geotiff: GPU + dask+GPU backend coverage for float16 read (#1941)

Issue #1941 added float16 auto-promotion on read and gated the GPU
GDS path off for (bps=16, sf=float). The eager numpy and dask paths
are covered by test_float16_read_1941.py; the cupy and dask+cupy
paths had no targeted tests. A regression dropping the bps_mismatch
fallback at _backends/gpu.py:357 or the float16 gate in
_gds_chunk_path_available would silently mis-decode half-precision
tiles and ship under existing CI.

Adds 13 tests, all passing on a CUDA host:
- read_geotiff_gpu on stripped + tiled (deflate, uncompressed) float16
- open_geotiff(gpu=True) dispatcher thread-through
- windowed GPU reads on stripped + tiled float16
- open_geotiff(chunks=, gpu=True) and read_geotiff_gpu(chunks=)
- _gds_chunk_path_available structural pin for (bps=16, sf=3) -> False
  plus a sanity check that float32 tiled files still allow GDS
- cross-backend pixel-exact parity (numpy vs GPU, numpy vs dask+GPU,
  dask+numpy vs dask+GPU)
- predictor=3 + float16 GPU round trip

Mutation against bps_mismatch flipped 5 tests red; mutation against
the GDS float16 gate flipped the structural test red.

Author: brendancol

xrspatial/geotiff/tests/test_float16_read_gpu_1941.py

@@ -0,0 +1,333 @@
+"""GPU backend coverage for issue #1941 (float16 read).
+
+#1941 added float16 auto-promotion on read by making
+``tiff_dtype_to_numpy(16, SAMPLE_FORMAT_FLOAT)`` return ``float32`` and
+adding the on-disk ``tiff_storage_dtype`` companion. The eager numpy and
+dask paths are covered by ``test_float16_read_1941.py``; this module
+closes the GPU and dask+GPU coverage gap.
+
+A regression that:
+
+* dropped the ``bps_mismatch`` stripped/odd-bps fallback at
+  ``_backends/gpu.py:357`` would route float16 stripped reads through
+  the tiled GPU decoder and mis-decode the half-precision samples;
+* dropped the ``bps_first == 16 and sample_format == SAMPLE_FORMAT_FLOAT``
+  early-out at ``_backends/gpu.py:791`` in ``_gds_chunk_path_available``
+  would send tiled float16 chunked reads down the kvikIO GDS path and
+  mis-stride the buffer;
+* dropped the entry at ``(16, SAMPLE_FORMAT_FLOAT) -> float32`` in
+  ``tiff_dtype_to_numpy`` would surface as ``ValueError("Unsupported
+  BitsPerSample=16, SampleFormat=3")`` from the GPU read paths.
+
+Every test ships through ``read_geotiff_gpu`` directly or through
+``open_geotiff(..., gpu=True)`` so the dispatcher path is also wired in.
+``cuda-unavailable`` builds skip the suite via the project's standard
+``CUDA_AVAILABLE`` gate.
+"""
+from __future__ import annotations
+
+import importlib.util
+import os
+
+import numpy as np
+import pytest
+import xarray as xr
+
+
+def _gpu_available() -> bool:
+    if importlib.util.find_spec("cupy") is None:
+        return False
+    try:
+        import cupy
+
+        return bool(cupy.cuda.is_available())
+    except Exception:
+        return False
+
+
+_HAS_GPU = _gpu_available()
+pytestmark = pytest.mark.skipif(
+    not _HAS_GPU, reason="cupy + CUDA required for GPU float16 read tests",
+)
+
+
+@pytest.fixture
+def float16_stripped_tif(tmp_path):
+    """Stripped float16 GeoTIFF: triggers the bps_mismatch CPU fallback.
+
+    ``tifffile.imwrite`` without ``tile=`` produces a stripped layout, so
+    the GPU reader hits ``bps_mismatch=True`` (file_dtype.itemsize*8 == 32
+    but bps == 16) and falls back to ``_read_to_array`` on CPU before
+    copying to device.
+    """
+    tifffile = pytest.importorskip("tifffile")
+    arr = np.array(
+        [[0.0, 1.0, 2.0, 3.0],
+         [-1.0, -2.0, -3.0, -4.0],
+         [0.5, 1.5, 2.5, 3.5],
+         [100.0, 200.0, 300.0, 400.0]],
+        dtype=np.float16,
+    )
+    path = tmp_path / "f16_stripped.tif"
+    tifffile.imwrite(str(path), arr, compression=None)
+    return path, arr
+
+
+@pytest.fixture
+def float16_tiled_tif(tmp_path):
+    """Tiled float16 GeoTIFF: triggers the bps_mismatch tiled CPU fallback.
+
+    Tiled with a tile size matching the image, deflate-compressed.
+    ``bps_mismatch`` short-circuits the tiled GPU decode path and routes
+    through the CPU decoder; the GDS path is also gated off via
+    ``_gds_chunk_path_available`` returning False for (bps=16, sf=3).
+    """
+    tifffile = pytest.importorskip("tifffile")
+    arr = np.arange(256, dtype=np.float16).reshape(16, 16)
+    path = tmp_path / "f16_tiled.tif"
+    tifffile.imwrite(
+        str(path), arr, compression="deflate", tile=(16, 16))
+    return path, arr
+
+
+@pytest.fixture
+def float16_tiled_uncompressed_tif(tmp_path):
+    """Tiled uncompressed float16 GeoTIFF.
+
+    Mirrors ``float16_tiled_tif`` but with ``compression=None`` so the
+    tile-decode path is exercised without an extra deflate codec call.
+    Tile size 16 is the smallest tifffile allows.
+    """
+    tifffile = pytest.importorskip("tifffile")
+    arr = np.arange(256, dtype=np.float16).reshape(16, 16)
+    path = tmp_path / "f16_tiled_none.tif"
+    tifffile.imwrite(
+        str(path), arr, compression=None, tile=(16, 16))
+    return path, arr
+
+
+class TestEagerGPUReadFloat16:
+    """``read_geotiff_gpu`` returns float32 for stripped float16 input."""
+
+    def test_read_geotiff_gpu_stripped_returns_float32(
+        self, float16_stripped_tif
+    ):
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_stripped_tif
+        result = read_geotiff_gpu(str(path))
+        assert result.dtype == np.float32, (
+            f"GPU read of float16 must return float32, got {result.dtype}"
+        )
+        np.testing.assert_array_equal(
+            result.data.get(), arr.astype(np.float32))
+
+    def test_read_geotiff_gpu_tiled_returns_float32(
+        self, float16_tiled_tif
+    ):
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_tiled_tif
+        result = read_geotiff_gpu(str(path))
+        assert result.dtype == np.float32
+        np.testing.assert_array_equal(
+            result.data.get(), arr.astype(np.float32))
+
+    def test_read_geotiff_gpu_tiled_uncompressed_returns_float32(
+        self, float16_tiled_uncompressed_tif
+    ):
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_tiled_uncompressed_tif
+        result = read_geotiff_gpu(str(path))
+        assert result.dtype == np.float32
+        np.testing.assert_array_equal(
+            result.data.get(), arr.astype(np.float32))
+
+    def test_open_geotiff_gpu_dispatcher_float16(self, float16_tiled_tif):
+        """``open_geotiff(gpu=True)`` dispatches correctly for float16."""
+        from xrspatial.geotiff import open_geotiff
+
+        path, arr = float16_tiled_tif
+        result = open_geotiff(str(path), gpu=True)
+        assert result.dtype == np.float32
+        np.testing.assert_array_equal(
+            result.data.get(), arr.astype(np.float32))
+
+
+class TestGPUWindowedFloat16:
+    """Windowed GPU reads honour the bps_mismatch fallback path."""
+
+    def test_read_geotiff_gpu_windowed_stripped(self, float16_stripped_tif):
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_stripped_tif
+        result = read_geotiff_gpu(str(path), window=(0, 0, 2, 2))
+        assert result.dtype == np.float32
+        assert result.shape == (2, 2)
+        np.testing.assert_array_equal(
+            result.data.get(), arr[:2, :2].astype(np.float32))
+
+    def test_read_geotiff_gpu_windowed_tiled(self, float16_tiled_tif):
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_tiled_tif
+        result = read_geotiff_gpu(str(path), window=(0, 0, 8, 8))
+        assert result.dtype == np.float32
+        assert result.shape == (8, 8)
+        np.testing.assert_array_equal(
+            result.data.get(), arr[:8, :8].astype(np.float32))
+
+
+class TestDaskGPUFloat16:
+    """``open_geotiff(chunks=, gpu=True)`` decodes float16 correctly."""
+
+    def test_dask_gpu_tiled_float16(self, float16_tiled_tif):
+        from xrspatial.geotiff import open_geotiff
+
+        path, arr = float16_tiled_tif
+        result = open_geotiff(str(path), chunks=8, gpu=True)
+        assert result.dtype == np.float32, (
+            f"dask+GPU read of float16 must return float32, got {result.dtype}"
+        )
+        # Compute the dask array; under dask+cupy, .compute() yields a
+        # cupy-backed DataArray, so the .data.get() step pulls to host.
+        computed = result.compute()
+        np.testing.assert_array_equal(
+            computed.data.get(), arr.astype(np.float32))
+
+    def test_read_geotiff_gpu_chunks_kwarg_float16(self, float16_tiled_tif):
+        """``read_geotiff_gpu(chunks=)`` also routes correctly."""
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        path, arr = float16_tiled_tif
+        result = read_geotiff_gpu(str(path), chunks=8)
+        assert result.dtype == np.float32
+        computed = result.compute()
+        np.testing.assert_array_equal(
+            computed.data.get(), arr.astype(np.float32))
+
+
+class TestGDSPathGatedOffForFloat16:
+    """``_gds_chunk_path_available`` returns False for (bps=16, sf=3).
+
+    Direct structural test of the gating logic added in #1941 to keep the
+    KvikIO GDS chunked path from mis-decoding half-precision tiles. A
+    regression dropping the float16 guard would silently corrupt every
+    chunked GPU read of a float16 source.
+    """
+
+    def test_gds_path_gated_off_for_float16(self, float16_tiled_tif):
+        from xrspatial.geotiff._backends.gpu import _gds_chunk_path_available
+        from xrspatial.geotiff._header import parse_all_ifds, parse_header
+
+        path, _ = float16_tiled_tif
+        with open(str(path), "rb") as f:
+            data = f.read()
+        header = parse_header(data)
+        ifds = parse_all_ifds(data, header)
+        ifd = ifds[0]
+
+        # Sanity-check fixture: tiled, bps=16, sample_format=3 (float)
+        from xrspatial.geotiff._dtypes import SAMPLE_FORMAT_FLOAT
+        assert ifd.is_tiled, "fixture sanity: tiled layout expected"
+        bps = ifd.bits_per_sample
+        if isinstance(bps, tuple):
+            bps = bps[0]
+        assert bps == 16, "fixture sanity: bps=16 expected"
+        assert ifd.sample_format == SAMPLE_FORMAT_FLOAT
+
+        result = _gds_chunk_path_available(
+            str(path), ifd, has_sparse_tile=False, orientation=1)
+        assert result is False, (
+            "_gds_chunk_path_available must return False for "
+            "(bps=16, sf=float) so the GDS chunked path does not "
+            "mis-decode half-precision tiles."
+        )
+
+    def test_gds_path_allowed_for_float32_tiled(self, tmp_path):
+        """Sanity: GDS path remains allowed for a float32 tiled file.
+
+        Pins that the float16 guard at gpu.py:791 fires only on
+        (bps=16, sf=float), not on every tiled float file. A regression
+        widening the guard to all floats would silently disable the
+        GDS path on every float32 tiled COG.
+        """
+        tifffile = pytest.importorskip("tifffile")
+        pytest.importorskip("kvikio")
+
+        arr = np.arange(256, dtype=np.float32).reshape(16, 16)
+        path = tmp_path / "f32_tiled.tif"
+        tifffile.imwrite(
+            str(path), arr, compression="deflate", tile=(16, 16))
+
+        from xrspatial.geotiff._backends.gpu import _gds_chunk_path_available
+        from xrspatial.geotiff._header import parse_all_ifds, parse_header
+
+        with open(str(path), "rb") as f:
+            data = f.read()
+        header = parse_header(data)
+        ifds = parse_all_ifds(data, header)
+
+        result = _gds_chunk_path_available(
+            str(path), ifds[0], has_sparse_tile=False, orientation=1)
+        assert result is True, (
+            "_gds_chunk_path_available must remain True for "
+            "(bps=32, sf=float) tiled files so the kvikio GDS chunk "
+            "path still applies."
+        )
+
+
+class TestBackendParityFloat16:
+    """All four backends agree pixel-exact on float16 input."""
+
+    def test_eager_numpy_equals_gpu(self, float16_tiled_tif):
+        from xrspatial.geotiff import open_geotiff
+
+        path, _ = float16_tiled_tif
+        cpu = open_geotiff(str(path))
+        gpu = open_geotiff(str(path), gpu=True)
+
+        assert cpu.dtype == gpu.dtype == np.float32
+        np.testing.assert_array_equal(np.asarray(cpu), gpu.data.get())
+
+    def test_eager_numpy_equals_dask_gpu(self, float16_tiled_tif):
+        from xrspatial.geotiff import open_geotiff
+
+        path, _ = float16_tiled_tif
+        cpu = open_geotiff(str(path))
+        dask_gpu = open_geotiff(str(path), chunks=8, gpu=True).compute()
+
+        assert cpu.dtype == dask_gpu.dtype == np.float32
+        np.testing.assert_array_equal(
+            np.asarray(cpu), dask_gpu.data.get())
+
+    def test_dask_numpy_equals_dask_gpu(self, float16_tiled_tif):
+        from xrspatial.geotiff import open_geotiff, read_geotiff_dask
+
+        path, _ = float16_tiled_tif
+        dask_cpu = read_geotiff_dask(str(path), chunks=8).compute()
+        dask_gpu = open_geotiff(str(path), chunks=8, gpu=True).compute()
+
+        np.testing.assert_array_equal(
+            np.asarray(dask_cpu), dask_gpu.data.get())
+
+
+class TestPredictor3Float16GPU:
+    """Predictor=3 + float16 on disk also decodes correctly on GPU."""
+
+    def test_predictor3_float16_gpu_round_trip(self, tmp_path):
+        tifffile = pytest.importorskip("tifffile")
+        pytest.importorskip("imagecodecs")  # required for predictor=3
+
+        from xrspatial.geotiff import read_geotiff_gpu
+
+        arr = np.linspace(-1.0, 1.0, 16).astype(np.float16).reshape(4, 4)
+        path = tmp_path / "pred3_f16.tif"
+        tifffile.imwrite(
+            str(path), arr, predictor=3, compression="deflate")
+
+        result = read_geotiff_gpu(str(path))
+        assert result.dtype == np.float32
+        np.testing.assert_array_equal(
+            result.data.get(), arr.astype(np.float32))