Support differing subpixel alignment for nonaffine distortion table

ayshih · ayshih · commit acbedc617fb5 · 2026-01-08T10:46:54.000-05:00
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1668,6 +1668,11 @@ def test__resample_valid_output():
       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)),
+     # Test at the subpixel level
+     (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,
+                      np.full(256, 0.9)]).reshape(1, -1)),
     ]
 )
 def test_resample_nonaffine(data, interpolation, expected):
diff --git a/src/_image_resample.h b/src/_image_resample.h
@@ -569,23 +569,28 @@ class lookup_distortion
 {
 public:
     lookup_distortion(const double *mesh, int in_width, int in_height,
-                      int out_width, int out_height) :
+                      int out_width, int out_height, bool edge_aligned_subpixels) :
         m_mesh(mesh),
         m_in_width(in_width),
         m_in_height(in_height),
         m_out_width(out_width),
-        m_out_height(out_height)
+        m_out_height(out_height),
+        m_edge_aligned_subpixels(edge_aligned_subpixels)
     {}
 
     void calculate(int* x, int* y) {
         if (m_mesh) {
+            // Nearest-neighbor interpolation needs edge-aligned subpixels
+            // All other interpolation approaches need center-aligned subpixels
+            double offset = m_edge_aligned_subpixels ? 0 : 0.5;
+
             double dx = double(*x) / agg::image_subpixel_scale;
             double dy = double(*y) / agg::image_subpixel_scale;
             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 + 0.5); // round
-                *y = int(coord[1] * agg::image_subpixel_scale + 0.5); // round
+                *x = int(coord[0] * agg::image_subpixel_scale + offset);
+                *y = int(coord[1] * agg::image_subpixel_scale + offset);
             }
         }
     }
@@ -596,6 +601,7 @@ class lookup_distortion
     int m_in_height;
     int m_out_width;
     int m_out_height;
+    bool m_edge_aligned_subpixels;
 };
 
 
@@ -781,7 +787,7 @@ void resample(
             using span_conv_t = agg::span_converter<span_gen_t, span_conv_alpha_t>;
             using nn_renderer_t = agg::renderer_scanline_aa<renderer_t, span_alloc_t, span_conv_t>;
             lookup_distortion dist(
-                params.transform_mesh, in_width, in_height, out_width, out_height);
+                params.transform_mesh, in_width, in_height, out_width, out_height, true);
             arbitrary_interpolator_t interpolator(inverted, dist);
             span_gen_t span_gen(input_accessor, interpolator);
             span_conv_t span_conv(span_gen, conv_alpha);
@@ -806,7 +812,7 @@ void resample(
             using span_conv_t = agg::span_converter<span_gen_t, span_conv_alpha_t>;
             using int_renderer_t = agg::renderer_scanline_aa<renderer_t, span_alloc_t, span_conv_t>;
             lookup_distortion dist(
-                params.transform_mesh, in_width, in_height, out_width, out_height);
+                params.transform_mesh, in_width, in_height, out_width, out_height, false);
             arbitrary_interpolator_t interpolator(inverted, dist);
             span_gen_t span_gen(input_accessor, interpolator, filter);
             span_conv_t span_conv(span_gen, conv_alpha);

Original file line number	Diff line number	Diff line change
`@@ -1668,6 +1668,11 @@ def test__resample_valid_output():`
`1668`	`1668`	`np.array([[0.1, 0.1, 0.1, 0.1, 0.1, 0.14, 0.22, 0.3, 0.38, 0.46,`
`1669`	`1669`	`0.54, 0.62, 0.7, 0.78, 0.86, 0.9, 0.9, 0.9, 0.9, 0.9]])),`
`1670`	`1670`	`(np.array([[0.1, 0.1]]), mimage.BILINEAR, np.full((1, 10), 0.1)),`
	`1671`	`+ # Test at the subpixel level`
	`1672`	`+ (np.array([[0.1, 0.9]]), mimage.BILINEAR,`
	`1673`	`+ np.concatenate([np.full(256, 0.1),`
	`1674`	`+ np.linspace(0.5, 256, 512).astype(int) / 256 * 0.8 + 0.1,`
	`1675`	`+ np.full(256, 0.9)]).reshape(1, -1)),`
`1671`	`1676`	`]`
`1672`	`1677`	`)`
`1673`	`1678`	`def test_resample_nonaffine(data, interpolation, expected):`