Fix resample code to round half down

ayshih · ayshih · commit f0fec8bc4d1a · 2026-03-20T00:35:07.000-04:00
The code previously performed the flipping in output space, but the
flipping needs to be in input space because that is where the rounding
to nearest input pixel takes place.  Also, flipping in the vertical
direction already happens if origin='lower', so flip vertically only if
origin='upper'.
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -209,9 +209,15 @@ def _resample(
 
     # When an output pixel falls exactly on the edge between two input pixels, the Agg
     # resampler will use the right input pixel as the nearest neighbor.  We want the
-    # left input pixel to be chosen instead, so we flip the supplied transform.
+    # left input pixel to be chosen instead, so we flip the input data and the supplied
+    # transform.  If origin != 'upper', the transform will already include a flip in the
+    # vertical direction.
     if interpolation == 'nearest':
-        transform += Affine2D().translate(-out.shape[1], -out.shape[0]).scale(-1, -1)
+        transform = Affine2D().translate(-data.shape[1], 0).scale(-1, 1) + transform
+        data = np.flip(data, axis=1)
+        if image_obj.origin == 'upper':
+            transform = Affine2D().translate(0, -data.shape[0]).scale(1, -1) + transform
+            data = np.flip(data, axis=0)
 
     _image.resample(data, out, transform,
                     _interpd_[interpolation],
@@ -220,10 +226,6 @@ def _resample(
                     image_obj.get_filternorm(),
                     image_obj.get_filterrad())
 
-    # Because we flipped the supplied transform, we then flip the output image back.
-    if interpolation == 'nearest':
-        out = np.flip(out, axis=(0, 1))
-
     return out
 
 
