Fix Cloth Brush grab deformation mode

The grab mode was not correctly implemented, so the way it was working
was confusing for users.
- Grab delta was calculated in increments from the last stroke position, so it did not match the behavior of a grab brush. I refactored the grab delta calculation to make this change more explicit.
- Grab displacement was not calculated from the original coordinates
- Grab was using an incorrect strength

Grab is now setting the position of the affected vertices directly and
the constraints solve the rest of the cloth. I also tried to implement
an alternative version based on applying forces to move the vertices to
the grab position, but I think this is more controllable and the grab
falloff can be adjusted by tweaking the simulation falloff.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7756

1 files changed, 58 insertions, 43 deletions

diff --git a/source/blender/editors/sculpt_paint/sculpt.c b/source/blender/editors/sculpt_paint/sculpt.c
index 8443608cfa5..c42170695c2 100644
--- a/source/blender/editors/sculpt_paint/sculpt.c
+++ b/source/blender/editors/sculpt_paint/sculpt.c
@@ -2091,9 +2091,13 @@ static float brush_strength(const Sculpt *sd,
     case SCULPT_TOOL_LAYER:
       return alpha * flip * pressure * overlap * feather;
     case SCULPT_TOOL_CLOTH:
-      /* Expand is more sensible to strength as it keeps expanding the cloth when sculpting over
-       * the same vertices. */
-      if (brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_EXPAND) {
+      if (brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_GRAB) {
+        /* Grab deform uses the same falloff as a regular grab brush. */
+        return root_alpha * feather;
+      }
+      else if (brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_EXPAND) {
+        /* Expand is more sensible to strength as it keeps expanding the cloth when sculpting over
+         * the same vertices. */
         return 0.1f * alpha * flip * pressure * overlap * feather;
       }
       else {
@@ -6199,6 +6203,34 @@ static float sculpt_brush_dynamic_size_get(Brush *brush, StrokeCache *cache, flo
   }
 }
 
+/* In these brushes the grab delta is calculated always from the initial stroke location, which is
+ * generally used to create grab deformations. */
+static bool sculpt_needs_delta_from_anchored_origin(Brush *brush)
+{
+  return ELEM(brush->sculpt_tool,
+              SCULPT_TOOL_GRAB,
+              SCULPT_TOOL_POSE,
+              SCULPT_TOOL_THUMB,
+              SCULPT_TOOL_ELASTIC_DEFORM) ||
+         SCULPT_is_cloth_deform_brush(brush);
+}
+
+/* In these brushes the grab delta is calculated from the previous stroke location, which is used
+ * to calculate to orientate the brush tip and deformation towards the stroke direction. */
+static bool sculpt_needs_delta_for_tip_orientation(Brush *brush)
+{
+  if (brush->sculpt_tool == SCULPT_TOOL_CLOTH) {
+    return !SCULPT_is_cloth_deform_brush(brush);
+  }
+  return ELEM(brush->sculpt_tool,
+              SCULPT_TOOL_CLAY_STRIPS,
+              SCULPT_TOOL_PINCH,
+              SCULPT_TOOL_MULTIPLANE_SCRAPE,
+              SCULPT_TOOL_CLAY_THUMB,
+              SCULPT_TOOL_NUDGE,
+              SCULPT_TOOL_SNAKE_HOOK);
+}
+
 static void sculpt_update_brush_delta(UnifiedPaintSettings *ups, Object *ob, Brush *brush)
 {
   SculptSession *ss = ob->sculpt;
@@ -6242,38 +6274,27 @@ static void sculpt_update_brush_delta(UnifiedPaintSettings *ups, Object *ob, Bru
 
     /* Compute delta to move verts by. */
     if (!cache->first_time) {
-      switch (tool) {
-        case SCULPT_TOOL_GRAB:
-        case SCULPT_TOOL_POSE:
-        case SCULPT_TOOL_THUMB:
-        case SCULPT_TOOL_ELASTIC_DEFORM:
-          sub_v3_v3v3(delta, grab_location, cache->old_grab_location);
-          invert_m4_m4(imat, ob->obmat);
-          mul_mat3_m4_v3(imat, delta);
-          add_v3_v3(cache->grab_delta, delta);
-          break;
-        case SCULPT_TOOL_CLAY_STRIPS:
-        case SCULPT_TOOL_PINCH:
-        case SCULPT_TOOL_CLOTH:
-        case SCULPT_TOOL_MULTIPLANE_SCRAPE:
-        case SCULPT_TOOL_CLAY_THUMB:
-        case SCULPT_TOOL_NUDGE:
-        case SCULPT_TOOL_SNAKE_HOOK:
-          if (brush->flag & BRUSH_ANCHORED) {
-            float orig[3];
-            mul_v3_m4v3(orig, ob->obmat, cache->orig_grab_location);
-            sub_v3_v3v3(cache->grab_delta, grab_location, orig);
-          }
-          else {
-            sub_v3_v3v3(cache->grab_delta, grab_location, cache->old_grab_location);
-          }
-          invert_m4_m4(imat, ob->obmat);
-          mul_mat3_m4_v3(imat, cache->grab_delta);
-          break;
-        default:
-          /* Use for 'Brush.topology_rake_factor'. */
+      if (sculpt_needs_delta_from_anchored_origin(brush)) {
+        sub_v3_v3v3(delta, grab_location, cache->old_grab_location);
+        invert_m4_m4(imat, ob->obmat);
+        mul_mat3_m4_v3(imat, delta);
+        add_v3_v3(cache->grab_delta, delta);
+      }
+      else if (sculpt_needs_delta_for_tip_orientation(brush)) {
+        if (brush->flag & BRUSH_ANCHORED) {
+          float orig[3];
+          mul_v3_m4v3(orig, ob->obmat, cache->orig_grab_location);
+          sub_v3_v3v3(cache->grab_delta, grab_location, orig);
+        }
+        else {
           sub_v3_v3v3(cache->grab_delta, grab_location, cache->old_grab_location);
-          break;
+        }
+        invert_m4_m4(imat, ob->obmat);
+        mul_mat3_m4_v3(imat, cache->grab_delta);
+      }
+      else {
+        /* Use for 'Brush.topology_rake_factor'. */
+        sub_v3_v3v3(cache->grab_delta, grab_location, cache->old_grab_location);
       }
     }
     else {
@@ -6294,18 +6315,14 @@ static void sculpt_update_brush_delta(UnifiedPaintSettings *ups, Object *ob, Bru
         copy_v3_v3(cache->anchored_location, cache->true_location);
       }
     }
-    else if (tool == SCULPT_TOOL_ELASTIC_DEFORM) {
+    else if (tool == SCULPT_TOOL_ELASTIC_DEFORM || SCULPT_is_cloth_deform_brush(brush)) {
       copy_v3_v3(cache->anchored_location, cache->true_location);
     }
     else if (tool == SCULPT_TOOL_THUMB) {
       copy_v3_v3(cache->anchored_location, cache->orig_grab_location);
     }
 
-    if (ELEM(tool,
-             SCULPT_TOOL_GRAB,
-             SCULPT_TOOL_THUMB,
-             SCULPT_TOOL_ELASTIC_DEFORM,
-             SCULPT_TOOL_POSE)) {
+    if (sculpt_needs_delta_from_anchored_origin(brush)) {
       /* Location stays the same for finding vertices in brush radius. */
       copy_v3_v3(cache->true_location, cache->orig_grab_location);
 
@@ -6376,9 +6393,7 @@ static void sculpt_update_cache_variants(bContext *C, Sculpt *sd, Object *ob, Po
 
   if (cache->first_time ||
       !((brush->flag & BRUSH_ANCHORED) || (brush->sculpt_tool == SCULPT_TOOL_SNAKE_HOOK) ||
-        (brush->sculpt_tool == SCULPT_TOOL_ROTATE) ||
-        (brush->sculpt_tool == SCULPT_TOOL_CLOTH &&
-         brush->cloth_deform_type == BRUSH_CLOTH_DEFORM_GRAB))) {
+        (brush->sculpt_tool == SCULPT_TOOL_ROTATE) || SCULPT_is_cloth_deform_brush(brush))) {
     RNA_float_get_array(ptr, "location", cache->true_location);
   }