Cycles: Stitching of subdivided and displaced meshes

This patch stitches the vertices along patch edges so that cracks can
no longer form when applying subdivision or displacement a mesh.

Subpatches are now formed in a way that ensures vertex indices along
subpatch edges are equal for adjacent subpatches. A mapping of vertices
along patch edges is built to preform stitching. Overall performance is
roughly the same, some gains were made in splitting, but some was lost
in stitching.

This fixes:
- T49049 (cracks between patches from material and uv seams)
- T49048 (discontinuous normals with true displacement)

Reviewers: sergey, brecht

Differential Revision: https://developer.blender.org/D3692

1 files changed, 73 insertions, 2 deletions

diff --git a/intern/cycles/render/mesh_displace.cpp b/intern/cycles/render/mesh_displace.cpp
index 5ae9348d83e..6a6c2fbb3eb 100644
--- a/intern/cycles/render/mesh_displace.cpp
+++ b/intern/cycles/render/mesh_displace.cpp
@@ -22,7 +22,9 @@
 #include "render/shader.h"
 
 #include "util/util_foreach.h"
+#include "util/util_map.h"
 #include "util/util_progress.h"
+#include "util/util_set.h"
 
 CCL_NAMESPACE_BEGIN
 
@@ -184,6 +186,38 @@ bool MeshManager::displace(
 
   d_output.free();
 
+  /* stitch */
+  unordered_set<int> stitch_keys;
+  for (pair<int, int> i : mesh->vert_to_stitching_key_map) {
+    stitch_keys.insert(i.second); /* stitching index */
+  }
+
+  typedef unordered_multimap<int, int>::iterator map_it_t;
+
+  for (int key : stitch_keys) {
+    pair<map_it_t, map_it_t> verts = mesh->vert_stitching_map.equal_range(key);
+
+    float3 pos = make_float3(0.0f, 0.0f, 0.0f);
+    int num = 0;
+
+    for (map_it_t v = verts.first; v != verts.second; ++v) {
+      int vert = v->second;
+
+      pos += mesh->verts[vert];
+      num++;
+    }
+
+    if (num <= 1) {
+      continue;
+    }
+
+    pos *= 1.0f / num;
+
+    for (map_it_t v = verts.first; v != verts.second; ++v) {
+      mesh->verts[v->second] = pos;
+    }
+  }
+
   /* for displacement method both, we only need to recompute the face
    * normals, as bump mapping in the shader will already alter the
    * vertex normal, so we start from the non-displaced vertex normals
@@ -238,7 +272,25 @@ bool MeshManager::displace(
     for (size_t i = 0; i < num_triangles; i++) {
       if (tri_has_true_disp[i]) {
         for (size_t j = 0; j < 3; j++) {
-          vN[mesh->get_triangle(i).v[j]] += fN[i];
+          int vert = mesh->get_triangle(i).v[j];
+          vN[vert] += fN[i];
+
+          /* add face normals to stitched vertices */
+          if (stitch_keys.size()) {
+            map_it_t key = mesh->vert_to_stitching_key_map.find(vert);
+
+            if (key != mesh->vert_to_stitching_key_map.end()) {
+              pair<map_it_t, map_it_t> verts = mesh->vert_stitching_map.equal_range(key->second);
+
+              for (map_it_t v = verts.first; v != verts.second; ++v) {
+                if (v->second == vert) {
+                  continue;
+                }
+
+                vN[v->second] += fN[i];
+              }
+            }
+          }
         }
       }
     }
@@ -289,8 +341,27 @@ bool MeshManager::displace(
         for (size_t i = 0; i < num_triangles; i++) {
           if (tri_has_true_disp[i]) {
             for (size_t j = 0; j < 3; j++) {
+              int vert = mesh->get_triangle(i).v[j];
               float3 fN = compute_face_normal(mesh->get_triangle(i), mP);
-              mN[mesh->get_triangle(i).v[j]] += fN;
+              mN[vert] += fN;
+
+              /* add face normals to stitched vertices */
+              if (stitch_keys.size()) {
+                map_it_t key = mesh->vert_to_stitching_key_map.find(vert);
+
+                if (key != mesh->vert_to_stitching_key_map.end()) {
+                  pair<map_it_t, map_it_t> verts = mesh->vert_stitching_map.equal_range(
+                      key->second);
+
+                  for (map_it_t v = verts.first; v != verts.second; ++v) {
+                    if (v->second == vert) {
+                      continue;
+                    }
+
+                    mN[v->second] += fN;
+                  }
+                }
+              }
             }
           }
         }