new parameter in assign_material() to specify where material shall be assigned: object, obdata, by userpref(default) (as discussed with ideasman_42)

source/blender/blenkernel/BKE_material.h

@@ -67,10 +67,16 @@ short *give_totcolp(struct Object *ob);
 struct Material ***give_matarar_id(struct ID *id); /* same but for ID's */
 short *give_totcolp_id(struct ID *id);
 
+enum {
+	BKE_MAT_ASSIGN_USERPREF,
+	BKE_MAT_ASSIGN_OBDATA,
+	BKE_MAT_ASSIGN_OBJECT
+};
+
 struct Material *give_current_material(struct Object *ob, short act);
 struct ID *material_from(struct Object *ob, short act);
 void assign_material_id(struct ID *id, struct Material *ma, short act);
-void assign_material(struct Object *ob, struct Material *ma, short act);
+void assign_material(struct Object *ob, struct Material *ma, short act, int assign_type);
 void assign_matarar(struct Object *ob, struct Material ***matar, short totcol);
 
 short find_material_index(struct Object *ob, struct Material *ma);

source/blender/blenkernel/intern/material.c

@@ -774,11 +774,12 @@ void assign_material_id(ID *id, Material *ma, short act)
 	test_object_materials(id);
 }
 
-void assign_material(Object *ob, Material *ma, short act)
+void assign_material(Object *ob, Material *ma, short act, int assign_type)
 {
 	Material *mao, **matar, ***matarar;
 	char *matbits;
 	short *totcolp;
+	char bit=0;
 
 	if (act > MAXMAT) return;
 	if (act < 1) act = 1;
@@ -805,8 +806,29 @@ void assign_material(Object *ob, Material *ma, short act)
 		*matarar = matar;
 		*totcolp = act;
 	}
-	
+
+	// Determine the object/mesh linking
+	if (assign_type == BKE_MAT_ASSIGN_USERPREF && ob->actcol) {
+		/* copy from previous material */
+		bit = ob->matbits[ob->actcol - 1];
+	}
+	else {
+		switch(assign_type) {
+			case BKE_MAT_ASSIGN_OBDATA:
+				bit = 0;
+				break;
+			case BKE_MAT_ASSIGN_OBJECT:
+				bit = 1;
+				break;
+			case BKE_MAT_ASSIGN_USERPREF:
+			default:
+				bit = (U.flag & USER_MAT_ON_OB) ? 1 : 0;
+				break;
+		}
+	}
+
 	if (act > ob->totcol) {
+		/* Need more space in the material arrays */
 		matar = MEM_callocN(sizeof(void *) * act, "matarray2");
 		matbits = MEM_callocN(sizeof(char) * act, "matbits1");
 		if (ob->totcol) {
@@ -818,17 +840,12 @@ void assign_material(Object *ob, Material *ma, short act)
 		ob->mat = matar;
 		ob->matbits = matbits;
 		ob->totcol = act;
-
-		/* copy object/mesh linking, or assign based on userpref */
-		if (ob->actcol)
-			ob->matbits[act - 1] = ob->matbits[ob->actcol - 1];
-		else
-			ob->matbits[act - 1] = (U.flag & USER_MAT_ON_OB) ? 1 : 0;
 	}
 	
 	/* do it */
 
-	if (ob->matbits[act - 1]) {   /* in object */
+	ob->matbits[act - 1] = bit;
+	if (bit == 1) {   /* in object */
 		mao = ob->mat[act - 1];
 		if (mao) mao->id.us--;
 		ob->mat[act - 1] = ma;
@@ -854,7 +871,7 @@ void assign_matarar(struct Object *ob, struct Material ***matar, short totcol)
 
 	/* now we have the right number of slots */
 	for (i = 0; i < totcol; i++)
-		assign_material(ob, (*matar)[i], i + 1);
+		assign_material(ob, (*matar)[i], i + 1, BKE_MAT_ASSIGN_USERPREF);
 
 	if (actcol_orig > ob->totcol)
 		actcol_orig = ob->totcol;
@@ -888,7 +905,7 @@ int object_add_material_slot(Object *ob)
 	if (ob == NULL) return FALSE;
 	if (ob->totcol >= MAXMAT) return FALSE;
 	
-	assign_material(ob, NULL, ob->totcol + 1);
+	assign_material(ob, NULL, ob->totcol + 1, BKE_MAT_ASSIGN_USERPREF);
 	ob->actcol = ob->totcol;
 	return TRUE;
 }

source/blender/editors/object/object_relations.c

@@ -1368,7 +1368,7 @@ static int make_links_data_exec(bContext *C, wmOperator *op)
 						/* new approach, using functions from kernel */
 						for (a = 0; a < ob_src->totcol; a++) {
 							Material *ma = give_current_material(ob_src, a + 1);
-							assign_material(ob_dst, ma, a + 1); /* also works with ma==NULL */
+							assign_material(ob_dst, ma, a + 1, BKE_MAT_ASSIGN_USERPREF); /* also works with ma==NULL */
 						}
 						break;
 					case MAKE_LINKS_ANIMDATA:
@@ -1692,7 +1692,7 @@ static void single_mat_users(Scene *scene, int flag, int do_textures)
 						BKE_copy_animdata_id_action(&man->id);
 						
 						man->id.us = 0;
-						assign_material(ob, man, a);
+						assign_material(ob, man, a, BKE_MAT_ASSIGN_USERPREF);
 
 						if (do_textures) {
 							for (b = 0; b < MAX_MTEX; b++) {
@@ -2044,7 +2044,7 @@ static int drop_named_material_invoke(bContext *C, wmOperator *op, wmEvent *even
 	if (base == NULL || ma == NULL)
 		return OPERATOR_CANCELLED;
 	
-	assign_material(base->object, ma, 1);
+	assign_material(base->object, ma, 1, BKE_MAT_ASSIGN_USERPREF);
 	
 	DAG_ids_flush_update(bmain, 0);
 	WM_event_add_notifier(C, NC_SPACE | ND_SPACE_VIEW3D, CTX_wm_view3d(C));

source/blender/editors/space_outliner/outliner_edit.c

@@ -1851,7 +1851,7 @@ static int material_drop_invoke(bContext *C, wmOperator *op, wmEvent *event)
 			return OPERATOR_CANCELLED;
 		}
 
-		assign_material(ob, ma, ob->totcol + 1);
+		assign_material(ob, ma, ob->totcol + 1, BKE_MAT_ASSIGN_USERPREF);
 
 		DAG_ids_flush_update(bmain, 0);
 		WM_event_add_notifier(C, NC_SPACE | ND_SPACE_VIEW3D, CTX_wm_view3d(C));		

source/blender/makesrna/intern/rna_object.c

@@ -663,7 +663,7 @@ static void rna_Object_active_material_set(PointerRNA *ptr, PointerRNA value)
 	Object *ob = (Object *)ptr->id.data;
 
 	DAG_id_tag_update(value.data, 0);
-	assign_material(ob, value.data, ob->actcol);
+	assign_material(ob, value.data, ob->actcol, BKE_MAT_ASSIGN_USERPREF);
 }
 
 static void rna_Object_active_particle_system_index_range(PointerRNA *ptr, int *min, int *max,
@@ -820,7 +820,7 @@ static void rna_MaterialSlot_material_set(PointerRNA *ptr, PointerRNA value)
 	Object *ob = (Object *)ptr->id.data;
 	int index = (Material **)ptr->data - ob->mat;
 
-	assign_material(ob, value.data, index + 1);
+	assign_material(ob, value.data, index + 1, BKE_MAT_ASSIGN_USERPREF);
 }
 
 static int rna_MaterialSlot_link_get(PointerRNA *ptr)

source/blender/modifiers/intern/MOD_boolean_util.c

@@ -632,7 +632,7 @@ int NewBooleanMesh(Scene *scene, Base *base, Base *base_select, int int_op_type)
 
 	/* add materials to object */
 	for (a = 0; a < totmat; a++)
-		assign_material(ob_new, mat[a], a + 1);
+		assign_material(ob_new, mat[a], a + 1, BKE_MAT_ASSIGN_USERPREF);
 
 	MEM_freeN(mat);