img: add _IHDRChunk

Steve Canny · Steve Canny · commit 921afe8ceb15 · 2014-03-01T01:35:30.000-08:00
diff --git a/docx/image/png.py b/docx/image/png.py
@@ -283,6 +283,10 @@ class _Chunk(object):
     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):
         """
@@ -295,13 +299,28 @@ 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*.
         """
-        raise NotImplementedError
+        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):
diff --git a/tests/image/test_png.py b/tests/image/test_png.py
@@ -504,3 +504,22 @@ def phys_chunk_(self, request):
     @pytest.fixture
     def stream_rdr_(self, request):
         return instance_mock(request, StreamReader)
+
+
+class Describe_IHDRChunk(object):
+
+    def it_can_construct_from_a_stream_and_offset(self, from_offset_fixture):
+        stream_rdr, offset, px_width, px_height = from_offset_fixture
+        ihdr_chunk = _IHDRChunk.from_offset(None, stream_rdr, offset)
+        assert isinstance(ihdr_chunk, _IHDRChunk)
+        assert ihdr_chunk.px_width == px_width
+        assert ihdr_chunk.px_height == px_height
+
+    # fixtures -------------------------------------------------------
+
+    @pytest.fixture
+    def from_offset_fixture(self):
+        bytes_ = b'\x00\x00\x00\x2A\x00\x00\x00\x18'
+        stream_rdr = StreamReader(BytesIO(bytes_), BIG_ENDIAN)
+        offset, px_width, px_height = 0, 42, 24
+        return stream_rdr, offset, px_width, px_height
