Ensure mask layers are always present in sculpt mode.

source/blender/editors/include/ED_sculpt.h

@@ -32,6 +32,7 @@
 
 struct ARegion;
 struct bContext;
+struct MultiresModifierData;
 struct Object;
 struct RegionView3D;
 struct wmKeyConfig;
@@ -44,6 +45,8 @@ void sculpt_get_redraw_planes(float planes[4][4], struct ARegion *ar,
 void ED_sculpt_force_update(struct bContext *C);
 float *ED_sculpt_get_last_stroke(struct Object *ob);
 int ED_sculpt_minmax(struct bContext *C, float *min, float *max);
+void ED_sculpt_mask_layers_ensure(struct Object *ob,
+								  struct MultiresModifierData *mmd);
 
 /* paint_ops.c */
 void ED_operatortypes_paint(void);

source/blender/editors/object/object_modifier.c

@@ -76,6 +76,7 @@
 #include "ED_armature.h"
 #include "ED_object.h"
 #include "ED_screen.h"
+#include "ED_sculpt.h"
 #include "ED_mesh.h"
 
 #include "WM_api.h"
@@ -147,9 +148,13 @@ ModifierData *ED_object_modifier_add(ReportList *reports, Main *bmain, Scene *sc
 		}
 		else if (type == eModifierType_Surface)
 			DAG_scene_sort(bmain, scene);
-		else if (type == eModifierType_Multires)
+		else if (type == eModifierType_Multires) {
 			/* set totlvl from existing MDISPS layer if object already had it */
 			multiresModifier_set_levels_from_disps((MultiresModifierData *)new_md, ob);
+
+			/* ensure that grid paint mask layer is created */
+			ED_sculpt_mask_layers_ensure(ob, (MultiresModifierData*)new_md);
+		}
 	}
 
 	DAG_id_tag_update(&ob->id, OB_RECALC_DATA);
@@ -610,6 +615,9 @@ int ED_object_modifier_apply(ReportList *reports, Scene *scene, Object *ob, Modi
 	BLI_remlink(&ob->modifiers, md);
 	modifier_free(md);
 
+	/* ensure mesh paint mask layer remains after applying */
+	ED_sculpt_mask_layers_ensure(ob, NULL);
+
 	return 1;
 }
 

source/blender/editors/sculpt_paint/sculpt.c

@@ -3703,6 +3703,71 @@ static void sculpt_init_session(Scene *scene, Object *ob)
 	sculpt_update_mesh_elements(scene, scene->toolsettings->sculpt, ob, 0);
 }
 
+void ED_sculpt_mask_layers_ensure(Object *ob, MultiresModifierData *mmd)
+{
+	float *paint_mask;
+	Mesh *me = ob->data;
+
+	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 = MAX2(1, mmd->sculptlvl);
+		int gridsize = 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];
+				}
+			}
+		}
+	}
+
+	/* 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);
+	}
+}
+
 static int sculpt_toggle_mode(bContext *C, wmOperator *UNUSED(op))
 {
 	Scene *scene = CTX_data_scene(C);
@@ -3749,6 +3814,9 @@ static int sculpt_toggle_mode(bContext *C, wmOperator *UNUSED(op))
 
 		sculpt_init_session(scene, ob);
 
+		/* Mask layer is required */
+		ED_sculpt_mask_layers_ensure(ob, mmd);
+
 		paint_init(&ts->sculpt->paint, PAINT_CURSOR_SCULPT);
 		
 		paint_cursor_start(C, sculpt_poll);