Sculpt: Elastic deform type for Snake Hook

This adds deformation types to snake hook and the elastic deformation
type. This mode deforms the mesh using a kelvinlet instead of applying
the displacement directly inside the brush radius, which is great for
stylized shapes sketching.

Changes in rake rotation when using elastic are too strong when set
to 1, so I'll add a nicer way to support rake rotations with smoother
transitions in the future.

Reviewed By: sergey, JulienKaspar

Differential Revision: https://developer.blender.org/D9560

2 files changed, 67 insertions, 20 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt.c b/source/blender/editors/sculpt_paint/sculpt.c
index b9427677745..fd7ec1da497 100644
--- a/source/blender/editors/sculpt_paint/sculpt.c
+++ b/source/blender/editors/sculpt_paint/sculpt.c
@@ -4232,6 +4232,8 @@ static void do_snake_hook_brush_task_cb_ex(void *__restrict userdata,
                                   (len_v3(grab_delta) / ss->cache->radius)) :
                                  0.0f;
 
+  const bool do_elastic = brush->snake_hook_deform_type == BRUSH_SNAKE_HOOK_DEFORM_ELASTIC;
+
   proxy = BKE_pbvh_node_add_proxy(ss->pbvh, data->nodes[n])->co;
 
   SculptBrushTest test;
@@ -4239,18 +4241,28 @@ static void do_snake_hook_brush_task_cb_ex(void *__restrict userdata,
       ss, &test, data->brush->falloff_shape);
   const int thread_id = BLI_task_parallel_thread_id(tls);
 
+  KelvinletParams params;
+  BKE_kelvinlet_init_params(&params, ss->cache->radius, bstrength, 1.0f, 0.4f);
+
   BKE_pbvh_vertex_iter_begin(ss->pbvh, data->nodes[n], vd, PBVH_ITER_UNIQUE)
   {
-    if (sculpt_brush_test_sq_fn(&test, vd.co)) {
-      const float fade = bstrength * SCULPT_brush_strength_factor(ss,
-                                                                  brush,
-                                                                  vd.co,
-                                                                  sqrtf(test.dist),
-                                                                  vd.no,
-                                                                  vd.fno,
-                                                                  vd.mask ? *vd.mask : 0.0f,
-                                                                  vd.index,
-                                                                  thread_id);
+    if (do_elastic || sculpt_brush_test_sq_fn(&test, vd.co)) {
+
+      float fade;
+      if (do_elastic) {
+        fade = 1.0f;
+      }
+      else {
+        fade = bstrength * SCULPT_brush_strength_factor(ss,
+                                                        brush,
+                                                        vd.co,
+                                                        sqrtf(test.dist),
+                                                        vd.no,
+                                                        vd.fno,
+                                                        vd.mask ? *vd.mask : 0.0f,
+                                                        vd.index,
+                                                        thread_id);
+      }
 
       mul_v3_v3fl(proxy[vd.i], grab_delta, fade);
 
@@ -4289,6 +4301,17 @@ static void do_snake_hook_brush_task_cb_ex(void *__restrict userdata,
         add_v3_v3(proxy[vd.i], delta_rotate);
       }
 
+      if (do_elastic) {
+        float disp[3];
+        BKE_kelvinlet_grab_triscale(disp, &params, vd.co, ss->cache->location, proxy[vd.i]);
+        mul_v3_fl(disp, bstrength * 20.0f);
+        if (vd.mask) {
+          mul_v3_fl(disp, 1.0f - *vd.mask);
+        }
+        mul_v3_fl(disp, SCULPT_automasking_factor_get(ss->cache->automasking, ss, vd.index));
+        copy_v3_v3(proxy[vd.i], disp);
+      }
+
       if (vd.mvert) {
         vd.mvert->flag |= ME_VERT_PBVH_UPDATE;
       }
@@ -5714,16 +5737,8 @@ static void do_brush_action(Sculpt *sd, Object *ob, Brush *brush, UnifiedPaintSe
 
   /* Build a list of all nodes that are potentially within the brush's area of influence */
 
-  /* These brushes need to update all nodes as they are not constrained by the brush radius */
-  /* Elastic deform needs all nodes to avoid artifacts as the effect of the brush is not
-   * constrained by the radius. */
-  /* Pose needs all nodes because it applies all symmetry iterations at the same time and the IK
-   * chain can grow to any area of the model. */
-  /* This can be optimized by filtering the nodes after calculating the chain. */
-  if (ELEM(brush->sculpt_tool,
-           SCULPT_TOOL_ELASTIC_DEFORM,
-           SCULPT_TOOL_POSE,
-           SCULPT_TOOL_BOUNDARY)) {
+  if (SCULPT_tool_needs_all_pbvh_nodes(brush)) {
+    /* These brushes need to update all nodes as they are not constrained by the brush radius */
     BKE_pbvh_search_gather(ss->pbvh, NULL, NULL, &nodes, &totnode);
   }
   else if (brush->sculpt_tool == SCULPT_TOOL_CLOTH) {
diff --git a/source/blender/editors/sculpt_paint/sculpt_intern.h b/source/blender/editors/sculpt_paint/sculpt_intern.h
index 99ee22328ea..3b48207f461 100644
--- a/source/blender/editors/sculpt_paint/sculpt_intern.h
+++ b/source/blender/editors/sculpt_paint/sculpt_intern.h
@@ -434,6 +434,38 @@ BLI_INLINE bool SCULPT_is_cloth_deform_brush(const Brush *brush)
           brush->deform_target == BRUSH_DEFORM_TARGET_CLOTH_SIM);
 }
 
+BLI_INLINE bool SCULPT_tool_needs_all_pbvh_nodes(const Brush *brush)
+{
+  if (brush->sculpt_tool == SCULPT_TOOL_ELASTIC_DEFORM) {
+    /* Elastic deformations in any brush need all nodes to avoid artifacts as the effect
+     * of the Kelvinlet is not constrained by the radius. */
+    return true;
+  }
+
+  if (brush->sculpt_tool == SCULPT_TOOL_POSE) {
+    /* Pose needs all nodes because it applies all symmetry iterations at the same time
+     * and the IK chain can grow to any area of the model. */
+    /* TODO: This can be optimized by filtering the nodes after calculating the chain. */
+    return true;
+  }
+
+  if (brush->sculpt_tool == SCULPT_TOOL_BOUNDARY) {
+    /* Boundary needs all nodes because it is not possible to know where the boundary
+     * deformation is going to be propagated before calculating it. */
+    /* TODO: after calculating the boudnary info in the first iteration, it should be
+     * possible to get the nodes that have vertices included in any boundary deformation
+     * and cache them. */
+    return true;
+  }
+
+  if (brush->sculpt_tool == SCULPT_TOOL_SNAKE_HOOK &&
+      brush->snake_hook_deform_type == BRUSH_SNAKE_HOOK_DEFORM_ELASTIC) {
+    /* Snake hook in elastic deform type has same requirements as the elastic deform tool. */
+    return true;
+  }
+  return false;
+}
+
 /* Pose Brush. */
 void SCULPT_do_pose_brush(struct Sculpt *sd,
                           struct Object *ob,