Avoid affine code path when scaling will make subpixels visible

ayshih · ayshih · commit 86100aa3900f · 2026-01-08T10:46:54.000-05:00
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1669,6 +1669,8 @@ def test__resample_valid_output():
                  0.54, 0.62, 0.7, 0.78, 0.86, 0.9, 0.9, 0.9, 0.9, 0.9]])),
      (np.array([[0.1, 0.1]]), mimage.BILINEAR, np.full((1, 10), 0.1)),
      # Test at the subpixel level
+     (np.array([[0.1, 0.9]]), mimage.NEAREST,
+      np.concatenate([np.full(512, 0.1), np.full(512, 0.9)]).reshape(1, -1)),
      (np.array([[0.1, 0.9]]), mimage.BILINEAR,
       np.concatenate([np.full(256, 0.1),
                       np.linspace(0.5, 256, 512).astype(int) / 256 * 0.8 + 0.1,
diff --git a/src/_image_wrapper.cpp b/src/_image_wrapper.cpp
@@ -167,12 +167,17 @@ image_resample(py::array input_array,
 
         if (is_affine) {
             convert_trans_affine(transform, params.affine);
-            params.is_affine = true;
-        } else {
+            // If affine parameters will make subpixels visible, treat as nonaffine instead
+            if (params.affine.sx >= agg::image_subpixel_scale / 2 || params.affine.sy >= agg::image_subpixel_scale / 2) {
+                is_affine = false;
+                params.affine = agg::trans_affine();  // reset to identity affine parameters
+            }
+        }
+        if (!is_affine) {
             transform_mesh = _get_transform_mesh(transform, output_array.shape());
             params.transform_mesh = transform_mesh.data();
-            params.is_affine = false;
         }
+        params.is_affine = is_affine;
     }
 
     if (auto resampler =
