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Backport PR #24970 on branch v3.7.x (FIX: Handle uint8 indices properly for colormap lookups) #24977

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Merged
merged 1 commit into from
Jan 14, 2023
Merged

Backport PR #24970 on branch v3.7.x (FIX: Handle uint8 indices properly for colormap lookups) #24977

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21 changes: 11 additions & 10 deletions lib/matplotlib/colors.py
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Original file line number Diff line number Diff line change
Expand Up @@ -715,16 +715,17 @@ def __call__(self, X, alpha=None, bytes=False):
if not xa.dtype.isnative:
xa = xa.byteswap().newbyteorder() # Native byteorder is faster.
if xa.dtype.kind == "f":
with np.errstate(invalid="ignore"):
xa *= self.N
# Negative values are out of range, but astype(int) would
# truncate them towards zero.
xa[xa < 0] = -1
# xa == 1 (== N after multiplication) is not out of range.
xa[xa == self.N] = self.N - 1
# Avoid converting large positive values to negative integers.
np.clip(xa, -1, self.N, out=xa)
xa = xa.astype(int)
xa *= self.N
# Negative values are out of range, but astype(int) would
# truncate them towards zero.
xa[xa < 0] = -1
# xa == 1 (== N after multiplication) is not out of range.
xa[xa == self.N] = self.N - 1
# Avoid converting large positive values to negative integers.
np.clip(xa, -1, self.N, out=xa)
with np.errstate(invalid="ignore"):
# We need this cast for unsigned ints as well as floats
xa = xa.astype(int)
# 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
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7 changes: 7 additions & 0 deletions lib/matplotlib/tests/test_colors.py
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Expand Up @@ -30,6 +30,13 @@ def test_create_lookup_table(N, result):
assert_array_almost_equal(mcolors._create_lookup_table(N, data), result)


@pytest.mark.parametrize("dtype", [np.uint8, int, np.float16, float])
def test_index_dtype(dtype):
# We use subtraction in the indexing, so need to verify that uint8 works
cm = mpl.colormaps["viridis"]
assert_array_equal(cm(dtype(0)), cm(0))


def test_resampled():
"""
GitHub issue #6025 pointed to incorrect ListedColormap.resampled;
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