Fix part of #34640: colors darkening when using the vertex paint blur tool.

The problem was that vertex colors only have 8 bits of precision, and integer
division always rounds down, so after some color blending iterations everything
gets darker. Instead use integer division that behaves like round() instead of
floor() for blending operations.

1 files changed, 36 insertions, 33 deletions

diff --git a/source/blender/editors/sculpt_paint/paint_vertex.c b/source/blender/editors/sculpt_paint/paint_vertex.c
index d0f1bb882a3..7cc224db085 100644
--- a/source/blender/editors/sculpt_paint/paint_vertex.c
+++ b/source/blender/editors/sculpt_paint/paint_vertex.c
@@ -256,9 +256,9 @@ static void do_shared_vertex_tesscol(Mesh *me)
 	scol = scolmain;
 	while (a--) {
 		if (scol[0] > 1) {
-			scol[1] /= scol[0];
-			scol[2] /= scol[0];
-			scol[3] /= scol[0];
+			scol[1] = divide_round_i(scol[1], scol[0]);
+			scol[2] = divide_round_i(scol[2], scol[0]);
+			scol[3] = divide_round_i(scol[3], scol[0]);
 		}
 		scol += 4;
 	}
@@ -287,7 +287,7 @@ static void do_shared_vertexcol(Mesh *me, int do_tessface)
 {
 	const int use_face_sel = (me->editflag & ME_EDIT_PAINT_FACE_SEL);
 	MPoly *mp;
-	float (*scol)[4];
+	int (*scol)[4];
 	int i, j, has_shared = 0;
 
 	/* if no mloopcol: do not do */
@@ -295,7 +295,7 @@ static void do_shared_vertexcol(Mesh *me, int do_tessface)
 
 	if (me->mloopcol == NULL || me->totvert == 0 || me->totpoly == 0) return;
 
-	scol = MEM_callocN(sizeof(float) * me->totvert * 5, "scol");
+	scol = MEM_callocN(sizeof(int) * me->totvert * 5, "scol");
 
 	for (i = 0, mp = me->mpoly; i < me->totpoly; i++, mp++) {
 		if ((use_face_sel == FALSE) || (mp->flag & ME_FACE_SEL)) {
@@ -305,7 +305,7 @@ static void do_shared_vertexcol(Mesh *me, int do_tessface)
 				scol[ml->v][0] += lcol->r;
 				scol[ml->v][1] += lcol->g;
 				scol[ml->v][2] += lcol->b;
-				scol[ml->v][3] += 1.0f;
+				scol[ml->v][3] += 1;
 				has_shared = 1;
 			}
 		}
@@ -313,8 +313,10 @@ static void do_shared_vertexcol(Mesh *me, int do_tessface)
 
 	if (has_shared) {
 		for (i = 0; i < me->totvert; i++) {
-			if (scol[i][3] != 0.0f) {
-				mul_v3_fl(scol[i], 1.0f / scol[i][3]);
+			if (scol[i][3] != 0) {
+				scol[i][0] = divide_round_i(scol[i][0], scol[i][3]);
+				scol[i][1] = divide_round_i(scol[i][1], scol[i][3]);
+				scol[i][2] = divide_round_i(scol[i][2], scol[i][3]);
 			}
 		}
 
@@ -613,9 +615,9 @@ BLI_INLINE unsigned int mcol_blend(unsigned int col1, unsigned int col2, int fac
 	cp2 = (unsigned char *)&col2;
 	cp  = (unsigned char *)&col;
 
-	cp[0] = (mfac * cp1[0] + fac * cp2[0]) / 255;
-	cp[1] = (mfac * cp1[1] + fac * cp2[1]) / 255;
-	cp[2] = (mfac * cp1[2] + fac * cp2[2]) / 255;
+	cp[0] = divide_round_i((mfac * cp1[0] + fac * cp2[0]), 255);
+	cp[1] = divide_round_i((mfac * cp1[1] + fac * cp2[1]), 255);
+	cp[2] = divide_round_i((mfac * cp1[2] + fac * cp2[2]), 255);
 	cp[3] = 255;
 
 	return col;
@@ -635,11 +637,11 @@ BLI_INLINE unsigned int mcol_add(unsigned int col1, unsigned int col2, int fac)
 	cp2 = (unsigned char *)&col2;
 	cp  = (unsigned char *)&col;
 
-	temp = cp1[0] + ((fac * cp2[0]) / 255);
+	temp = cp1[0] + divide_round_i((fac * cp2[0]), 255);
 	cp[0] = (temp > 254) ? 255 : temp;
-	temp = cp1[1] + ((fac * cp2[1]) / 255);
+	temp = cp1[1] + divide_round_i((fac * cp2[1]), 255);
 	cp[1] = (temp > 254) ? 255 : temp;
-	temp = cp1[2] + ((fac * cp2[2]) / 255);
+	temp = cp1[2] + divide_round_i((fac * cp2[2]), 255);
 	cp[2] = (temp > 254) ? 255 : temp;
 	cp[3] = 255;
 	
@@ -660,11 +662,11 @@ BLI_INLINE unsigned int mcol_sub(unsigned int col1, unsigned int col2, int fac)
 	cp2 = (unsigned char *)&col2;
 	cp  = (unsigned char *)&col;
 
-	temp = cp1[0] - ((fac * cp2[0]) / 255);
+	temp = cp1[0] - divide_round_i((fac * cp2[0]), 255);
 	cp[0] = (temp < 0) ? 0 : temp;
-	temp = cp1[1] - ((fac * cp2[1]) / 255);
+	temp = cp1[1] - divide_round_i((fac * cp2[1]), 255);
 	cp[1] = (temp < 0) ? 0 : temp;
-	temp = cp1[2] - ((fac * cp2[2]) / 255);
+	temp = cp1[2] - divide_round_i((fac * cp2[2]), 255);
 	cp[2] = (temp < 0) ? 0 : temp;
 	cp[3] = 255;
 
@@ -688,9 +690,9 @@ BLI_INLINE unsigned int mcol_mul(unsigned int col1, unsigned int col2, int fac)
 	cp  = (unsigned char *)&col;
 
 	/* first mul, then blend the fac */
-	cp[0] = (mfac * cp1[0] + fac * ((cp2[0] * cp1[0]) / 255)) / 255;
-	cp[1] = (mfac * cp1[1] + fac * ((cp2[1] * cp1[1]) / 255)) / 255;
-	cp[2] = (mfac * cp1[2] + fac * ((cp2[2] * cp1[2]) / 255)) / 255;
+	cp[0] = divide_round_i(mfac * cp1[0] * 255 + fac * cp2[0] * cp1[0], 255*255);
+	cp[1] = divide_round_i(mfac * cp1[1] * 255 + fac * cp2[1] * cp1[1], 255*255);
+	cp[2] = divide_round_i(mfac * cp1[2] * 255 + fac * cp2[2] * cp1[2], 255*255);
 	cp[3] = 255;
 
 	return col;
@@ -721,9 +723,9 @@ BLI_INLINE unsigned int mcol_lighten(unsigned int col1, unsigned int col2, int f
 		return col1;
 	}
 
-	cp[0] = (mfac * cp1[0] + fac * cp2[0]) / 255;
-	cp[1] = (mfac * cp1[1] + fac * cp2[1]) / 255;
-	cp[2] = (mfac * cp1[2] + fac * cp2[2]) / 255;
+	cp[0] = divide_round_i(mfac * cp1[0] + fac * cp2[0], 255);
+	cp[1] = divide_round_i(mfac * cp1[1] + fac * cp2[1], 255);
+	cp[2] = divide_round_i(mfac * cp1[2] + fac * cp2[2], 255);
 	cp[3] = 255;
 
 	return col;
@@ -754,9 +756,9 @@ BLI_INLINE unsigned int mcol_darken(unsigned int col1, unsigned int col2, int fa
 		return col1;
 	}
 
-	cp[0] = (mfac * cp1[0] + fac * cp2[0]) / 255;
-	cp[1] = (mfac * cp1[1] + fac * cp2[1]) / 255;
-	cp[2] = (mfac * cp1[2] + fac * cp2[2]) / 255;
+	cp[0] = divide_round_i((mfac * cp1[0] + fac * cp2[0]), 255);
+	cp[1] = divide_round_i((mfac * cp1[1] + fac * cp2[1]), 255);
+	cp[2] = divide_round_i((mfac * cp1[2] + fac * cp2[2]), 255);
 	cp[3] = 255;
 	return col;
 }
@@ -2790,6 +2792,7 @@ static void vpaint_paint_poly(VPaint *vp, VPaintData *vpd, Mesh *me,
 	unsigned int *lcolorig = ((unsigned int *)vp->vpaint_prev) + mpoly->loopstart;
 	float alpha;
 	int i, j;
+	int totloop = mpoly->totloop;
 
 	int brush_alpha_pressure_i = (int)(brush_alpha_pressure * 255.0f);
 
@@ -2798,7 +2801,7 @@ static void vpaint_paint_poly(VPaint *vp, VPaintData *vpd, Mesh *me,
 		unsigned int tcol;
 		char *col;
 
-		for (j = 0; j < mpoly->totloop; j++) {
+		for (j = 0; j < totloop; j++) {
 			col = (char *)(lcol + j);
 			blend[0] += col[0];
 			blend[1] += col[1];
@@ -2806,10 +2809,10 @@ static void vpaint_paint_poly(VPaint *vp, VPaintData *vpd, Mesh *me,
 			blend[3] += col[3];
 		}
 
-		blend[0] /= mpoly->totloop;
-		blend[1] /= mpoly->totloop;
-		blend[2] /= mpoly->totloop;
-		blend[3] /= mpoly->totloop;
+		blend[0] = divide_round_i(blend[0], totloop);
+		blend[1] = divide_round_i(blend[1], totloop);
+		blend[2] = divide_round_i(blend[2], totloop);
+		blend[3] = divide_round_i(blend[3], totloop);
 		col = (char *)&tcol;
 		col[0] = blend[0];
 		col[1] = blend[1];
@@ -2820,7 +2823,7 @@ static void vpaint_paint_poly(VPaint *vp, VPaintData *vpd, Mesh *me,
 	}
 
 	ml = me->mloop + mpoly->loopstart;
-	for (i = 0; i < mpoly->totloop; i++, ml++) {
+	for (i = 0; i < totloop; i++, ml++) {
 		float rgba[4];
 		unsigned int paintcol;
 		alpha = calc_vp_alpha_col_dl(vp, vc, vpd->vpimat,
@@ -2851,7 +2854,7 @@ static void vpaint_paint_poly(VPaint *vp, VPaintData *vpd, Mesh *me,
 
 			ml = me->mloop + mpoly->loopstart;
 			mlc = me->mloopcol + mpoly->loopstart;
-			for (j = 0; j < mpoly->totloop; j++, ml++, mlc++) {
+			for (j = 0; j < totloop; j++, ml++, mlc++) {
 				if (ml->v == mf->v1) {
 					MESH_MLOOPCOL_TO_MCOL(mlc, mc + 0);
 				}