detailed comments what is going on

prevent setting the `k[-1]` item

libImaging/Resample.c

@@ -176,12 +176,19 @@ precompute_coeffs(int inSize, int outSize, struct filter *filterp,
         k = &kk[xx * kmax];
         for (x = 0; x < xmax; x++) {
             double w = filterp->filter((x + xmin - center + 0.5) * ss);
+            // We can skip extreme coefficients if they are zeroes.
             if (w == 0) {
+                // Skip from the start.
                 if (x == 0) {
+                    // At next loop `x` will be 0.
                     x -= 1;
+                    // But `w` will not be 0, because it based on `xmin`.
                     xmin += 1;
                     xmax -= 1;
+                    // Prevent setting the `k[-1]` item.
+                    continue;
                 } else if (x == xmax - 1) {
+                    // Truncate the last coefficient for current `xx`.
                     xmax -= 1;
                 }
             }