GridGeoSampler: change stride of last patch to sample entire ROI (#630)

* Adjust minx/miny with a smaller stride for the last sample per row/col and issue warning

* style and mypy fixes

* black test fix

* Adjust minx/miny with a smaller stride for the last sample per row/col and issue warning

* style and mypy fixes

* black test fix

* single.py: adapt gridgeosampler to sample beyond limit of ROI for a partial patch (to be padded)
test_single.py: add tests for multiple limit cases (see issue #448)

* format for black and flake8

* format for black and flake8

* once again, format for black and flake8

* Revert "Adjust minx/miny with a smaller stride for the last sample per row/col and issue warning"

This reverts commit cb554c6

* adapt unit tests, remove warnings

* flake8: remove warnings import

* Address some comments

* Simplify computation of # rows/cols

* Document this new feature

* Fix size of ceiling symbol

* Simplify tests

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

tests/samplers/test_single.py

@@ -182,9 +182,9 @@ def test_iter(self, sampler: GridGeoSampler) -> None:
             )
 
     def test_len(self, sampler: GridGeoSampler) -> None:
-        rows = ((100 - sampler.size[0]) // sampler.stride[0]) + 1
-        cols = ((100 - sampler.size[1]) // sampler.stride[1]) + 1
-        length = rows * cols * 2
+        rows = math.ceil((100 - sampler.size[0]) / sampler.stride[0]) + 1
+        cols = math.ceil((100 - sampler.size[1]) / sampler.stride[1]) + 1
+        length = rows * cols * 2  # two items in dataset
         assert len(sampler) == length
 
     def test_roi(self, dataset: CustomGeoDataset) -> None:
@@ -194,12 +194,35 @@ def test_roi(self, dataset: CustomGeoDataset) -> None:
             assert query in roi
 
     def test_small_area(self) -> None:
+        ds = CustomGeoDataset()
+        ds.index.insert(0, (0, 1, 0, 1, 0, 1))
+        sampler = GridGeoSampler(ds, 2, 10)
+        assert len(sampler) == 0
+
+    def test_tiles_side_by_side(self) -> None:
         ds = CustomGeoDataset()
         ds.index.insert(0, (0, 10, 0, 10, 0, 10))
-        ds.index.insert(1, (20, 21, 20, 21, 20, 21))
+        ds.index.insert(0, (0, 10, 10, 20, 0, 10))
         sampler = GridGeoSampler(ds, 2, 10)
-        for _ in sampler:
-            continue
+        for bbox in sampler:
+            assert bbox.area > 0
+
+    def test_integer_multiple(self) -> None:
+        ds = CustomGeoDataset()
+        ds.index.insert(0, (0, 10, 0, 10, 0, 10))
+        sampler = GridGeoSampler(ds, 10, 10, units=Units.CRS)
+        iterator = iter(sampler)
+        assert len(sampler) == 1
+        assert next(iterator) == BoundingBox(0, 10, 0, 10, 0, 10)
+
+    def test_float_multiple(self) -> None:
+        ds = CustomGeoDataset()
+        ds.index.insert(0, (0, 6, 0, 5, 0, 10))
+        sampler = GridGeoSampler(ds, 5, 5, units=Units.CRS)
+        iterator = iter(sampler)
+        assert len(sampler) == 2
+        assert next(iterator) == BoundingBox(0, 5, 0, 5, 0, 10)
+        assert next(iterator) == BoundingBox(1, 6, 0, 5, 0, 10)
 
     @pytest.mark.slow
     @pytest.mark.parametrize("num_workers", [0, 1, 2])

torchgeo/samplers/single.py

@@ -4,6 +4,7 @@
 """TorchGeo samplers."""
 
 import abc
+import math
 from typing import Callable, Iterable, Iterator, Optional, Tuple, Union
 
 import torch
@@ -146,7 +147,7 @@ def __len__(self) -> int:
 
 
 class GridGeoSampler(GeoSampler):
-    """Samples elements in a grid-like fashion.
+    r"""Samples elements in a grid-like fashion.
 
     This is particularly useful during evaluation when you want to make predictions for
     an entire region of interest. You want to minimize the amount of redundant
@@ -158,6 +159,21 @@ class GridGeoSampler(GeoSampler):
     The overlap between each chip (``chip_size - stride``) should be approximately equal
     to the `receptive field <https://distill.pub/2019/computing-receptive-fields/>`_ of
     the CNN.
+
+    Note that the stride of the final set of chips in each row/column may be adjusted so
+    that the entire :term:`tile` is sampled without exceeding the bounds of the dataset.
+
+    Let :math:`i` be the size of the input tile. Let :math:`k` be the requested size of
+    the output patch. Let :math:`s` be the requested stride. Let :math:`o` be the number
+    of output rows/columns sampled from each tile. :math:`o` can then be computed as:
+
+    .. math::
+
+       o = \left\lceil \frac{i - k}{s} \right\rceil + 1
+
+    This is almost identical to relationship 5 in
+    https://doi.org/10.48550/arXiv.1603.07285. However, we use ceiling instead of floor
+    because we want to include the final remaining chip.
     """
 
     def __init__(
@@ -200,17 +216,23 @@ def __init__(
         for hit in self.index.intersection(tuple(self.roi), objects=True):
             bounds = BoundingBox(*hit.bounds)
             if (
-                bounds.maxx - bounds.minx > self.size[1]
-                and bounds.maxy - bounds.miny > self.size[0]
+                bounds.maxx - bounds.minx >= self.size[1]
+                and bounds.maxy - bounds.miny >= self.size[0]
             ):
                 self.hits.append(hit)
 
         self.length = 0
         for hit in self.hits:
             bounds = BoundingBox(*hit.bounds)
 
-            rows = int((bounds.maxy - bounds.miny - self.size[0]) // self.stride[0]) + 1
-            cols = int((bounds.maxx - bounds.minx - self.size[1]) // self.stride[1]) + 1
+            rows = (
+                math.ceil((bounds.maxy - bounds.miny - self.size[0]) / self.stride[0])
+                + 1
+            )
+            cols = (
+                math.ceil((bounds.maxx - bounds.minx - self.size[1]) / self.stride[1])
+                + 1
+            )
             self.length += rows * cols
 
     def __iter__(self) -> Iterator[BoundingBox]:
@@ -223,8 +245,14 @@ def __iter__(self) -> Iterator[BoundingBox]:
         for hit in self.hits:
             bounds = BoundingBox(*hit.bounds)
 
-            rows = int((bounds.maxy - bounds.miny - self.size[0]) // self.stride[0]) + 1
-            cols = int((bounds.maxx - bounds.minx - self.size[1]) // self.stride[1]) + 1
+            rows = (
+                math.ceil((bounds.maxy - bounds.miny - self.size[0]) / self.stride[0])
+                + 1
+            )
+            cols = (
+                math.ceil((bounds.maxx - bounds.minx - self.size[1]) / self.stride[1])
+                + 1
+            )
 
             mint = bounds.mint
             maxt = bounds.maxt
@@ -233,11 +261,17 @@ def __iter__(self) -> Iterator[BoundingBox]:
             for i in range(rows):
                 miny = bounds.miny + i * self.stride[0]
                 maxy = miny + self.size[0]
+                if maxy > bounds.maxy:
+                    maxy = bounds.maxy
+                    miny = bounds.maxy - self.size[0]
 
                 # For each column...
                 for j in range(cols):
                     minx = bounds.minx + j * self.stride[1]
                     maxx = minx + self.size[1]
+                    if maxx > bounds.maxx:
+                        maxx = bounds.maxx
+                        minx = bounds.maxx - self.size[1]
 
                     yield BoundingBox(minx, maxx, miny, maxy, mint, maxt)