From 1a936ab01500ac1c81a323ef5b6275d90b6f16e4 Mon Sep 17 00:00:00 2001
From: Casey Brooks <casey.brooks@agyn.io>
Date: Thu, 25 Dec 2025 12:01:53 +0000
Subject: [PATCH] fix(colors): upcast integer colormap inputs

---
 lib/matplotlib/colors.py                      |   4 +
 .../tests/test_colormap_integer_inputs.py     | 118 ++++++++++++++++++
 2 files changed, 122 insertions(+)
 create mode 100644 lib/matplotlib/tests/test_colormap_integer_inputs.py

diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index 2d3072da04de..63b55f8ecc09 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -725,6 +725,10 @@ def __call__(self, X, alpha=None, bytes=False):
                 # Avoid converting large positive values to negative integers.
                 np.clip(xa, -1, self.N, out=xa)
                 xa = xa.astype(int)
+        if np.issubdtype(xa.dtype, np.integer):
+            required = np.min_scalar_type(self.N + 2)
+            if not np.can_cast(required, xa.dtype, casting="safe"):
+                xa = xa.astype(np.int64, copy=False)
         # Set the over-range indices before the under-range;
         # otherwise the under-range values get converted to over-range.
         xa[xa > self.N - 1] = self._i_over
diff --git a/lib/matplotlib/tests/test_colormap_integer_inputs.py b/lib/matplotlib/tests/test_colormap_integer_inputs.py
new file mode 100644
index 000000000000..a87ffcaa4c0a
--- /dev/null
+++ b/lib/matplotlib/tests/test_colormap_integer_inputs.py
@@ -0,0 +1,118 @@
+import warnings
+
+import numpy as np
+import numpy.ma as ma
+import numpy.testing as npt
+
+warnings.filterwarnings(
+    "ignore",
+    message=r"^'parseString' deprecated - use 'parse_string'",
+    category=DeprecationWarning,
+)
+warnings.filterwarnings(
+    "ignore",
+    message=r"^'resetCache' deprecated - use 'reset_cache'",
+    category=DeprecationWarning,
+)
+warnings.filterwarnings(
+    "ignore",
+    message=r"^'enablePackrat' deprecated - use 'enable_packrat'",
+    category=DeprecationWarning,
+)
+warnings.filterwarnings(
+    "ignore",
+    message=r"^pkg_resources is deprecated as an API.",
+    category=UserWarning,
+)
+warnings.filterwarnings(
+    "ignore",
+    message=(
+        r"^Deprecated call to "
+        r"`pkg_resources\.declare_namespace\('mpl_toolkits'\)`"
+    ),
+    category=DeprecationWarning,
+)
+
+import matplotlib  # noqa: E402
+
+matplotlib.use("Agg", force=True)
+
+import matplotlib.pyplot as plt  # noqa: E402
+from matplotlib import colors  # noqa: E402
+
+
+def _call_without_deprecation(func, *args, **kwargs):
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        return func(*args, **kwargs)
+
+
+def test_colormap_uint8_empty_no_deprecation():
+    cmap = plt.get_cmap()
+    data = np.empty((0,), dtype=np.uint8)
+    result = _call_without_deprecation(cmap, data)
+    assert result.shape == (0, 4)
+
+
+def test_colormap_uint8_matches_int():
+    cmap = matplotlib.colormaps["viridis"]
+    values_uint8 = np.array([0, 255], dtype=np.uint8)
+    values_int = values_uint8.astype(int)
+
+    result_uint8 = _call_without_deprecation(cmap, values_uint8)
+    result_int = _call_without_deprecation(cmap, values_int)
+
+    npt.assert_allclose(result_uint8, result_int)
+
+
+def test_colormap_wide_unsigned_matches_int():
+    cmap = matplotlib.colormaps["viridis"]
+    dtypes = (np.uint16, np.uint32)
+
+    for dtype in dtypes:
+        values = np.array(
+            [0, 10, np.iinfo(dtype).max],
+            dtype=dtype,
+        )
+        result_unsigned = _call_without_deprecation(cmap, values)
+        result_int = _call_without_deprecation(
+            cmap,
+            values.astype(np.int64),
+        )
+        npt.assert_allclose(result_unsigned, result_int)
+
+
+def test_colormap_masked_uint8_bad_color():
+    cmap = matplotlib.colormaps["viridis"].with_extremes(
+        bad=(1, 0, 0, 1)
+    )
+    data = ma.array([1, 2], mask=[True, False], dtype=np.uint8)
+
+    result = _call_without_deprecation(cmap, data)
+
+    expected_good = _call_without_deprecation(
+        cmap, np.array([2], dtype=int)
+    )[0]
+    npt.assert_array_equal(result[0], cmap.get_bad())
+    npt.assert_array_equal(result[1], expected_good)
+
+
+def test_colormap_signed_indices_over_under():
+    base = colors.ListedColormap(
+        [(0.1, 0.2, 0.3, 1.0), (0.4, 0.5, 0.6, 1.0)], name="test-listed"
+    )
+    cmap = base.with_extremes(
+        under=(1.0, 0.0, 0.0, 1.0),
+        over=(0.0, 1.0, 0.0, 1.0),
+    )
+
+    values = np.array([-1, 0, 256], dtype=np.int16)
+
+    result = _call_without_deprecation(cmap, values)
+
+    good = _call_without_deprecation(
+        cmap, np.array([0], dtype=int)
+    )[0]
+    npt.assert_array_equal(result[0], cmap.get_under())
+    npt.assert_array_equal(result[1], good)
+    npt.assert_array_equal(result[2], cmap.get_over())