Frame: gate fold refinement by Jacobian

tests/test_frame.c

@@ -193,24 +193,32 @@ int main(void) {
     goto done;
   }
   frame_lens_mesh_destroy(&adaptive_mesh);
-  /* A zero discrete Jacobian is an independent fold/critical-band trigger:
-   * the probe below agrees exactly with linear endpoint interpolation, so the
-   * direction-error criterion cannot request this split. */
-  if (frame_lens_mesh_build_coarse(&adaptive_mesh, width, height, 100, 30.0))
-    goto done;
-  for (size_t i = 0; i < adaptive_mesh.vertex_count; ++i) {
-    adaptive_mesh.vertices[i].traced = 1;
-    adaptive_mesh.vertices[i].status = RAY_ENDPOINT_ESCAPED;
-    adaptive_mesh.vertices[i].n_infinity[0] = 1.0;
-  }
   const RayEndpoint flat_probe = {.n_infinity = {1.0, 0.0, 0.0},
                                   .frequency_ratio = 1.0,
                                   .status = RAY_ENDPOINT_ESCAPED};
+  /* Opposite nonzero discrete-Jacobian signs on the two sides of the shared
+   * diagonal require a sufficiently small magnitude before requesting it. */
+  refine.jacobian_minimum = 10.0;
+  refine.angle_absolute_rad = 3.14159265358979323846;
+  refine.angle_relative = 1e6;
+  if (frame_lens_mesh_build_coarse(&adaptive_mesh, width, height, 100, 30.0))
+    goto done;
+  const double source_directions[4][3] = {
+      {1.0, 0.0, 0.0},
+      {sqrt(0.99), 0.0, 0.1},
+      {sqrt(0.99), 0.0, 0.1},
+      {sqrt(0.98), 0.1, 0.1}};
+  for (size_t i = 0; i < adaptive_mesh.vertex_count; ++i) {
+    adaptive_mesh.vertices[i].traced = 1;
+    adaptive_mesh.vertices[i].status = RAY_ENDPOINT_ESCAPED;
+    memcpy(adaptive_mesh.vertices[i].n_infinity, source_directions[i],
+           sizeof source_directions[i]);
+  }
   if (frame_lens_mesh_prepare_generation(&adaptive_mesh, &refine) != 1 ||
       frame_lens_mesh_install_sample(&adaptive_mesh, 0, &flat_probe) ||
       frame_lens_mesh_finish_generation(&adaptive_mesh, &refine) != 1 ||
       adaptive_mesh.vertex_count != 5 || adaptive_mesh.triangle_count != 4) {
-    fputs("adaptive discrete-Jacobian split regression failed\n", stderr);
+    fputs("adaptive fold-parity split regression failed\n", stderr);
     frame_lens_mesh_destroy(&adaptive_mesh);
     goto done;
   }