Fix accurate interpolator for affine nearest-neighbor

ayshih · ayshih · commit 38d9ed5133d7 · 2026-03-20T00:35:07.000-04:00
The interpolator was previously written to minimize bias with each step,
effectively equivalent to rounding half even.  However, rendering
alignment requires there to be bias equivalent to rounding half up.
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1677,6 +1677,8 @@ def test__resample_valid_output():
 @pytest.mark.parametrize("data, interpolation, expected",
     [(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, 0.2, 0.3, 0.4, 0.5, 0.6]]), mimage.NEAREST,
+      np.array([[0.1, 0.2, 0.2, 0.3, 0.4, 0.4, 0.5, 0.6, 0.6]])),
      (np.array([[0.1, 0.3, 0.2]]), mimage.BILINEAR,
       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,
diff --git a/src/agg_workaround.h b/src/agg_workaround.h
@@ -80,6 +80,7 @@ class accurate_interpolator_affine_nn
     void begin(double x, double y, unsigned len)
     {
         m_len = len - 1;
+        m_cur = 0;
 
         m_stx1 = x;
         m_sty1 = y;
@@ -98,6 +99,7 @@ class accurate_interpolator_affine_nn
     void resynchronize(double xe, double ye, unsigned len)
     {
         m_len = len - 1;
+        m_cur = 0;
 
         m_trans->transform(&xe, &ye);
         m_stx2 = xe * subpixel_scale;
@@ -107,23 +109,30 @@ class accurate_interpolator_affine_nn
     //----------------------------------------------------------------
     void operator++()
     {
-        m_stx1 += (m_stx2 - m_stx1) / m_len;
-        m_sty1 += (m_sty2 - m_sty1) / m_len;
-        m_len--;
+        m_cur++;
     }
 
     //----------------------------------------------------------------
     void coordinates(int* x, int* y) const
     {
         // Truncate instead of round because this interpolator needs to
         // match the definitions for nearest-neighbor interpolation
-        *x = int(m_stx1);
-        *y = int(m_sty1);
+        if (m_cur == m_len)
+        {
+            *x = int(m_stx2);
+            *y = int(m_sty2);
+        }
+        else
+        {
+            // Add a tiny fudge amount to account for numerical precision loss
+            *x = int(m_stx1 + (m_stx2 - m_stx1) * m_cur / m_len + 1e-8);
+            *y = int(m_sty1 + (m_sty2 - m_sty1) * m_cur / m_len + 1e-8);
+        }
     }
 
 private:
     trans_type* m_trans;
-    unsigned m_len;
+    unsigned m_len, m_cur;
     double m_stx1, m_sty1, m_stx2, m_sty2;
 };
 #endif
