add warning when no materialized overview are found

docs/supported_format.md

@@ -0,0 +1,13 @@
+
+`rio-tiler` can work with all raster formats supported by [GDAL](https://gdal.org). That's beeing said, `rio-tiler` works better with data format that supports **partial reading**, like [Cloud Optimized GeoTIFF](http://cogeo.org).
+
+On interesting feature of Cloud Optimized GeoTIFF is the internal overviews which enable fast preview of the data. For example, when using the `COGReader.preview` method, rio-tiler will only fetch the internal overviews instead of the whole data, to be able to contruct the output array. Doing this reduce the amount of data transfert and thus increase the process speed.
+
+### VRT
+
+GDAL's [Virtual format](https://gdal.org/drivers/raster/vrt.html#raster-vrt) is also supported by rio-tiler.
+
+!!! warnings
+    Map Tile reading from VRT might not be efficient if overviews are not present, because GDAL will try to open a lot of files.
+
+![](img/vrt_tile.png)

mkdocs.yml

@@ -30,6 +30,7 @@ nav:
     - Models: 'models.md'
     - Mosaic: 'mosaic.md'
     - Colormaps: 'colormap.md'
+    - Supported file formats: 'supported_format.md'
   - Advanced Topics:
     - Read Polygon-shaped regions: 'advanced/feature.md'
     - Base classes and custom readers: 'advanced/custom_readers.md'

rio_tiler/io/cogeo.py

@@ -18,7 +18,7 @@
 
 from .. import reader
 from ..constants import WEB_MERCATOR_TMS, WGS84_CRS, BBox, Indexes, NoData
-from ..errors import ExpressionMixingWarning, TileOutsideBounds
+from ..errors import ExpressionMixingWarning, NoOverviewWarning, TileOutsideBounds
 from ..expression import apply_expression, parse_expression
 from ..models import ImageData, ImageStatistics, Info
 from ..utils import create_cutline, has_alpha_band, has_mask_band
@@ -109,6 +109,14 @@ def __attrs_post_init__(self):
         if self.colormap is None:
             self._get_colormap()
 
+        if min(
+            self.dataset.width, self.dataset.height
+        ) > 512 and not self.dataset.overviews(1):
+            warnings.warn(
+                "The dataset has no Overviews. rio-tiler performances might be impacted.",
+                NoOverviewWarning,
+            )
+
     def close(self):
         """Close rasterio dataset."""
         if self.filepath:

tests/test_io_cogeo.py

@@ -14,6 +14,7 @@
 from rio_tiler.errors import (
     AlphaBandWarning,
     ExpressionMixingWarning,
+    NoOverviewWarning,
     TileOutsideBounds,
 )
 from rio_tiler.io import COGReader, GCPCOGReader
@@ -29,6 +30,7 @@
 COG_GCPS = os.path.join(PREFIX, "cog_gcps.tif")
 COG_DLINE = os.path.join(PREFIX, "cog_dateline.tif")
 COG_EARTH = os.path.join(PREFIX, "cog_fullearth.tif")
+GEOTIFF = os.path.join(PREFIX, "nocog.tif")
 
 KEY_ALPHA = "hro_sources/colorado/201404_13SED190110_201404_0x1500m_CL_1_alpha.tif"
 COG_ALPHA = os.path.join(PREFIX, "my-bucket", KEY_ALPHA)
@@ -672,3 +674,10 @@ def test_read():
 
         img = cog.read(indexes=(1, 1,))
         assert img.data.shape == (2, 2667, 2658)
+
+
+def test_no_overviews():
+    """Should warns when no overviews are found."""
+    with pytest.warns(NoOverviewWarning):
+        with COGReader(GEOTIFF):
+            pass