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# encoding: utf-8
"""
Objects related to parsing headers of JPEG image streams, both JFIF and Exif
sub-formats.
"""
from __future__ import absolute_import, division, print_function
from ..compat import BytesIO
from .constants import JPEG_MARKER_CODE, MIME_TYPE
from .helpers import BIG_ENDIAN, StreamReader
from .image import BaseImageHeader
from .tiff import Tiff
class Jpeg(BaseImageHeader):
"""
Base class for JFIF and EXIF subclasses.
"""
@property
def content_type(self):
"""
MIME content type for this image, unconditionally `image/jpeg` for
JPEG images.
"""
return MIME_TYPE.JPEG
@property
def default_ext(self):
"""
Default filename extension, always 'jpg' for JPG images.
"""
return 'jpg'
class Exif(Jpeg):
"""
Image header parser for Exif image format
"""
@classmethod
def from_stream(cls, stream):
"""
Return |Exif| instance having header properties parsed from Exif
image in *stream*.
"""
markers = _JfifMarkers.from_stream(stream)
# print('\n%s' % markers)
px_width = markers.sof.px_width
px_height = markers.sof.px_height
horz_dpi = markers.app1.horz_dpi
vert_dpi = markers.app1.vert_dpi
return cls(px_width, px_height, horz_dpi, vert_dpi)
class Jfif(Jpeg):
"""
Image header parser for JFIF image format
"""
@classmethod
def from_stream(cls, stream):
"""
Return a |Jfif| instance having header properties parsed from image
in *stream*.
"""
markers = _JfifMarkers.from_stream(stream)
px_width = markers.sof.px_width
px_height = markers.sof.px_height
horz_dpi = markers.app0.horz_dpi
vert_dpi = markers.app0.vert_dpi
return cls(px_width, px_height, horz_dpi, vert_dpi)
class _JfifMarkers(object):
"""
Sequence of markers in a JPEG file, perhaps truncated at first SOS marker
for performance reasons.
"""
def __init__(self, markers):
super(_JfifMarkers, self).__init__()
self._markers = list(markers)
def __str__(self): # pragma: no cover
"""
Returns a tabular listing of the markers in this instance, which can
be handy for debugging and perhaps other uses.
"""
header = ' offset seglen mc name\n======= ====== == ====='
tmpl = '%7d %6d %02X %s'
rows = []
for marker in self._markers:
rows.append(tmpl % (
marker.offset, marker.segment_length,
ord(marker.marker_code), marker.name
))
lines = [header] + rows
return '\n'.join(lines)
@classmethod
def from_stream(cls, stream):
"""
Return a |_JfifMarkers| instance containing a |_JfifMarker| subclass
instance for each marker in *stream*.
"""
marker_parser = _MarkerParser.from_stream(stream)
markers = []
for marker in marker_parser.iter_markers():
markers.append(marker)
if marker.marker_code == JPEG_MARKER_CODE.SOS:
break
return cls(markers)
@property
def app0(self):
"""
First APP0 marker in image markers.
"""
for m in self._markers:
if m.marker_code == JPEG_MARKER_CODE.APP0:
return m
raise KeyError('no APP0 marker in image')
@property
def app1(self):
"""
First APP1 marker in image markers.
"""
for m in self._markers:
if m.marker_code == JPEG_MARKER_CODE.APP1:
return m
raise KeyError('no APP1 marker in image')
@property
def sof(self):
"""
First start of frame (SOFn) marker in this sequence.
"""
for m in self._markers:
if m.marker_code in JPEG_MARKER_CODE.SOF_MARKER_CODES:
return m
raise KeyError('no start of frame (SOFn) marker in image')
class _MarkerParser(object):
"""
Service class that knows how to parse a JFIF stream and iterate over its
markers.
"""
def __init__(self, stream_reader):
super(_MarkerParser, self).__init__()
self._stream = stream_reader
@classmethod
def from_stream(cls, stream):
"""
Return a |_MarkerParser| instance to parse JFIF markers from
*stream*.
"""
stream_reader = StreamReader(stream, BIG_ENDIAN)
return cls(stream_reader)
def iter_markers(self):
"""
Generate a (marker_code, segment_offset) 2-tuple for each marker in
the JPEG *stream*, in the order they occur in the stream.
"""
marker_finder = _MarkerFinder.from_stream(self._stream)
start = 0
marker_code = None
while marker_code != JPEG_MARKER_CODE.EOI:
marker_code, segment_offset = marker_finder.next(start)
marker = _MarkerFactory(
marker_code, self._stream, segment_offset
)
yield marker
start = segment_offset + marker.segment_length
class _MarkerFinder(object):
"""
Service class that knows how to find the next JFIF marker in a stream.
"""
def __init__(self, stream):
super(_MarkerFinder, self).__init__()
self._stream = stream
@classmethod
def from_stream(cls, stream):
"""
Return a |_MarkerFinder| instance to find JFIF markers in *stream*.
"""
return cls(stream)
def next(self, start):
"""
Return a (marker_code, segment_offset) 2-tuple identifying and
locating the first marker in *stream* occuring after offset *start*.
The returned *segment_offset* points to the position immediately
following the 2-byte marker code, the start of the marker segment,
for those markers that have a segment.
"""
position = start
while True:
# skip over any non-\xFF bytes
position = self._offset_of_next_ff_byte(start=position)
# skip over any \xFF padding bytes
position, byte_ = self._next_non_ff_byte(start=position+1)
# 'FF 00' sequence is not a marker, start over if found
if byte_ == b'\x00':
continue
# this is a marker, gather return values and break out of scan
marker_code, segment_offset = byte_, position+1
break
return marker_code, segment_offset
def _next_non_ff_byte(self, start):
"""
Return an offset, byte 2-tuple for the next byte in *stream* that is
not '\xFF', starting with the byte at offset *start*. If the byte at
offset *start* is not '\xFF', *start* and the returned *offset* will
be the same.
"""
self._stream.seek(start)
byte_ = self._read_byte()
while byte_ == b'\xFF':
byte_ = self._read_byte()
offset_of_non_ff_byte = self._stream.tell() - 1
return offset_of_non_ff_byte, byte_
def _offset_of_next_ff_byte(self, start):
"""
Return the offset of the next '\xFF' byte in *stream* starting with
the byte at offset *start*. Returns *start* if the byte at that
offset is a hex 255; it does not necessarily advance in the stream.
"""
self._stream.seek(start)
byte_ = self._read_byte()
while byte_ != b'\xFF':
byte_ = self._read_byte()
offset_of_ff_byte = self._stream.tell() - 1
return offset_of_ff_byte
def _read_byte(self):
"""
Return the next byte read from stream. Raise Exception if stream is
at end of file.
"""
byte_ = self._stream.read(1)
if not byte_: # pragma: no cover
raise Exception('unexpected end of file')
return byte_
def _MarkerFactory(marker_code, stream, offset):
"""
Return |_Marker| or subclass instance appropriate for marker at *offset*
in *stream* having *marker_code*.
"""
if marker_code == JPEG_MARKER_CODE.APP0:
marker_cls = _App0Marker
elif marker_code == JPEG_MARKER_CODE.APP1:
marker_cls = _App1Marker
elif marker_code in JPEG_MARKER_CODE.SOF_MARKER_CODES:
marker_cls = _SofMarker
else:
marker_cls = _Marker
return marker_cls.from_stream(stream, marker_code, offset)
class _Marker(object):
"""
Base class for JFIF marker classes. Represents a marker and its segment
occuring in a JPEG byte stream.
"""
def __init__(self, marker_code, offset, segment_length):
super(_Marker, self).__init__()
self._marker_code = marker_code
self._offset = offset
self._segment_length = segment_length
@classmethod
def from_stream(cls, stream, marker_code, offset):
"""
Return a generic |_Marker| instance for the marker at *offset* in
*stream* having *marker_code*.
"""
if JPEG_MARKER_CODE.is_standalone(marker_code):
segment_length = 0
else:
segment_length = stream.read_short(offset)
return cls(marker_code, offset, segment_length)
@property
def marker_code(self):
"""
The single-byte code that identifies the type of this marker, e.g.
``'\xE0'`` for start of image (SOI).
"""
return self._marker_code
@property
def name(self): # pragma: no cover
return JPEG_MARKER_CODE.marker_names[self._marker_code]
@property
def offset(self): # pragma: no cover
return self._offset
@property
def segment_length(self):
"""
The length in bytes of this marker's segment
"""
return self._segment_length
class _App0Marker(_Marker):
"""
Represents a JFIF APP0 marker segment.
"""
def __init__(
self, marker_code, offset, length, density_units, x_density,
y_density):
super(_App0Marker, self).__init__(marker_code, offset, length)
self._density_units = density_units
self._x_density = x_density
self._y_density = y_density
@property
def horz_dpi(self):
"""
Horizontal dots per inch specified in this marker, defaults to 72 if
not specified.
"""
return self._dpi(self._x_density)
@property
def vert_dpi(self):
"""
Vertical dots per inch specified in this marker, defaults to 72 if
not specified.
"""
return self._dpi(self._y_density)
def _dpi(self, density):
"""
Return dots per inch corresponding to *density* value.
"""
if self._density_units == 1:
dpi = density
elif self._density_units == 2:
dpi = int(round(density * 2.54))
else:
dpi = 72
return dpi
@classmethod
def from_stream(cls, stream, marker_code, offset):
"""
Return an |_App0Marker| instance for the APP0 marker at *offset* in
*stream*.
"""
# field off type notes
# ------------------ --- ----- -------------------
# segment length 0 short
# JFIF identifier 2 5 chr 'JFIF\x00'
# major JPEG version 7 byte typically 1
# minor JPEG version 8 byte typically 1 or 2
# density units 9 byte 1=inches, 2=cm
# horz dots per unit 10 short
# vert dots per unit 12 short
# ------------------ --- ----- -------------------
segment_length = stream.read_short(offset)
density_units = stream.read_byte(offset, 9)
x_density = stream.read_short(offset, 10)
y_density = stream.read_short(offset, 12)
return cls(
marker_code, offset, segment_length, density_units, x_density,
y_density
)
class _App1Marker(_Marker):
"""
Represents a JFIF APP1 (Exif) marker segment.
"""
def __init__(self, marker_code, offset, length, horz_dpi, vert_dpi):
super(_App1Marker, self).__init__(marker_code, offset, length)
self._horz_dpi = horz_dpi
self._vert_dpi = vert_dpi
@classmethod
def from_stream(cls, stream, marker_code, offset):
"""
Extract the horizontal and vertical dots-per-inch value from the APP1
header at *offset* in *stream*.
"""
# field off len type notes
# -------------------- --- --- ----- ----------------------------
# segment length 0 2 short
# Exif identifier 2 6 6 chr 'Exif\x00\x00'
# TIFF byte order 8 2 2 chr 'II'=little 'MM'=big endian
# meaning of universe 10 2 2 chr '*\x00' or '\x00*' depending
# IFD0 off fr/II or MM 10 16 long relative to ...?
# -------------------- --- --- ----- ----------------------------
segment_length = stream.read_short(offset)
if cls._is_non_Exif_APP1_segment(stream, offset):
return cls(marker_code, offset, segment_length, 72, 72)
tiff = cls._tiff_from_exif_segment(stream, offset, segment_length)
return cls(
marker_code, offset, segment_length, tiff.horz_dpi, tiff.vert_dpi
)
@property
def horz_dpi(self):
"""
Horizontal dots per inch specified in this marker, defaults to 72 if
not specified.
"""
return self._horz_dpi
@property
def vert_dpi(self):
"""
Vertical dots per inch specified in this marker, defaults to 72 if
not specified.
"""
return self._vert_dpi
@classmethod
def _is_non_Exif_APP1_segment(cls, stream, offset):
"""
Return True if the APP1 segment at *offset* in *stream* is NOT an
Exif segment, as determined by the ``'Exif\x00\x00'`` signature at
offset 2 in the segment.
"""
stream.seek(offset+2)
exif_signature = stream.read(6)
return exif_signature != b'Exif\x00\x00'
@classmethod
def _tiff_from_exif_segment(cls, stream, offset, segment_length):
"""
Return a |Tiff| instance parsed from the Exif APP1 segment of
*segment_length* at *offset* in *stream*.
"""
# wrap full segment in its own stream and feed to Tiff()
stream.seek(offset+8)
segment_bytes = stream.read(segment_length-8)
substream = BytesIO(segment_bytes)
return Tiff.from_stream(substream)
class _SofMarker(_Marker):
"""
Represents a JFIF start of frame (SOFx) marker segment.
"""
def __init__(
self, marker_code, offset, segment_length, px_width, px_height):
super(_SofMarker, self).__init__(marker_code, offset, segment_length)
self._px_width = px_width
self._px_height = px_height
@classmethod
def from_stream(cls, stream, marker_code, offset):
"""
Return an |_SofMarker| instance for the SOFn marker at *offset* in
stream.
"""
# field off type notes
# ------------------ --- ----- ----------------------------
# segment length 0 short
# Data precision 2 byte
# Vertical lines 3 short px_height
# Horizontal lines 5 short px_width
# ------------------ --- ----- ----------------------------
segment_length = stream.read_short(offset)
px_height = stream.read_short(offset, 3)
px_width = stream.read_short(offset, 5)
return cls(marker_code, offset, segment_length, px_width, px_height)
@property
def px_height(self):
"""
Image height in pixels
"""
return self._px_height
@property
def px_width(self):
"""
Image width in pixels
"""
return self._px_width