new weight paint draw option to display unweighted vertices with the option to check on the active group or all groups.

notes:
- vertices with zero weights are considered the same as vertices outside of a group.
- currently these show black but this can be made a theme color.
- multi-paint overrides this option (noted in description)

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -1057,6 +1057,11 @@ class VIEW3D_PT_tools_weightpaint_options(Panel, View3DPaintPanel):
 
         col.prop(wpaint, "input_samples")
 
+        col.label("Show Zero Weights:")
+        rowsub = col.row()
+        rowsub.active = (not tool_settings.use_multipaint)
+        rowsub.prop(tool_settings, "vertex_group_user", text="Show Group Use", expand=True)
+
         self.unified_paint_settings(col, context)
 
 # Commented out because the Apply button isn't an operator yet, making these settings useless

source/blender/blenkernel/BKE_deform.h

@@ -55,6 +55,7 @@ void                     defvert_add_index_notest(struct MDeformVert *dv, int de
 void                     defvert_remove_group(struct MDeformVert *dvert, struct MDeformWeight *dw);
 void                     defvert_clear(struct MDeformVert *dvert);
 int                      defvert_find_shared(const struct MDeformVert *dvert_a, const struct MDeformVert *dvert_b);
+bool                     defvert_is_weight_zero(const struct MDeformVert *dvert, const int defgroup_tot);
 
 void BKE_defvert_array_free_elems(struct MDeformVert *dvert, int totvert);
 void BKE_defvert_array_free(struct MDeformVert *dvert, int totvert);

source/blender/blenkernel/intern/DerivedMesh.c

@@ -1017,8 +1017,12 @@ void weight_to_rgb(float r_rgb[3], const float weight)
 
 /* draw_flag's for calc_weightpaint_vert_color */
 enum {
-	CALC_WP_MULTIPAINT = (1 << 0),
-	CALC_WP_AUTO_NORMALIZE = (1 << 1)
+	/* only one of these should be set, keep first (for easy bit-shifting) */
+	CALC_WP_GROUP_USER_ACTIVE   = (1 << 1),
+	CALC_WP_GROUP_USER_ALL      = (1 << 2),
+
+	CALC_WP_MULTIPAINT          = (1 << 3),
+	CALC_WP_AUTO_NORMALIZE      = (1 << 4)
 };
 
 static void weightpaint_color(unsigned char r_col[4], ColorBand *coba, const float input)
@@ -1050,7 +1054,7 @@ static void calc_weightpaint_vert_color(
 {
 	float input = 0.0f;
 	
-	int make_black = FALSE;
+	bool make_black = false;
 
 	if ((defbase_sel_tot > 1) && (draw_flag & CALC_WP_MULTIPAINT)) {
 		int was_a_nonzero = FALSE;
@@ -1072,7 +1076,7 @@ static void calc_weightpaint_vert_color(
 
 		/* make it black if the selected groups have no weight on a vertex */
 		if (was_a_nonzero == FALSE) {
-			make_black = TRUE;
+			make_black = true;
 		}
 		else if ((draw_flag & CALC_WP_AUTO_NORMALIZE) == FALSE) {
 			input /= defbase_sel_tot; /* get the average */
@@ -1081,6 +1085,17 @@ static void calc_weightpaint_vert_color(
 	else {
 		/* default, non tricky behavior */
 		input = defvert_find_weight(dv, defbase_act);
+
+		if (draw_flag & CALC_WP_GROUP_USER_ACTIVE) {
+			if (input == 0.0f) {
+				make_black = true;
+			}
+		}
+		else if (draw_flag & CALC_WP_GROUP_USER_ALL) {
+			if (input == 0.0f) {
+				make_black = defvert_is_weight_zero(dv, defbase_tot);
+			}
+		}
 	}
 
 	if (make_black) { /* TODO, theme color */
@@ -1138,7 +1153,12 @@ static unsigned char *calc_weightpaint_vert_array(Object *ob, DerivedMesh *dm, i
 	}
 	else {
 		int col_i;
-		weightpaint_color((unsigned char *)&col_i, coba, 0.0f);
+		if (draw_flag & (CALC_WP_GROUP_USER_ACTIVE | CALC_WP_GROUP_USER_ALL)) {
+			col_i = 0;
+		}
+		else {
+			weightpaint_color((unsigned char *)&col_i, coba, 0.0f);
+		}
 		fill_vn_i((int *)wtcol_v, numVerts, col_i);
 	}
 
@@ -1354,8 +1374,11 @@ static void mesh_calc_modifiers(Scene *scene, Object *ob, float (*inputVertexCos
 	int sculpt_mode = ob->mode & OB_MODE_SCULPT && ob->sculpt;
 	int sculpt_dyntopo = (sculpt_mode && ob->sculpt->bm);
 
-	const int draw_flag = ((scene->toolsettings->multipaint ? CALC_WP_MULTIPAINT : 0) |
+	const int draw_flag = ((scene->toolsettings->multipaint ? CALC_WP_MULTIPAINT :
+	                                                          /* CALC_WP_GROUP_USER_ACTIVE or CALC_WP_GROUP_USER_ALL*/
+	                                                          (1 << scene->toolsettings->weightuser)) |
 	                       (scene->toolsettings->auto_normalize ? CALC_WP_AUTO_NORMALIZE : 0));
+
 	/* Generic preview only in object mode! */
 	const int do_mod_mcol = (ob->mode == OB_MODE_OBJECT);
 #if 0 /* XXX Will re-enable this when we have global mod stack options. */

source/blender/blenkernel/intern/deform.c

@@ -41,14 +41,13 @@
 #include "DNA_meshdata_types.h"
 #include "DNA_object_types.h"
 
-#include "BKE_deform.h"
-
 #include "BLI_listbase.h"
 #include "BLI_math.h"
 #include "BLI_path_util.h"
 #include "BLI_string.h"
 #include "BLI_utildefines.h"
 
+#include "BKE_deform.h"  /* own include */
 
 void defgroup_copy_list(ListBase *outbase, ListBase *inbase)
 {
@@ -787,6 +786,24 @@ int defvert_find_shared(const MDeformVert *dvert_a, const MDeformVert *dvert_b)
 	return -1;
 }
 
+/**
+ * return true if has no weights
+ */
+bool defvert_is_weight_zero(const struct MDeformVert *dvert, const int defgroup_tot)
+{
+	MDeformWeight *dw = dvert->dw;
+	unsigned int i;
+	for (i = dvert->totweight; i != 0; i--, dw++) {
+		if (dw->weight != 0.0f) {
+			/* check the group is in-range, happens on rare situations */
+			if (LIKELY(dw->def_nr < defgroup_tot)) {
+				return false;
+			}
+		}
+	}
+	return true;
+}
+
 /* -------------------------------------------------------------------- */
 /* Defvert Array functions */
 

source/blender/makesdna/DNA_scene_types.h

@@ -1036,10 +1036,11 @@ typedef struct ToolSettings {
 	short proportional, prop_mode;
 	char proportional_objects; /* proportional edit, object mode */
 	char proportional_mask; /* proportional edit, object mode */
-	char pad4[2];
+	char pad4[1];
 
 	char auto_normalize; /*auto normalizing mode in wpaint*/
 	char multipaint; /* paint multiple bones in wpaint */
+	char weightuser;
 
 	/* UV painting */
 	int use_uv_sculpt;
@@ -1445,6 +1446,13 @@ typedef struct Scene {
 #define PROP_EDIT_ON			1
 #define PROP_EDIT_CONNECTED	2
 
+/* toolsettings->weightuser */
+enum {
+	OB_DRAW_GROUPUSER_NONE      = 0,
+	OB_DRAW_GROUPUSER_ACTIVE    = 1,
+	OB_DRAW_GROUPUSER_ALL       = 2
+};
+
 /* sce->flag */
 #define SCE_DS_SELECTED			(1<<0)
 #define SCE_DS_COLLAPSED		(1<<1)

source/blender/makesrna/intern/rna_scene.c

@@ -1528,6 +1528,13 @@ static void rna_def_tool_settings(BlenderRNA  *brna)
 		{0, NULL, 0, NULL, NULL}
 	};
 
+	static EnumPropertyItem draw_groupuser_items[] = {
+		{OB_DRAW_GROUPUSER_NONE, "NONE", 0, "None", ""},
+		{OB_DRAW_GROUPUSER_ACTIVE, "ACTIVE", 0, "Active", "Show vertices with no weights in the actuve group"},
+		{OB_DRAW_GROUPUSER_ALL, "ALL", 0, "All", "Show vertices with no weights in the any group"},
+		{0, NULL, 0, NULL, NULL}
+	};
+
 	srna = RNA_def_struct(brna, "ToolSettings", NULL);
 	RNA_def_struct_path_func(srna, "rna_ToolSettings_path");
 	RNA_def_struct_ui_text(srna, "Tool Settings", "");
@@ -1550,6 +1557,13 @@ static void rna_def_tool_settings(BlenderRNA  *brna)
 	                         "weight painting");
 	RNA_def_property_update(prop, 0, "rna_Scene_update_active_object_data");
 
+	prop = RNA_def_property(srna, "vertex_group_user", PROP_ENUM, PROP_NONE);
+	RNA_def_property_enum_sdna(prop, NULL, "weightuser");
+	RNA_def_property_enum_items(prop, draw_groupuser_items);
+	RNA_def_property_ui_text(prop, "Mask Non-Group Vertices", "Display unweighted vertices (multi-paint overrides)");
+	RNA_def_property_update(prop, 0, "rna_Scene_update_active_object_data");
+
+
 	prop = RNA_def_property(srna, "vertex_paint", PROP_POINTER, PROP_NONE);
 	RNA_def_property_pointer_sdna(prop, NULL, "vpaint");
 	RNA_def_property_ui_text(prop, "Vertex Paint", "");