream
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"""
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def __init__(self, chunk_iterable):
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super(_Chunks, self).__init__()
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self._chunks = list(chunk_iterable)
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@classmethod
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def from_stream(cls, stream):
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"""
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Return a |_Chunks| instance containing the PNG chunks in *stream*.
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"""
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chunk_parser = _ChunkParser.from_stream(stream)
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chunks = [chunk for chunk in chunk_parser.iter_chunks()]
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return cls(chunks)
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@property
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def IHDR(self):
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"""
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IHDR chunk in PNG image
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"""
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match = lambda chunk: chunk.type_name == PNG_CHUNK_TYPE.IHDR # noqa
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IHDR = self._find_first(match)
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if IHDR is None:
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raise InvalidImageStreamError('no IHDR chunk in PNG image')
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return IHDR
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@property
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def pHYs(self):
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"""
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pHYs chunk in PNG image, or |None| if not present
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"""
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match = lambda chunk: chunk.type_name == PNG_CHUNK_TYPE.pHYs # noqa
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return self._find_first(match)
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def _find_first(self, match):
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"""
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Return first chunk in stream order returning True for function
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*match*.
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"""
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for chunk in self._chunks:
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if match(chunk):
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return chunk
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return None
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class _ChunkParser(object):
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"""
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Extracts chunks from a PNG image stream
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"""
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def __init__(self, stream_rdr):
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super(_ChunkParser, self).__init__()
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self._stream_rdr = stream_rdr
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@classmethod
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def from_stream(cls, stream):
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"""
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Return a |_ChunkParser| instance that can extract the chunks from the
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PNG image in *stream*.
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"""
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stream_rdr = StreamReader(stream, BIG_ENDIAN)
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return cls(stream_rdr)
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def iter_chunks(self):
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"""
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Generate a |_Chunk| subclass instance for each chunk in this parser's
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PNG stream, in the order encountered in the stream.
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"""
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for chunk_type, offset in self._iter_chunk_offsets():
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chunk = _ChunkFactory(chunk_type, self._stream_rdr, offset)
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yield chunk
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def _iter_chunk_offsets(self):
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"""
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Generate a (chunk_type, chunk_offset) 2-tuple for each of the chunks
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in the PNG image stream. Iteration stops after the IEND chunk is
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returned.
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"""
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chunk_offset = 8
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while True:
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chunk_data_len = self._stream_rdr.read_long(chunk_offset)
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chunk_type = self._stream_rdr.read_str(4, chunk_offset, 4)
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data_offset = chunk_offset + 8
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yield chunk_type, data_offset
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if chunk_type == 'IEND':
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break
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# incr offset for chunk len long, chunk type, chunk data, and CRC
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chunk_offset += (4 + 4 + chunk_data_len + 4)
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def _ChunkFactory(chunk_type, stream_rdr, offset):
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"""
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Return a |_Chunk| subclass instance appropriate to *chunk_type* parsed
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from *stream_rdr* at *offset*.
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"""
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chunk_cls_map = {
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PNG_CHUNK_TYPE.IHDR: _IHDRChunk,
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PNG_CHUNK_TYPE.pHYs: _pHYsChunk,
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}
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chunk_cls = chunk_cls_map.get(chunk_type, _Chunk)
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return chunk_cls.from_offset(chunk_type, stream_rdr, offset)
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class _Chunk(object):
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"""
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Base class for specific chunk types. Also serves as the default chunk
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type.
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"""
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def __init__(self, chunk_type):
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super(_Chunk, self).__init__()
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self._chunk_type = chunk_type
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@classmethod
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def from_offset(cls, chunk_type, stream_rdr, offset):
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"""
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Return a default _Chunk instance that only knows its chunk type.
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"""
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return cls(chunk_type)
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@property
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def type_name(self):
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"""
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The chunk type name, e.g. 'IHDR', 'pHYs', etc.
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"""
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return self._chunk_type
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class _IHDRChunk(_Chunk):
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"""
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IHDR chunk, contains the image dimensions
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"""
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def __init__(self, chunk_type, px_width, px_height):
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super(_IHDRChunk, self).__init__(chunk_type)
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self._px_width = px_width
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self._px_height = px_height
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@classmethod
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def from_offset(cls, chunk_type, stream_rdr, offset):
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"""
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Return an _IHDRChunk instance containing the image dimensions
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extracted from the IHDR chunk in *stream* at *offset*.
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"""
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px_width = stream_rdr.read_long(offset)
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px_height = stream_rdr.read_long(offset, 4)
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return cls(chunk_type, px_width, px_height)
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@property
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def px_width(self):
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return self._px_width
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@property
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def px_height(self):
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return self._px_height
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class _pHYsChunk(_Chunk):
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"""
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pYHs chunk, contains the image dpi information
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"""
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def __init__(self, chunk_type, horz_px_per_unit, vert_px_per_unit,
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units_specifier):
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super(_pHYsChunk, self).__init__(chunk_type)
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self._horz_px_per_unit = horz_px_per_unit
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self._vert_px_per_unit = vert_px_per_unit
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self._units_specifier = units_specifier
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@classmethod
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def from_offset(cls, chunk_type, stream_rdr, offset):
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"""
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Return a _pHYsChunk instance containing the image resolution
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extracted from the pHYs chunk in *stream* at *offset*.
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"""
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horz_px_per_unit = stream_rdr.read_long(offset)
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vert_px_per_unit = stream_rdr.read_long(offset, 4)
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units_specifier = stream_rdr.read_byte(offset, 8)
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return cls(
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chunk_type, horz_px_per_unit, vert_px_per_unit, units_specifier
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)
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@property
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def horz_px_per_unit(self):
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return self._horz_px_per_unit
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@property
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def vert_px_per_unit(self):
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return self._vert_px_per_unit
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@property
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def units_specifier(self):
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return self._units_specifier
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