Fix issue discovered while investigating T39950:

Sculpt mode drawing fails after deleting a subsurf modifier
in sculpt mode and undoing.

This was quite difficult to spot. Main cause was that mesh data was not
synchronized properly between undo and sculpt code because we generated
a pbvh on derivedmesh invalidation without really refreshing the rest of
the data. This could result in undo and drawing operating on different
data.

To solve this and avoid bad level calls I had to move quite some code
around. Crazyspace is now moved to blenkernel, as did some sculpt calls
that make sure sculpt data are properly refreshed.

6 files changed, 759 insertions, 2 deletions

diff --git a/source/blender/blenkernel/BKE_crazyspace.h b/source/blender/blenkernel/BKE_crazyspace.h
new file mode 100644
index 00000000000..89cbd9ec119
--- /dev/null
+++ b/source/blender/blenkernel/BKE_crazyspace.h
@@ -0,0 +1,54 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2008 Blender Foundation.
+ * All rights reserved.
+ *
+ *
+ * Contributor(s): Blender Foundation
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file BKE_crazyspace.h
+ *  \ingroup bke
+ */
+
+#ifndef __BKE_CRAZYSPACE_H__
+#define __BKE_CRAZYSPACE_H__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+struct Scene;
+struct Object;
+struct BMEditMesh;
+struct Mesh;
+
+/* crazyspace.c */
+float (*crazyspace_get_mapped_editverts(struct Scene *scene, struct Object *obedit))[3];
+void crazyspace_set_quats_editmesh(struct BMEditMesh *em, float (*origcos)[3], float (*mappedcos)[3], float (*quats)[4],
+								   const bool use_select);
+void crazyspace_set_quats_mesh(struct Mesh *me, float (*origcos)[3], float (*mappedcos)[3], float (*quats)[4]);
+int sculpt_get_first_deform_matrices(struct Scene *scene, struct Object *ob, float (**deformmats)[3][3], float (**deformcos)[3]);
+void crazyspace_build_sculpt(struct Scene *scene, struct Object *ob, float (**deformmats)[3][3], float (**deformcos)[3]);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/source/blender/blenkernel/BKE_paint.h b/source/blender/blenkernel/BKE_paint.h
index df72cb988b1..cb521382bd3 100644
--- a/source/blender/blenkernel/BKE_paint.h
+++ b/source/blender/blenkernel/BKE_paint.h
@@ -47,6 +47,7 @@ struct Object;
 struct Paint;
 struct PBVH;
 struct Scene;
+struct Sculpt;
 struct StrokeCache;
 struct Tex;
 struct ImagePool;
@@ -176,4 +177,14 @@ void free_sculptsession(struct Object *ob);
 void free_sculptsession_deformMats(struct SculptSession *ss);
 void sculptsession_bm_to_me(struct Object *ob, bool reorder);
 void sculptsession_bm_to_me_for_render(struct Object *object);
+void sculpt_update_mesh_elements(struct Scene *scene, struct Sculpt *sd, struct Object *ob,
+								 bool need_pmap, bool need_mask);
+struct MultiresModifierData *sculpt_multires_active(struct Scene *scene, struct Object *ob);
+int ED_sculpt_mask_layers_ensure(struct Object *ob,
+								  struct MultiresModifierData *mmd);
+
+enum {
+	ED_SCULPT_MASK_LAYER_CALC_VERT = (1 << 0),
+	ED_SCULPT_MASK_LAYER_CALC_LOOP = (1 << 1)
+};
 #endif
diff --git a/source/blender/blenkernel/CMakeLists.txt b/source/blender/blenkernel/CMakeLists.txt
index 749a5eb408d..0caf7d11c55 100644
--- a/source/blender/blenkernel/CMakeLists.txt
+++ b/source/blender/blenkernel/CMakeLists.txt
@@ -78,7 +78,8 @@ set(SRC
 	intern/colortools.c
 	intern/constraint.c
 	intern/context.c
-	intern/curve.c
+        intern/crazyspace.c
+        intern/curve.c
 	intern/customdata.c
 	intern/customdata_file.c
 	intern/deform.c
@@ -191,6 +192,7 @@ set(SRC
 	BKE_colortools.h
 	BKE_constraint.h
 	BKE_context.h
+        BKE_crazyspace.h
 	BKE_curve.h
 	BKE_customdata.h
 	BKE_customdata_file.h
diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 26cc70e1089..228f4ce7730 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -2251,7 +2251,8 @@ static void mesh_build_data(Scene *scene, Object *ob, CustomDataMask dataMask,
 	if ((ob->mode & OB_MODE_SCULPT) && ob->sculpt) {
 		/* create PBVH immediately (would be created on the fly too,
 		 * but this avoids waiting on first stroke) */
-		ob->sculpt->pbvh = ob->derivedFinal->getPBVH(ob, ob->derivedFinal);
+
+		sculpt_update_mesh_elements(scene, scene->toolsettings->sculpt, ob, false, false);
 	}
 
 	BLI_assert(!(ob->derivedFinal->dirty & DM_DIRTY_NORMALS));
diff --git a/source/blender/blenkernel/intern/crazyspace.c b/source/blender/blenkernel/intern/crazyspace.c
new file mode 100644
index 00000000000..610a0fe0fbc
--- /dev/null
+++ b/source/blender/blenkernel/intern/crazyspace.c
@@ -0,0 +1,448 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2005 Blender Foundation.
+ * All rights reserved.
+ *
+ *
+ * Contributor(s): Blender Foundation,
+ *                 Sergey Sharybin
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/blenkernel/intern/crazyspace.c
+ *  \ingroup bke
+ */
+
+
+#include "MEM_guardedalloc.h"
+
+#include "DNA_scene_types.h"
+#include "DNA_object_types.h"
+#include "DNA_modifier_types.h"
+#include "DNA_mesh_types.h"
+#include "DNA_meshdata_types.h"
+
+#include "BLI_utildefines.h"
+#include "BLI_math.h"
+#include "BLI_bitmap.h"
+
+#include "BKE_crazyspace.h"
+#include "BKE_DerivedMesh.h"
+#include "BKE_modifier.h"
+#include "BKE_multires.h"
+#include "BKE_mesh.h"
+#include "BKE_editmesh.h"
+
+typedef struct {
+	float (*vertexcos)[3];
+	BLI_bitmap *vertex_visit;
+} MappedUserData;
+
+BLI_INLINE void tan_calc_quat_v3(
+        float r_quat[4],
+        const float co_1[3], const float co_2[3], const float co_3[3])
+{
+	float vec_u[3], vec_v[3];
+	float nor[3];
+
+	sub_v3_v3v3(vec_u, co_1, co_2);
+	sub_v3_v3v3(vec_v, co_1, co_3);
+
+	cross_v3_v3v3(nor, vec_u, vec_v);
+
+	if (normalize_v3(nor) > FLT_EPSILON) {
+		const float zero_vec[3] = {0.0f};
+		tri_to_quat_ex(r_quat, zero_vec, vec_u, vec_v, nor);
+	}
+	else {
+		unit_qt(r_quat);
+	}
+}
+
+static void set_crazy_vertex_quat(
+        float r_quat[4],
+        const float co_1[3], const float co_2[3], const float co_3[3],
+        const float vd_1[3], const float vd_2[3], const float vd_3[3])
+{
+	float q1[4], q2[4];
+
+	tan_calc_quat_v3(q1, co_1, co_2, co_3);
+	tan_calc_quat_v3(q2, vd_1, vd_2, vd_3);
+
+	sub_qt_qtqt(r_quat, q2, q1);
+}
+
+static void make_vertexcos__mapFunc(void *userData, int index, const float co[3],
+                                    const float UNUSED(no_f[3]), const short UNUSED(no_s[3]))
+{
+	MappedUserData *mappedData = (MappedUserData *)userData;
+
+	if (BLI_BITMAP_GET(mappedData->vertex_visit, index) == 0) {
+		/* we need coord from prototype vertex, not from copies,
+		 * assume they stored in the beginning of vertex array stored in DM
+		 * (mirror modifier for eg does this) */
+		copy_v3_v3(mappedData->vertexcos[index], co);
+		BLI_BITMAP_SET(mappedData->vertex_visit, index);
+	}
+}
+
+static int modifiers_disable_subsurf_temporary(Object *ob)
+{
+	ModifierData *md;
+	int disabled = 0;
+
+	for (md = ob->modifiers.first; md; md = md->next)
+		if (md->type == eModifierType_Subsurf)
+			if (md->mode & eModifierMode_OnCage) {
+				md->mode ^= eModifierMode_DisableTemporary;
+				disabled = 1;
+			}
+
+	return disabled;
+}
+
+/* disable subsurf temporal, get mapped cos, and enable it */
+float (*crazyspace_get_mapped_editverts(Scene *scene, Object *obedit))[3]
+{
+	Mesh *me = obedit->data;
+	DerivedMesh *dm;
+	float (*vertexcos)[3];
+	int nverts = me->edit_btmesh->bm->totvert;
+	BLI_bitmap *vertex_visit;
+	MappedUserData userData;
+
+	/* disable subsurf temporal, get mapped cos, and enable it */
+	if (modifiers_disable_subsurf_temporary(obedit)) {
+		/* need to make new derivemesh */
+		makeDerivedMesh(scene, obedit, me->edit_btmesh, CD_MASK_BAREMESH, 0);
+	}
+
+	/* now get the cage */
+	dm = editbmesh_get_derived_cage(scene, obedit, me->edit_btmesh, CD_MASK_BAREMESH);
+
+	vertexcos = MEM_callocN(sizeof(*vertexcos) * nverts, "vertexcos map");
+	vertex_visit = BLI_BITMAP_NEW(nverts, "vertexcos flags");
+
+	userData.vertexcos = vertexcos;
+	userData.vertex_visit = vertex_visit;
+	dm->foreachMappedVert(dm, make_vertexcos__mapFunc, &userData, DM_FOREACH_NOP);
+
+	dm->release(dm);
+
+	/* set back the flag, no new cage needs to be built, transform does it */
+	modifiers_disable_subsurf_temporary(obedit);
+
+	MEM_freeN(vertex_visit);
+
+	return vertexcos;
+}
+
+void crazyspace_set_quats_editmesh(BMEditMesh *em, float (*origcos)[3], float (*mappedcos)[3], float (*quats)[4],
+                                   const bool use_select)
+{
+	BMFace *f;
+	BMIter iter;
+	int index;
+
+	{
+		BMVert *v;
+		BM_ITER_MESH_INDEX (v, &iter, em->bm, BM_VERTS_OF_MESH, index) {
+			BM_elem_flag_disable(v, BM_ELEM_TAG);
+			BM_elem_index_set(v, index);  /* set_inline */
+		}
+		em->bm->elem_index_dirty &= ~BM_VERT;
+	}
+
+	BM_ITER_MESH (f, &iter, em->bm, BM_FACES_OF_MESH) {
+		BMLoop *l_iter, *l_first;
+
+		l_iter = l_first = BM_FACE_FIRST_LOOP(f);
+		do {
+			if (BM_elem_flag_test(l_iter->v, BM_ELEM_HIDDEN) ||
+			    BM_elem_flag_test(l_iter->v, BM_ELEM_TAG) ||
+			    (use_select && !BM_elem_flag_test(l_iter->v, BM_ELEM_SELECT)))
+			{
+				continue;
+			}
+
+			if (!BM_elem_flag_test(l_iter->v, BM_ELEM_TAG)) {
+				const float *co_prev, *co_curr, *co_next;  /* orig */
+				const float *vd_prev, *vd_curr, *vd_next;  /* deform */
+
+				const int i_prev = BM_elem_index_get(l_iter->prev->v);
+				const int i_curr = BM_elem_index_get(l_iter->v);
+				const int i_next = BM_elem_index_get(l_iter->next->v);
+
+				/* retrieve mapped coordinates */
+				vd_prev = mappedcos[i_prev];
+				vd_curr = mappedcos[i_curr];
+				vd_next = mappedcos[i_next];
+
+				if (origcos) {
+					co_prev = origcos[i_prev];
+					co_curr = origcos[i_curr];
+					co_next = origcos[i_next];
+				}
+				else {
+					co_prev = l_iter->prev->v->co;
+					co_curr = l_iter->v->co;
+					co_next = l_iter->next->v->co;
+				}
+
+				set_crazy_vertex_quat(quats[i_curr],
+				                      co_curr, co_next, co_prev,
+				                      vd_curr, vd_next, vd_prev);
+
+				BM_elem_flag_enable(l_iter->v, BM_ELEM_TAG);
+			}
+		} while ((l_iter = l_iter->next) != l_first);
+	}
+}
+
+void crazyspace_set_quats_mesh(Mesh *me, float (*origcos)[3], float (*mappedcos)[3], float (*quats)[4])
+{
+	int i;
+	MVert *mvert;
+	MLoop *mloop;
+	MPoly *mp;
+
+	mvert = me->mvert;
+	for (i = 0; i < me->totvert; i++, mvert++)
+		mvert->flag &= ~ME_VERT_TMP_TAG;
+
+	/* first store two sets of tangent vectors in vertices, we derive it just from the face-edges */
+	mvert = me->mvert;
+	mp = me->mpoly;
+	mloop = me->mloop;
+
+	for (i = 0; i < me->totpoly; i++, mp++) {
+		MLoop *ml_prev, *ml_curr, *ml_next;
+		int j;
+
+		ml_next = &mloop[mp->loopstart];
+		ml_curr = &ml_next[mp->totloop - 1];
+		ml_prev = &ml_next[mp->totloop - 2];
+
+		for (j = 0; j < mp->totloop; j++) {
+			if ((mvert[ml_curr->v].flag & ME_VERT_TMP_TAG) == 0) {
+				const float *co_prev, *co_curr, *co_next;  /* orig */
+				const float *vd_prev, *vd_curr, *vd_next;  /* deform */
+
+				/* retrieve mapped coordinates */
+				vd_prev = mappedcos[ml_prev->v];
+				vd_curr = mappedcos[ml_curr->v];
+				vd_next = mappedcos[ml_next->v];
+
+				if (origcos) {
+					co_prev = origcos[ml_prev->v];
+					co_curr = origcos[ml_curr->v];
+					co_next = origcos[ml_next->v];
+				}
+				else {
+					co_prev = mvert[ml_prev->v].co;
+					co_curr = mvert[ml_curr->v].co;
+					co_next = mvert[ml_next->v].co;
+				}
+
+				set_crazy_vertex_quat(quats[ml_curr->v],
+				                      co_curr, co_next, co_prev,
+				                      vd_curr, vd_next, vd_prev);
+
+				mvert[ml_curr->v].flag |= ME_VERT_TMP_TAG;
+			}
+
+			ml_prev = ml_curr;
+			ml_curr = ml_next;
+			ml_next++;
+		}
+	}
+}
+
+int editbmesh_get_first_deform_matrices(Scene *scene, Object *ob, BMEditMesh *em, 
+                                        float (**deformmats)[3][3], float (**deformcos)[3])
+{
+	ModifierData *md;
+	DerivedMesh *dm;
+	int i, a, numleft = 0, numVerts = 0;
+	int cageIndex = modifiers_getCageIndex(scene, ob, NULL, 1);
+	float (*defmats)[3][3] = NULL, (*deformedVerts)[3] = NULL;
+	VirtualModifierData virtualModifierData;
+
+	modifiers_clearErrors(ob);
+
+	dm = NULL;
+	md = modifiers_getVirtualModifierList(ob, &virtualModifierData);
+
+	/* compute the deformation matrices and coordinates for the first
+	 * modifiers with on cage editing that are enabled and support computing
+	 * deform matrices */
+	for (i = 0; md && i <= cageIndex; i++, md = md->next) {
+		ModifierTypeInfo *mti = modifierType_getInfo(md->type);
+
+		if (!editbmesh_modifier_is_enabled(scene, md, dm))
+			continue;
+
+		if (mti->type == eModifierTypeType_OnlyDeform && mti->deformMatricesEM) {
+			if (!defmats) {
+				dm = getEditDerivedBMesh(em, ob, NULL);
+				deformedVerts = editbmesh_get_vertex_cos(em, &numVerts);
+				defmats = MEM_mallocN(sizeof(*defmats) * numVerts, "defmats");
+
+				for (a = 0; a < numVerts; a++)
+					unit_m3(defmats[a]);
+			}
+
+			mti->deformMatricesEM(md, ob, em, dm, deformedVerts, defmats,
+			                      numVerts);
+		}
+		else
+			break;
+	}
+
+	for (; md && i <= cageIndex; md = md->next, i++)
+		if (editbmesh_modifier_is_enabled(scene, md, dm) && modifier_isCorrectableDeformed(md))
+			numleft++;
+
+	if (dm)
+		dm->release(dm);
+
+	*deformmats = defmats;
+	*deformcos = deformedVerts;
+
+	return numleft;
+}
+
+int sculpt_get_first_deform_matrices(Scene *scene, Object *ob, float (**deformmats)[3][3], float (**deformcos)[3])
+{
+	ModifierData *md;
+	DerivedMesh *dm;
+	int a, numVerts = 0;
+	float (*defmats)[3][3] = NULL, (*deformedVerts)[3] = NULL;
+	MultiresModifierData *mmd = get_multires_modifier(scene, ob, 0);
+	const bool has_multires = mmd != NULL && mmd->sculptlvl > 0;
+	int numleft = 0;
+	VirtualModifierData virtualModifierData;
+
+	if (has_multires) {
+		*deformmats = NULL;
+		*deformcos = NULL;
+		return numleft;
+	}
+
+	dm = NULL;
+	md = modifiers_getVirtualModifierList(ob, &virtualModifierData);
+
+	for (; md; md = md->next) {
+		ModifierTypeInfo *mti = modifierType_getInfo(md->type);
+
+		if (!modifier_isEnabled(scene, md, eModifierMode_Realtime)) continue;
+
+		if (mti->type == eModifierTypeType_OnlyDeform) {
+			if (!defmats) {
+				Mesh *me = (Mesh *)ob->data;
+				dm = mesh_create_derived(me, NULL);
+				deformedVerts = BKE_mesh_vertexCos_get(me, &numVerts);
+				defmats = MEM_callocN(sizeof(*defmats) * numVerts, "defmats");
+
+				for (a = 0; a < numVerts; a++)
+					unit_m3(defmats[a]);
+			}
+
+			if (mti->deformMatrices) mti->deformMatrices(md, ob, dm, deformedVerts, defmats, numVerts);
+			else break;
+		}
+	}
+
+	for (; md; md = md->next) {
+		ModifierTypeInfo *mti = modifierType_getInfo(md->type);
+
+		if (!modifier_isEnabled(scene, md, eModifierMode_Realtime)) continue;
+
+		if (mti->type == eModifierTypeType_OnlyDeform)
+			numleft++;
+	}
+
+	if (dm)
+		dm->release(dm);
+
+	*deformmats = defmats;
+	*deformcos = deformedVerts;
+
+	return numleft;
+}
+
+void crazyspace_build_sculpt(Scene *scene, Object *ob, float (**deformmats)[3][3], float (**deformcos)[3])
+{
+	int totleft = sculpt_get_first_deform_matrices(scene, ob, deformmats, deformcos);
+
+	if (totleft) {
+		/* there are deformation modifier which doesn't support deformation matrices
+		 * calculation. Need additional crazyspace correction */
+
+		float (*deformedVerts)[3] = *deformcos;
+		float (*origVerts)[3] = MEM_dupallocN(deformedVerts);
+		float (*quats)[4];
+		int i, deformed = 0;
+		VirtualModifierData virtualModifierData;
+		ModifierData *md = modifiers_getVirtualModifierList(ob, &virtualModifierData);
+		Mesh *me = (Mesh *)ob->data;
+
+		for (; md; md = md->next) {
+			ModifierTypeInfo *mti = modifierType_getInfo(md->type);
+
+			if (!modifier_isEnabled(scene, md, eModifierMode_Realtime)) continue;
+
+			if (mti->type == eModifierTypeType_OnlyDeform) {
+				/* skip leading modifiers which have been already
+				 * handled in sculpt_get_first_deform_matrices */
+				if (mti->deformMatrices && !deformed)
+					continue;
+
+				mti->deformVerts(md, ob, NULL, deformedVerts, me->totvert, 0);
+				deformed = 1;
+			}
+		}
+
+		quats = MEM_mallocN(me->totvert * sizeof(*quats), "crazy quats");
+
+		crazyspace_set_quats_mesh(me, origVerts, deformedVerts, quats);
+
+		for (i = 0; i < me->totvert; i++) {
+			float qmat[3][3], tmat[3][3];
+
+			quat_to_mat3(qmat, quats[i]);
+			mul_m3_m3m3(tmat, qmat, (*deformmats)[i]);
+			copy_m3_m3((*deformmats)[i], tmat);
+		}
+
+		MEM_freeN(origVerts);
+		MEM_freeN(quats);
+	}
+
+	if (*deformmats == NULL) {
+		int a, numVerts;
+		Mesh *me = (Mesh *)ob->data;
+
+		*deformcos = BKE_mesh_vertexCos_get(me, &numVerts);
+		*deformmats = MEM_callocN(sizeof(*(*deformmats)) * numVerts, "defmats");
+
+		for (a = 0; a < numVerts; a++)
+			unit_m3((*deformmats)[a]);
+	}
+}
diff --git a/source/blender/blenkernel/intern/paint.c b/source/blender/blenkernel/intern/paint.c
index 1189b5715bb..d5059566dab 100644
--- a/source/blender/blenkernel/intern/paint.c
+++ b/source/blender/blenkernel/intern/paint.c
@@ -37,6 +37,7 @@
 #include "DNA_object_types.h"
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
+#include "DNA_modifier_types.h"
 #include "DNA_scene_types.h"
 #include "DNA_brush_types.h"
 #include "DNA_space_types.h"
@@ -47,10 +48,14 @@
 
 #include "BKE_brush.h"
 #include "BKE_context.h"
+#include "BKE_crazyspace.h"
 #include "BKE_depsgraph.h"
 #include "BKE_global.h"
 #include "BKE_image.h"
+#include "BKE_key.h"
 #include "BKE_library.h"
+#include "BKE_mesh.h"
+#include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_paint.h"
 #include "BKE_pbvh.h"
@@ -499,3 +504,239 @@ void free_sculptsession(Object *ob)
 		ob->sculpt = NULL;
 	}
 }
+
+/* Sculpt mode handles multires differently from regular meshes, but only if
+ * it's the last modifier on the stack and it is not on the first level */
+MultiresModifierData *sculpt_multires_active(Scene *scene, Object *ob)
+{
+	Mesh *me = (Mesh *)ob->data;
+	ModifierData *md;
+	VirtualModifierData virtualModifierData;
+
+	if (ob->sculpt && ob->sculpt->bm) {
+		/* can't combine multires and dynamic topology */
+		return NULL;
+	}
+
+	if (!CustomData_get_layer(&me->ldata, CD_MDISPS)) {
+		/* multires can't work without displacement layer */
+		return NULL;
+	}
+
+	for (md = modifiers_getVirtualModifierList(ob, &virtualModifierData); md; md = md->next) {
+		if (md->type == eModifierType_Multires) {
+			MultiresModifierData *mmd = (MultiresModifierData *)md;
+
+			if (!modifier_isEnabled(scene, md, eModifierMode_Realtime))
+				continue;
+
+			if (mmd->sculptlvl > 0) return mmd;
+			else return NULL;
+		}
+	}
+
+	return NULL;
+}
+
+
+/* Checks if there are any supported deformation modifiers active */
+static bool sculpt_modifiers_active(Scene *scene, Sculpt *sd, Object *ob)
+{
+	ModifierData *md;
+	Mesh *me = (Mesh *)ob->data;
+	MultiresModifierData *mmd = sculpt_multires_active(scene, ob);
+	VirtualModifierData virtualModifierData;
+
+	if (mmd || ob->sculpt->bm)
+		return 0;
+
+	/* non-locked shape keys could be handled in the same way as deformed mesh */
+	if ((ob->shapeflag & OB_SHAPE_LOCK) == 0 && me->key && ob->shapenr)
+		return 1;
+
+	md = modifiers_getVirtualModifierList(ob, &virtualModifierData);
+
+	/* exception for shape keys because we can edit those */
+	for (; md; md = md->next) {
+		ModifierTypeInfo *mti = modifierType_getInfo(md->type);
+		if (!modifier_isEnabled(scene, md, eModifierMode_Realtime)) continue;
+		if (md->type == eModifierType_ShapeKey) continue;
+
+		if (mti->type == eModifierTypeType_OnlyDeform) return 1;
+		else if ((sd->flags & SCULPT_ONLY_DEFORM) == 0) return 1;
+	}
+
+	return 0;
+}
+
+/**
+ * \param need_mask So the DerivedMesh thats returned has mask data
+ */
+void sculpt_update_mesh_elements(Scene *scene, Sculpt *sd, Object *ob,
+								 bool need_pmap, bool need_mask)
+{
+	DerivedMesh *dm;
+	SculptSession *ss = ob->sculpt;
+	Mesh *me = ob->data;
+	MultiresModifierData *mmd = sculpt_multires_active(scene, ob);
+
+	ss->modifiers_active = sculpt_modifiers_active(scene, sd, ob);
+	ss->show_diffuse_color = (sd->flags & SCULPT_SHOW_DIFFUSE) != 0;
+
+	if (need_mask) {
+		if (mmd == NULL) {
+			if (!CustomData_has_layer(&me->vdata, CD_PAINT_MASK)) {
+				ED_sculpt_mask_layers_ensure(ob, NULL);
+			}
+		}
+		else {
+			if (!CustomData_has_layer(&me->ldata, CD_GRID_PAINT_MASK)) {
+#if 1
+				ED_sculpt_mask_layers_ensure(ob, mmd);
+#else				/* if we wanted to support adding mask data while multi-res painting, we would need to do this */
+				if ((ED_sculpt_mask_layers_ensure(ob, mmd) & ED_SCULPT_MASK_LAYER_CALC_LOOP)) {
+					/* remake the derived mesh */
+					ob->recalc |= OB_RECALC_DATA;
+					BKE_object_handle_update(scene, ob);
+				}
+#endif
+			}
+		}
+	}
+
+	/* BMESH ONLY --- at some point we should move sculpt code to use polygons only - but for now it needs tessfaces */
+	BKE_mesh_tessface_ensure(me);
+
+	if (!mmd) ss->kb = BKE_keyblock_from_object(ob);
+	else ss->kb = NULL;
+
+	/* needs to be called after we ensure tessface */
+	dm = mesh_get_derived_final(scene, ob, CD_MASK_BAREMESH);
+
+	if (mmd) {
+		ss->multires = mmd;
+		ss->totvert = dm->getNumVerts(dm);
+		ss->totpoly = dm->getNumPolys(dm);
+		ss->mvert = NULL;
+		ss->mpoly = NULL;
+		ss->mloop = NULL;
+		ss->face_normals = NULL;
+	}
+	else {
+		ss->totvert = me->totvert;
+		ss->totpoly = me->totpoly;
+		ss->mvert = me->mvert;
+		ss->mpoly = me->mpoly;
+		ss->mloop = me->mloop;
+		ss->face_normals = NULL;
+		ss->multires = NULL;
+		ss->vmask = CustomData_get_layer(&me->vdata, CD_PAINT_MASK);
+	}
+
+	ss->pbvh = dm->getPBVH(ob, dm);
+	ss->pmap = (need_pmap && dm->getPolyMap) ? dm->getPolyMap(ob, dm) : NULL;
+
+	pbvh_show_diffuse_color_set(ss->pbvh, ss->show_diffuse_color);
+
+	if (ss->modifiers_active) {
+		if (!ss->orig_cos) {
+			int a;
+
+			free_sculptsession_deformMats(ss);
+
+			ss->orig_cos = (ss->kb) ? BKE_key_convert_to_vertcos(ob, ss->kb) : BKE_mesh_vertexCos_get(me, NULL);
+
+			crazyspace_build_sculpt(scene, ob, &ss->deform_imats, &ss->deform_cos);
+			BKE_pbvh_apply_vertCos(ss->pbvh, ss->deform_cos);
+
+			for (a = 0; a < me->totvert; ++a) {
+				invert_m3(ss->deform_imats[a]);
+			}
+		}
+	}
+	else {
+		free_sculptsession_deformMats(ss);
+	}
+
+	/* if pbvh is deformed, key block is already applied to it */
+	if (ss->kb && !BKE_pbvh_isDeformed(ss->pbvh)) {
+		float (*vertCos)[3] = BKE_key_convert_to_vertcos(ob, ss->kb);
+
+		if (vertCos) {
+			/* apply shape keys coordinates to PBVH */
+			BKE_pbvh_apply_vertCos(ss->pbvh, vertCos);
+			MEM_freeN(vertCos);
+		}
+	}
+}
+
+int ED_sculpt_mask_layers_ensure(Object *ob, MultiresModifierData *mmd)
+{
+	const float *paint_mask;
+	Mesh *me = ob->data;
+	int ret = 0;
+
+	paint_mask = CustomData_get_layer(&me->vdata, CD_PAINT_MASK);
+
+	/* if multires is active, create a grid paint mask layer if there
+	 * isn't one already */
+	if (mmd && !CustomData_has_layer(&me->ldata, CD_GRID_PAINT_MASK)) {
+		GridPaintMask *gmask;
+		int level = max_ii(1, mmd->sculptlvl);
+		int gridsize = BKE_ccg_gridsize(level);
+		int gridarea = gridsize * gridsize;
+		int i, j;
+
+		gmask = CustomData_add_layer(&me->ldata, CD_GRID_PAINT_MASK,
+									 CD_CALLOC, NULL, me->totloop);
+
+		for (i = 0; i < me->totloop; i++) {
+			GridPaintMask *gpm = &gmask[i];
+
+			gpm->level = level;
+			gpm->data = MEM_callocN(sizeof(float) * gridarea,
+									"GridPaintMask.data");
+		}
+
+		/* if vertices already have mask, copy into multires data */
+		if (paint_mask) {
+			for (i = 0; i < me->totpoly; i++) {
+				const MPoly *p = &me->mpoly[i];
+				float avg = 0;
+
+				/* mask center */
+				for (j = 0; j < p->totloop; j++) {
+					const MLoop *l = &me->mloop[p->loopstart + j];
+					avg += paint_mask[l->v];
+				}
+				avg /= (float)p->totloop;
+
+				/* fill in multires mask corner */
+				for (j = 0; j < p->totloop; j++) {
+					GridPaintMask *gpm = &gmask[p->loopstart + j];
+					const MLoop *l = &me->mloop[p->loopstart + j];
+					const MLoop *prev = ME_POLY_LOOP_PREV(me->mloop, p, j);
+					const MLoop *next = ME_POLY_LOOP_NEXT(me->mloop, p, j);
+
+					gpm->data[0] = avg;
+					gpm->data[1] = (paint_mask[l->v] +
+									paint_mask[next->v]) * 0.5f;
+					gpm->data[2] = (paint_mask[l->v] +
+									paint_mask[prev->v]) * 0.5f;
+					gpm->data[3] = paint_mask[l->v];
+				}
+			}
+		}
+
+		ret |= ED_SCULPT_MASK_LAYER_CALC_LOOP;
+	}
+
+	/* create vertex paint mask layer if there isn't one already */
+	if (!paint_mask) {
+		CustomData_add_layer(&me->vdata, CD_PAINT_MASK,
+							 CD_CALLOC, NULL, me->totvert);
+		ret |= ED_SCULPT_MASK_LAYER_CALC_VERT;
+	}
+
+	return ret;
+}