add a unit test for underflow

Author: hayatoikoma

Tests/test_image_resample.py

@@ -630,7 +630,7 @@ def test_skip_vertical(self, flt: Image.Resampling) -> None:
 
 
 class TestCoreResample16bpc:
-    def test_resampling_clamp(self) -> None:
+    def test_resampling_clamp_overflow(self) -> None:
         # Lanczos weighting during downsampling can push accumulated float sums
         # above 65535. These must be clamped to 65535, not corrupted byte-by-byte.
         ims = {}
@@ -655,3 +655,29 @@ def test_resampling_clamp(self) -> None:
                 assert (
                     value == expected
                 ), f"Pixel ({x}, {y}): expected {expected}, got {value}"
+
+    def test_resampling_clamp_underflow(self) -> None:
+        # Lanczos weighting during downsampling can push accumulated float sums
+        # below 0. These must be clamped to 0, not corrupted byte-by-byte.
+        ims = {}
+        width, height = 100, 10
+        for mode in ("I;16", "F"):
+            # Left half = 65535, right half = 0
+            im = Image.new(mode, (width, height))
+            for y in range(height):
+                for x in range(width // 2):
+                    im.putpixel((x, y), 65535)
+
+            # 5x downsampling with Lanczos creates ~8.7% undershoot at the step edge
+            ims[mode] = im.resize((20, height), Image.Resampling.LANCZOS)
+
+        for y in range(height):
+            for x in range(20):
+                v = ims["F"].getpixel((x, y))
+                assert isinstance(v, float)
+                expected = max(0, min(65535, round(v)))
+
+                value = ims["I;16"].getpixel((x, y))
+                assert (
+                    value == expected
+                ), f"Pixel ({x}, {y}): expected {expected}, got {value}"