3 files changed, 41 insertions, 20 deletions
diff --git a/source/blender/blenkernel/BKE_deform.h b/source/blender/blenkernel/BKE_deform.h
index a45893b00fa..e716a7bc8fe 100644
--- a/source/blender/blenkernel/BKE_deform.h
+++ b/source/blender/blenkernel/BKE_deform.h
@@ -68,6 +68,10 @@ void BKE_defvert_array_copy(struct MDeformVert *dst, const struct MDeformVert *s
 float  defvert_find_weight(const struct MDeformVert *dvert, const int defgroup);
 float  defvert_array_find_weight_safe(const struct MDeformVert *dvert, const int index, const int defgroup);
 
+float BKE_defvert_multipaint_collective_weight(
+        const struct MDeformVert *dv, int defbase_tot,
+        const bool *defbase_sel, int defbase_tot_sel, bool do_autonormalize);
+
 void defvert_copy(struct MDeformVert *dvert_dst, const struct MDeformVert *dvert_src);
 void defvert_copy_subset(struct MDeformVert *dvert_dst, const struct MDeformVert *dvert_src,
                          const bool *vgroup_subset, const int vgroup_tot);
diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 1577a818180..dc7ef3055ee 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -1373,30 +1373,13 @@ static void calc_weightpaint_vert_color(
 
 	if ((defbase_sel_tot > 1) && (draw_flag & CALC_WP_MULTIPAINT)) {
 		/* Multi-Paint feature */
-		bool was_a_nonzero = false;
-		unsigned int i;
-
-		const MDeformWeight *dw = dv->dw;
-		for (i = dv->totweight; i != 0; i--, dw++) {
-			/* in multipaint, get the average if auto normalize is inactive
-			 * get the sum if it is active */
-			if (dw->def_nr < defbase_tot) {
-				if (defbase_sel[dw->def_nr]) {
-					if (dw->weight) {
-						input += dw->weight;
-						was_a_nonzero = true;
-					}
-				}
-			}
-		}
+		input = BKE_defvert_multipaint_collective_weight(
+		        dv, defbase_tot, defbase_sel, defbase_sel_tot, (draw_flag & CALC_WP_AUTO_NORMALIZE) != 0);
 
 		/* make it black if the selected groups have no weight on a vertex */
-		if (was_a_nonzero == false) {
+		if (input == 0.0f) {
 			show_alert_color = true;
 		}
-		else if ((draw_flag & CALC_WP_AUTO_NORMALIZE) == false) {
-			input /= defbase_sel_tot; /* get the average */
-		}
 	}
 	else {
 		/* default, non tricky behavior */
diff --git a/source/blender/blenkernel/intern/deform.c b/source/blender/blenkernel/intern/deform.c
index b09a0e92edb..135bf3e02ac 100644
--- a/source/blender/blenkernel/intern/deform.c
+++ b/source/blender/blenkernel/intern/deform.c
@@ -919,6 +919,40 @@ bool defvert_is_weight_zero(const struct MDeformVert *dvert, const int defgroup_
 	return true;
 }
 
+
+/**
+ * \return The representative weight of a multipaint group, used for
+ * viewport colors and actual painting.
+ *
+ * Result equal to sum of weights with auto normalize, and average otherwise.
+ * Value is not clamped, since painting relies on multiplication being always
+ * commutative with the collective weight function.
+ */
+float BKE_defvert_multipaint_collective_weight(
+        const struct MDeformVert *dv, int defbase_tot,
+        const bool *defbase_sel, int defbase_tot_sel, bool do_autonormalize)
+{
+	int i;
+	float total = 0.0f;
+	const MDeformWeight *dw = dv->dw;
+
+	for (i = dv->totweight; i != 0; i--, dw++) {
+		/* in multipaint, get the average if auto normalize is inactive
+		 * get the sum if it is active */
+		if (dw->def_nr < defbase_tot) {
+			if (defbase_sel[dw->def_nr]) {
+				total += dw->weight;
+			}
+		}
+	}
+
+	if (do_autonormalize == false) {
+		total /= defbase_tot_sel;
+	}
+
+	return total;
+}
+
 /* -------------------------------------------------------------------- */
 /* Defvert Array functions */