png.py

from .constants import MIME_TYPE, PNG_CHUNK_TYPE
from .exceptions import InvalidImageStreamError
from .helpers import BIG_ENDIAN, StreamReader
from .image import BaseImageHeader


class Png(BaseImageHeader):
    """Image header parser for PNG images."""

    @property
    def content_type(self):
        """MIME content type for this image, unconditionally `image/png` for PNG
        images."""
        return MIME_TYPE.PNG

    @property
    def default_ext(self):
        """Default filename extension, always 'png' for PNG images."""
        return "png"

    @classmethod
    def from_stream(cls, stream):
        """Return a |Png| instance having header properties parsed from image in
        `stream`."""
        parser = _PngParser.parse(stream)

        px_width = parser.px_width
        px_height = parser.px_height
        horz_dpi = parser.horz_dpi
        vert_dpi = parser.vert_dpi

        return cls(px_width, px_height, horz_dpi, vert_dpi)


class _PngParser:
    """Parses a PNG image stream to extract the image properties found in its chunks."""

    def __init__(self, chunks):
        super(_PngParser, self).__init__()
        self._chunks = chunks

    @classmethod
    def parse(cls, stream):
        """Return a |_PngParser| instance containing the header properties parsed from
        the PNG image in `stream`."""
        chunks = _Chunks.from_stream(stream)
        return cls(chunks)

    @property
    def px_width(self):
        """The number of pixels in each row of the image."""
        IHDR = self._chunks.IHDR
        return IHDR.px_width

    @property
    def px_height(self):
        """The number of stacked rows of pixels in the image."""
        IHDR = self._chunks.IHDR
        return IHDR.px_height

    @property
    def horz_dpi(self):
        """Integer dots per inch for the width of this image.

        Defaults to 72 when not present in the file, as is often the case.
        """
        pHYs = self._chunks.pHYs
        if pHYs is None:
            return 72
        return self._dpi(pHYs.units_specifier, pHYs.horz_px_per_unit)

    @property
    def vert_dpi(self):
        """Integer dots per inch for the height of this image.

        Defaults to 72 when not present in the file, as is often the case.
        """
        pHYs = self._chunks.pHYs
        if pHYs is None:
            return 72
        return self._dpi(pHYs.units_specifier, pHYs.vert_px_per_unit)

    @staticmethod
    def _dpi(units_specifier, px_per_unit):
        """Return dots per inch value calculated from `units_specifier` and
        `px_per_unit`."""
        if units_specifier == 1 and px_per_unit:
            return int(round(px_per_unit * 0.0254))
        return 72


class _Chunks:
    """Collection of the chunks parsed from a PNG image stream."""

    def __init__(self, chunk_iterable):
        super(_Chunks, self).__init__()
        self._chunks = list(chunk_iterable)

    @classmethod
    def from_stream(cls, stream):
        """Return a |_Chunks| instance containing the PNG chunks in `stream`."""
        chunk_parser = _ChunkParser.from_stream(stream)
        chunks = list(chunk_parser.iter_chunks())
        return cls(chunks)

    @property
    def IHDR(self):
        """IHDR chunk in PNG image."""
        match = lambda chunk: chunk.type_name == PNG_CHUNK_TYPE.IHDR  # noqa
        IHDR = self._find_first(match)
        if IHDR is None:
            raise InvalidImageStreamError("no IHDR chunk in PNG image")
        return IHDR

    @property
    def pHYs(self):
        """PHYs chunk in PNG image, or |None| if not present."""
        match = lambda chunk: chunk.type_name == PNG_CHUNK_TYPE.pHYs  # noqa
        return self._find_first(match)

    def _find_first(self, match):
        """Return first chunk in stream order returning True for function `match`."""
        for chunk in self._chunks:
            if match(chunk):
                return chunk
        return None


class _ChunkParser:
    """Extracts chunks from a PNG image stream."""

    def __init__(self, stream_rdr):
        super(_ChunkParser, self).__init__()
        self._stream_rdr = stream_rdr

    @classmethod
    def from_stream(cls, stream):
        """Return a |_ChunkParser| instance that can extract the chunks from the PNG
        image in `stream`."""
        stream_rdr = StreamReader(stream, BIG_ENDIAN)
        return cls(stream_rdr)

    def iter_chunks(self):
        """Generate a |_Chunk| subclass instance for each chunk in this parser's PNG
        stream, in the order encountered in the stream."""
        for chunk_type, offset in self._iter_chunk_offsets():
            chunk = _ChunkFactory(chunk_type, self._stream_rdr, offset)
            yield chunk

    def _iter_chunk_offsets(self):
        """Generate a (chunk_type, chunk_offset) 2-tuple for each of the chunks in the
        PNG image stream.

        Iteration stops after the IEND chunk is returned.
        """
        chunk_offset = 8
        while True:
            chunk_data_len = self._stream_rdr.read_long(chunk_offset)
            chunk_type = self._stream_rdr.read_str(4, chunk_offset, 4)
            data_offset = chunk_offset + 8
            yield chunk_type, data_offset
            if chunk_type == "IEND":
                break
            # incr offset for chunk len long, chunk type, chunk data, and CRC
            chunk_offset += 4 + 4 + chunk_data_len + 4


def _ChunkFactory(chunk_type, stream_rdr, offset):
    """Return a |_Chunk| subclass instance appropriate to `chunk_type` parsed from
    `stream_rdr` at `offset`."""
    chunk_cls_map = {
        PNG_CHUNK_TYPE.IHDR: _IHDRChunk,
        PNG_CHUNK_TYPE.pHYs: _pHYsChunk,
    }
    chunk_cls = chunk_cls_map.get(chunk_type, _Chunk)
    return chunk_cls.from_offset(chunk_type, stream_rdr, offset)


class _Chunk:
    """Base class for specific chunk types.

    Also serves as the default chunk type.
    """

    def __init__(self, chunk_type):
        super(_Chunk, self).__init__()
        self._chunk_type = chunk_type

    @classmethod
    def from_offset(cls, chunk_type, stream_rdr, offset):
        """Return a default _Chunk instance that only knows its chunk type."""
        return cls(chunk_type)

    @property
    def type_name(self):
        """The chunk type name, e.g. 'IHDR', 'pHYs', etc."""
        return self._chunk_type


class _IHDRChunk(_Chunk):
    """IHDR chunk, contains the image dimensions."""

    def __init__(self, chunk_type, px_width, px_height):
        super(_IHDRChunk, self).__init__(chunk_type)
        self._px_width = px_width
        self._px_height = px_height

    @classmethod
    def from_offset(cls, chunk_type, stream_rdr, offset):
        """Return an _IHDRChunk instance containing the image dimensions extracted from
        the IHDR chunk in `stream` at `offset`."""
        px_width = stream_rdr.read_long(offset)
        px_height = stream_rdr.read_long(offset, 4)
        return cls(chunk_type, px_width, px_height)

    @property
    def px_width(self):
        return self._px_width

    @property
    def px_height(self):
        return self._px_height


class _pHYsChunk(_Chunk):
    """PYHs chunk, contains the image dpi information."""

    def __init__(self, chunk_type, horz_px_per_unit, vert_px_per_unit, units_specifier):
        super(_pHYsChunk, self).__init__(chunk_type)
        self._horz_px_per_unit = horz_px_per_unit
        self._vert_px_per_unit = vert_px_per_unit
        self._units_specifier = units_specifier

    @classmethod
    def from_offset(cls, chunk_type, stream_rdr, offset):
        """Return a _pHYsChunk instance containing the image resolution extracted from
        the pHYs chunk in `stream` at `offset`."""
        horz_px_per_unit = stream_rdr.read_long(offset)
        vert_px_per_unit = stream_rdr.read_long(offset, 4)
        units_specifier = stream_rdr.read_byte(offset, 8)
        return cls(chunk_type, horz_px_per_unit, vert_px_per_unit, units_specifier)

    @property
    def horz_px_per_unit(self):
        return self._horz_px_per_unit

    @property
    def vert_px_per_unit(self):
        return self._vert_px_per_unit

    @property
    def units_specifier(self):
        return self._units_specifier