Fix bugs in the calculation of the weight array

ayshih · ayshih · commit 71bb56fde11d · 2026-01-08T10:46:53.000-05:00
diff --git a/extern/agg24-svn/include/agg_image_filters.h b/extern/agg24-svn/include/agg_image_filters.h
@@ -20,6 +20,8 @@
 #ifndef AGG_IMAGE_FILTERS_INCLUDED
 #define AGG_IMAGE_FILTERS_INCLUDED
 
+#define MPL_FIX_IMAGE_FILTER_LUT_BUGS
+
 #include "agg_array.h"
 #include "agg_math.h"
 
@@ -53,6 +55,7 @@ namespace agg
             double r = filter.radius();
             realloc_lut(r);
             unsigned i;
+#ifndef MPL_FIX_IMAGE_FILTER_LUT_BUGS
             unsigned pivot = diameter() << (image_subpixel_shift - 1);
             for(i = 0; i < pivot; i++)
             {
@@ -63,6 +66,17 @@ namespace agg
             }
             unsigned end = (diameter() << image_subpixel_shift) - 1;
             m_weight_array[0] = m_weight_array[end];
+#else
+            unsigned pivot = (diameter() << (image_subpixel_shift - 1)) - 1;
+            for(i = 0; i <= pivot + 1; i++)
+            {
+                double x = double(i) / double(image_subpixel_scale);
+                double y = filter.calc_weight(x);
+                int16 value = iround(y * image_filter_scale);
+                m_weight_array[pivot + i] = value;
+                if(i <= pivot) m_weight_array[pivot - i] = value;
+            }
+#endif
             if(normalization) 
             {
                 normalize();
diff --git a/extern/agg24-svn/src/agg_image_filters.cpp b/extern/agg24-svn/src/agg_image_filters.cpp
@@ -88,6 +88,7 @@ namespace agg
             }
         }
 
+#ifndef MPL_FIX_IMAGE_FILTER_LUT_BUGS
         unsigned pivot = m_diameter << (image_subpixel_shift - 1);
 
         for(i = 0; i < pivot; i++)
@@ -96,6 +97,7 @@ namespace agg
         }
         unsigned end = (diameter() << image_subpixel_shift) - 1;
         m_weight_array[0] = m_weight_array[end];
+#endif
     }
 
 
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1663,19 +1663,26 @@ def test__resample_valid_output():
     [(np.array([[0.1, 0.3, 0.2]]), mimage.NEAREST,
       np.array([[0.1, 0.1, 0.1, 0.3, 0.3, 0.3, 0.3, 0.2, 0.2, 0.2]])),
      (np.array([[0.1, 0.3, 0.2]]), mimage.BILINEAR,
-      np.array([[0.1, 0.1, 0.15078125, 0.21096191, 0.27033691,
-                 0.28476562, 0.2546875, 0.22460938, 0.20002441, 0.20002441]])),
+      np.array([[0.1, 0.1, 0.15, 0.21, 0.27, 0.285, 0.255, 0.225, 0.2, 0.2]])),
+     (np.array([[0.1, 0.9]]), mimage.BILINEAR,
+      np.array([[0.1, 0.1, 0.1, 0.1, 0.1, 0.14, 0.22, 0.3, 0.38, 0.46,
+                 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)),
     ]
 )
 def test_resample_nonaffine(data, interpolation, expected):
-    # Test that equivalent affine and nonaffine transforms resample the same
+    # Test that both affine and nonaffine transforms resample to the correct answer
+
+    # If the array is constant, the tolerance can be tight
+    # Otherwise, the tolerance is limited by the subpixel approach in the agg backend
+    atol = 0 if np.all(data == data.ravel()[0]) else 2e-3
 
     # Create a simple affine transform for scaling the input array
     affine_transform = Affine2D().scale(sx=expected.shape[1] / data.shape[1], sy=1)
 
     affine_result = np.empty_like(expected)
     mimage.resample(data, affine_result, affine_transform, interpolation=interpolation)
-    assert_allclose(affine_result, expected)
+    assert_allclose(affine_result, expected, atol=atol)
 
     # Create a nonaffine version of the same transform
     # by compositing with a nonaffine identity transform
@@ -1684,13 +1691,13 @@ class NonAffineIdentityTransform(Transform):
         output_dims = 2
 
         def inverted(self):
-           return self
+            return self
     nonaffine_transform = NonAffineIdentityTransform() + affine_transform
 
     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=atol)
 
 
 def test_axesimage_get_shape():

Original file line number	Diff line number	Diff line change
`@@ -88,6 +88,7 @@ namespace agg`
`88`	`88`	`}`
`89`	`89`	`}`
`90`	`90`
	`91`	`+#ifndef MPL_FIX_IMAGE_FILTER_LUT_BUGS`
`91`	`92`	`unsigned pivot = m_diameter << (image_subpixel_shift - 1);`
`92`	`93`
`93`	`94`	`for(i = 0; i < pivot; i++)`
`@@ -96,6 +97,7 @@ namespace agg`
`96`	`97`	`}`
`97`	`98`	`unsigned end = (diameter() << image_subpixel_shift) - 1;`
`98`	`99`	`m_weight_array[0] = m_weight_array[end];`
	`100`	`+#endif`
`99`	`101`	`}`
`100`	`102`
`101`	`103`