img: add _TiffParser.horz_dpi and .vert_dpi

Steve Canny · Steve Canny · commit fdeb44f042c6 · 2014-02-27T12:08:36.000-08:00
diff --git a/docx/image/tiff.py b/docx/image/tiff.py
@@ -53,7 +53,16 @@ def horz_dpi(self):
         and ResolutionUnit tags of the IFD; defaults to 72 if those tags are
         not present.
         """
-        raise NotImplementedError
+        return self._dpi(TIFF_TAG.X_RESOLUTION)
+
+    @property
+    def vert_dpi(self):
+        """
+        The vertical dots per inch value calculated from the XResolution and
+        ResolutionUnit tags of the IFD; defaults to 72 if those tags are not
+        present.
+        """
+        return self._dpi(TIFF_TAG.Y_RESOLUTION)
 
     @property
     def px_height(self):
@@ -73,15 +82,6 @@ def px_width(self):
         """
         return self._ifd_entries.get(TIFF_TAG.IMAGE_WIDTH)
 
-    @property
-    def vert_dpi(self):
-        """
-        The vertical dots per inch value calculated from the XResolution and
-        ResolutionUnit tags of the IFD; defaults to 72 if those tags are not
-        present.
-        """
-        raise NotImplementedError
-
     @classmethod
     def _detect_endian(cls, stream):
         """
@@ -92,6 +92,23 @@ def _detect_endian(cls, stream):
         endian_str = stream.read(2)
         return BIG_ENDIAN if endian_str == b'MM' else LITTLE_ENDIAN
 
+    def _dpi(self, resolution_tag):
+        """
+        Return the dpi value calculated for *resolution_tag*, which can be
+        either TIFF_TAG.X_RESOLUTION or TIFF_TAG.Y_RESOLUTION. The
+        calculation is based on the values of both that tag and the
+        TIFF_TAG.RESOLUTION_UNIT tag in this parser's |_IfdEntries| instance.
+        """
+        if resolution_tag not in self._ifd_entries:
+            return 72
+        resolution_unit = self._ifd_entries[TIFF_TAG.RESOLUTION_UNIT]
+        if resolution_unit == 1:  # aspect ratio only
+            return 72
+        # resolution_unit == 2 for inches, 3 for centimeters
+        units_per_inch = 1 if resolution_unit == 2 else 2.54
+        dots_per_unit = self._ifd_entries[resolution_tag]
+        return int(round(dots_per_unit * units_per_inch))
+
     @classmethod
     def _make_stream_reader(cls, stream):
         """
@@ -112,6 +129,18 @@ def __init__(self, entries):
         super(_IfdEntries, self).__init__()
         self._entries = entries
 
+    def __contains__(self, key):
+        """
+        Provides ``in`` operator, e.g. ``tag in ifd_entries``
+        """
+        return self._entries.__contains__(key)
+
+    def __getitem__(self, key):
+        """
+        Provides indexed access, e.g. ``tag_value = ifd_entries[tag_code]``
+        """
+        return self._entries.__getitem__(key)
+
     @classmethod
     def from_stream(cls, stream, offset):
         """
diff --git a/tests/image/test_tiff.py b/tests/image/test_tiff.py
@@ -110,8 +110,32 @@ def it_knows_image_width_and_height_after_parsing(self):
         assert tiff_parser.px_width == px_width
         assert tiff_parser.px_height == px_height
 
+    def it_knows_the_horz_and_vert_dpi_after_parsing(self, dpi_fixture):
+        tiff_parser, expected_horz_dpi, expected_vert_dpi = dpi_fixture
+        assert tiff_parser.horz_dpi == expected_horz_dpi
+        assert tiff_parser.vert_dpi == expected_vert_dpi
+
     # fixtures -------------------------------------------------------
 
+    @pytest.fixture(params=[
+        (1,  150,  240,  72,  72),
+        (2,   42,   24,  42,  24),
+        (3,  100,  200, 254, 508),
+        (6, None, None,  72,  72),
+    ])
+    def dpi_fixture(self, request):
+        resolution_unit, x_resolution, y_resolution = request.param[:3]
+        expected_horz_dpi, expected_vert_dpi = request.param[3:]
+
+        entries = {TIFF_TAG.RESOLUTION_UNIT: resolution_unit}
+        if x_resolution is not None:
+            entries[TIFF_TAG.X_RESOLUTION] = x_resolution
+        if y_resolution is not None:
+            entries[TIFF_TAG.Y_RESOLUTION] = y_resolution
+
+        tiff_parser = _TiffParser(entries)
+        return tiff_parser, expected_horz_dpi, expected_vert_dpi
+
     @pytest.fixture
     def from_stream_fixture(
             self, stream_, _make_stream_reader_, _IfdEntries_, ifd0_offset_,
