calculate weight paint colors once per vertex rather then on every face corner (was doing the same deform vert lookup and color calculation multiple times per vertex),

Quick shows over 2x speedup in my tests, will give bigger speedup with more vertex groups.

If you happen to have vertices with no faces using them - vertex colors will be calculated unnecessarily, but this isnt a common use case for weight paint mode.

source/blender/blenkernel/BKE_texture.h

@@ -67,7 +67,7 @@ void free_plugin_tex(struct PluginTex *pit);
 
 void init_colorband(struct ColorBand *coba, int rangetype);
 struct ColorBand *add_colorband(int rangetype);
-int do_colorband(struct ColorBand *coba, float in, float out[4]);
+int do_colorband(const struct ColorBand *coba, float in, float out[4]);
 void colorband_table_RGBA(struct ColorBand *coba, float **array, int *size);
 int vergcband(const void *a1, const void *a2);
 struct CBData *colorband_element_add(struct ColorBand *coba, float position);

source/blender/blenkernel/intern/DerivedMesh.c

@@ -642,7 +642,7 @@ enum {
 	CALC_WP_AUTO_NORMALIZE= (1<<1)
 };
 
-void weightpaint_color(unsigned char r_col[4], ColorBand *coba, const float input)
+static void weightpaint_color(unsigned char r_col[4], ColorBand *coba, const float input)
 {
 	float colf[4];
 
@@ -669,10 +669,10 @@ static void calc_weightpaint_vert_color(
 
 	if ((selected > 1) && (draw_flag & CALC_WP_MULTIPAINT)) {
 		int was_a_nonzero= FALSE;
-		int i;
+		unsigned int i;
 
 		MDeformWeight *dw= dv->dw;
-		for (i = dv->totweight; i > 0; i--, 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) {
@@ -717,46 +717,63 @@ void vDM_ColorBand_store(ColorBand *coba)
 	stored_cb= coba;
 }
 
-static void add_weight_mcol_dm(Object *ob, DerivedMesh *dm, int const draw_flag)
+/* return an array of vertex weight colors */
+static unsigned char *calc_weightpaint_vert_array(Object *ob, int const draw_flag, ColorBand *coba)
 {
 	Mesh *me = ob->data;
-	ColorBand *coba= stored_cb;	/* warning, not a local var */
-
-	unsigned char *wtcol = MEM_mallocN (sizeof (unsigned char) * me->totface*4*4, "weightmap");
+	unsigned char *wtcol_v = MEM_callocN (sizeof(unsigned char) * me->totvert * 4, "weightmap_v");
 
 	if (me->dvert) {
-		MDeformVert *dvert= me->dvert;
-		MFace *mf = me->mface;
+		unsigned char *wc = wtcol_v;
+		MDeformVert *dv= me->dvert;
+		unsigned int i;
 
+		/* varisbles for multipaint */
 		const int defbase_tot = BLI_countlist(&ob->defbase);
 		const int defbase_act = ob->actdef-1;
 		char *dg_flags = MEM_mallocN(defbase_tot * sizeof(char), __func__);
 		const int selected = get_selected_defgroups(ob, dg_flags, defbase_tot);
 		/* const int unselected = defbase_tot - selected; */ /* UNUSED */
 
-		int i;
-
-		memset(wtcol, 0x55, sizeof (unsigned char) * me->totface*4*4);
-		for (i=0; i<me->totface; i++, mf++) {
-			unsigned int fidx= mf->v4 ? 3:2;
-			do {
-				calc_weightpaint_vert_color(&wtcol[(i*4 + fidx)*4],
-				                            &dvert[*(&mf->v1 + fidx)], coba,
-				                            defbase_tot, defbase_act,
-				                            dg_flags, selected, draw_flag);
-			} while (fidx--);
+		for (i = me->totvert; i != 0; i--, wc += 4, dv++) {
+			calc_weightpaint_vert_color(wc, dv, coba, defbase_tot, defbase_act, dg_flags, selected, draw_flag);
 		}
 
 		MEM_freeN(dg_flags);
 	}
 	else {
-		/* no weights, fill in zero */
 		int col_i;
 		weightpaint_color((unsigned char *)&col_i, coba, 0.0f);
-		fill_vn_i((int *)wtcol, me->totface*4, col_i);
+		fill_vn_i((int *)wtcol_v, me->totvert, col_i);
 	}
 
-	CustomData_add_layer(&dm->faceData, CD_WEIGHT_MCOL, CD_ASSIGN, wtcol, dm->numFaceData);
+	return wtcol_v;
+}
+
+#include "PIL_time.h"
+
+static void add_weight_mcol_dm(Object *ob, DerivedMesh *dm, int const draw_flag)
+{
+	ColorBand *coba= stored_cb;	/* warning, not a local var */
+
+	Mesh *me = ob->data;
+	unsigned char *wtcol_v = calc_weightpaint_vert_array(ob, draw_flag, coba);
+	unsigned char *wtcol_f = MEM_mallocN (sizeof(unsigned char) * me->totface*4*4, "weightmap_f");
+
+	MFace *mf = me->mface;
+	int i;
+
+	for (i=0; i<me->totface; i++, mf++) {
+		unsigned int fidx= mf->v4 ? 3:2;
+		do {
+			copy_v4_v4_char((char *)&wtcol_f[(4 * i + fidx) * 4],
+			                (char *)&wtcol_v[4 * (*(&mf->v1 + fidx))]);
+		} while (fidx--);
+	}
+
+	MEM_freeN(wtcol_v);
+
+	CustomData_add_layer(&dm->faceData, CD_WEIGHT_MCOL, CD_ASSIGN, wtcol_f, dm->numFaceData);
 }
 
 /* new value for useDeform -1  (hack for the gameengine):

source/blender/blenkernel/intern/texture.c

@@ -349,9 +349,9 @@ ColorBand *add_colorband(int rangetype)
 
 /* ------------------------------------------------------------------------- */
 
-int do_colorband(ColorBand *coba, float in, float out[4])
+int do_colorband(const ColorBand *coba, float in, float out[4])
 {
-	CBData *cbd1, *cbd2, *cbd0, *cbd3;
+	const CBData *cbd1, *cbd2, *cbd0, *cbd3;
 	float fac, mfac, t[4];
 	int a;