add method to create downsampled version of cog (#63)

aiocogeo/cog.py

@@ -269,6 +269,37 @@ async def point(self, x: Union[float, int], y: Union[float, int]) -> Union[np.nd
         return tile[:, xindex, yindex]
 
 
+    async def preview(
+        self,
+        max_size: int = 1024,
+        height: Optional[int] = None,
+        width: Optional[int] = None,
+        resample_method: int = Image.NEAREST
+    ) -> Union[np.ndarray, np.ma.masked_array]:
+        """
+        Create downsampled version of the COG
+
+        https://github.com/cogeotiff/rio-tiler/blob/master/rio_tiler/reader.py#L272-L315
+        """
+        ifd = self.ifds[0]
+        if not height and not width:
+            if max(ifd.ImageHeight.value, ifd.ImageWidth.value) < max_size:
+                height, width = ifd.ImageHeight.value, ifd.ImageWidth.value
+            else:
+                ratio = ifd.ImageHeight.value / ifd.ImageWidth.value
+                if ratio > 1:
+                    height = max_size
+                    width = math.ceil(height / ratio)
+                else:
+                    width = max_size
+                    height = math.ceil(width * ratio)
+        return await self.read(
+            bounds=self.bounds,
+            shape=(width, height),
+            resample_method=resample_method
+        )
+
+
     def create_tile_matrix_set(self, identifier: str = None) -> Dict[str, Any]:
         """Create an OGC TileMatrixSet where each TileMatrix corresponds to an overview"""
         matrices = []

tests/test_cog_reader.py

@@ -371,6 +371,29 @@ async def test_point(create_cog_reader, infile):
             assert np.equal(pt, tile[:,0,0]).all()
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "infile", TEST_DATA[:-1]
+)
+async def test_preview(create_cog_reader, infile):
+    async with create_cog_reader(infile) as cog:
+        profile = cog.profile
+        preview = await cog.preview(max_size=1024)
+
+        src_aspect_ratio = profile['height'] / profile['width']
+        dst_aspect_ratio = preview.shape[-2] / preview.shape[-1]
+        assert pytest.approx(src_aspect_ratio, 0.001) == dst_aspect_ratio
+        assert preview.shape[-2] <= 1024
+        assert preview.shape[-1] <= 1024
+
+
+@pytest.mark.asyncio
+async def test_preview_width_height(create_cog_reader):
+    async with create_cog_reader("https://async-cog-reader-test-data.s3.amazonaws.com/webp_cog.tif") as cog:
+        preview = await cog.preview(width=512, height=512)
+        assert preview.shape == (3, 512, 512)
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "infile", TEST_DATA