Fix T81392: subdivision surface poor shading without limit surface

When the limit surface is disabled OpenSubdiv generates a set of linear
patches which are only C0 continuous, not C1. This makes it impossible to
evaluate derivatives at vertices which, in this mode, are by definition put
at boundaries of patches. Normals are calculated from those derivatives.

Solution is to disable normal calculation and let it be done downstream, as
for other modifiers. This limitation is also the reason that non feature
adaptive subdivision is badly suited for GPU evaluation.

Differential Revision: https://developer.blender.org/D9103

1 files changed, 7 insertions, 4 deletions

diff --git a/source/blender/blenkernel/intern/subdiv_mesh.c b/source/blender/blenkernel/intern/subdiv_mesh.c
index 8f97fb82db7..b0adec4e194 100644
--- a/source/blender/blenkernel/intern/subdiv_mesh.c
+++ b/source/blender/blenkernel/intern/subdiv_mesh.c
@@ -499,7 +499,9 @@ static void subdiv_accumulate_vertex_normal_and_displacement(SubdivMeshContext *
     BKE_subdiv_eval_displacement(subdiv, ptex_face_index, u, v, dPdu, dPdv, D);
     add_v3_v3(subdiv_vert->co, D);
   }
-  ++ctx->accumulated_counters[subdiv_vertex_index];
+  if (ctx->accumulated_counters) {
+    ++ctx->accumulated_counters[subdiv_vertex_index];
+  }
 }
 
 /** \} */
@@ -573,11 +575,11 @@ static void evaluate_vertex_and_apply_displacement_copy(const SubdivMeshContext
                                                         MVert *subdiv_vert)
 {
   const int subdiv_vertex_index = subdiv_vert - ctx->subdiv_mesh->mvert;
-  const float inv_num_accumulated = 1.0f / ctx->accumulated_counters[subdiv_vertex_index];
   /* Displacement is accumulated in subdiv vertex position.
    * Needs to be backed up before copying data from original vertex. */
   float D[3] = {0.0f, 0.0f, 0.0f};
   if (ctx->have_displacement) {
+    const float inv_num_accumulated = 1.0f / ctx->accumulated_counters[subdiv_vertex_index];
     copy_v3_v3(D, subdiv_vert->co);
     mul_v3_fl(D, inv_num_accumulated);
   }
@@ -606,11 +608,11 @@ static void evaluate_vertex_and_apply_displacement_interpolate(
     MVert *subdiv_vert)
 {
   const int subdiv_vertex_index = subdiv_vert - ctx->subdiv_mesh->mvert;
-  const float inv_num_accumulated = 1.0f / ctx->accumulated_counters[subdiv_vertex_index];
   /* Displacement is accumulated in subdiv vertex position.
    * Needs to be backed up before copying data from original vertex. */
   float D[3] = {0.0f, 0.0f, 0.0f};
   if (ctx->have_displacement) {
+    const float inv_num_accumulated = 1.0f / ctx->accumulated_counters[subdiv_vertex_index];
     copy_v3_v3(D, subdiv_vert->co);
     mul_v3_fl(D, inv_num_accumulated);
   }
@@ -621,6 +623,7 @@ static void evaluate_vertex_and_apply_displacement_interpolate(
   add_v3_v3(subdiv_vert->co, D);
   /* Copy normal from accumulated storage. */
   if (ctx->can_evaluate_normals) {
+    const float inv_num_accumulated = 1.0f / ctx->accumulated_counters[subdiv_vertex_index];
     float N[3];
     copy_v3_v3(N, ctx->accumulated_normals[subdiv_vertex_index]);
     mul_v3_fl(N, inv_num_accumulated);
@@ -1213,7 +1216,7 @@ Mesh *BKE_subdiv_to_mesh(Subdiv *subdiv,
   subdiv_context.coarse_mesh = coarse_mesh;
   subdiv_context.subdiv = subdiv;
   subdiv_context.have_displacement = (subdiv->displacement_evaluator != NULL);
-  subdiv_context.can_evaluate_normals = !subdiv_context.have_displacement;
+  subdiv_context.can_evaluate_normals = !subdiv_context.have_displacement && subdiv_context.subdiv->settings.is_adaptive;
   /* Multi-threaded traversal/evaluation. */
   BKE_subdiv_stats_begin(&subdiv->stats, SUBDIV_STATS_SUBDIV_TO_MESH_GEOMETRY);
   SubdivForeachContext foreach_context;