Fix bug with using the distortion table for nonaffine transformations

ayshih · ayshih · commit d2fa1e5ba3a7 · 2025-06-18T00:20:54.000-04:00
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1675,7 +1675,7 @@ def inverted(self):
     nonaffine_result = np.empty_like(expected)
     mimage.resample(data, nonaffine_result, nonaffine_transform,
                     interpolation=interpolation)
-    assert_allclose(nonaffine_result, expected, atol=5e-3)
+    assert_allclose(nonaffine_result, expected, atol=2e-3)
 
 
 def test_axesimage_get_shape():
diff --git a/src/_image_resample.h b/src/_image_resample.h
@@ -584,8 +584,8 @@ class lookup_distortion
             if (dx >= 0 && dx < m_out_width &&
                 dy >= 0 && dy < m_out_height) {
                 const double *coord = m_mesh + (int(dy) * m_out_width + int(dx)) * 2;
-                *x = int(coord[0] * agg::image_subpixel_scale);
-                *y = int(coord[1] * agg::image_subpixel_scale);
+                *x = int(coord[0] * agg::image_subpixel_scale + 0.5); // round
+                *y = int(coord[1] * agg::image_subpixel_scale + 0.5); // round
             }
         }
     }

Original file line number	Diff line number	Diff line change
`@@ -584,8 +584,8 @@ class lookup_distortion`
`584`	`584`	`if (dx >= 0 && dx < m_out_width &&`
`585`	`585`	`dy >= 0 && dy < m_out_height) {`
`586`	`586`	`const double coord = m_mesh + (int(dy) m_out_width + int(dx)) * 2;`
`587`		`- x = int(coord[0] agg::image_subpixel_scale);`
`588`		`- y = int(coord[1] agg::image_subpixel_scale);`
	`587`	`+ x = int(coord[0] agg::image_subpixel_scale + 0.5); // round`
	`588`	`+ y = int(coord[1] agg::image_subpixel_scale + 0.5); // round`
`589`	`589`	`}`
`590`	`590`	`}`
`591`	`591`	`}`