Fix bug 692377: Incorrect rendering of blending groups.

A combination of constant alpha and non-zero blending modes resulted
in incorrect rendering.

Various fixes seem to have made it work:

 * Allow for non-full alpha when blending non-isolated groups back.

 * Clamp blended pixels to allow for overflow.

 * Allow for alpha in the shape blending.

2 files changed, 35 insertions, 15 deletions

diff --git a/draw/draw_blend.c b/draw/draw_blend.c
index 0634f944..20ad8a28 100644
--- a/draw/draw_blend.c
+++ b/draw/draw_blend.c
@@ -330,14 +330,14 @@ fz_blend_nonseparable(byte * restrict bp, byte * restrict sp, int w, int blendmo
 	}
 }
 
-void
-fz_blend_separable_isolated(byte * restrict bp, byte * restrict sp, int n, int w, int blendmode, byte * restrict hp)
+static void
+fz_blend_separable_nonisolated(byte * restrict bp, byte * restrict sp, int n, int w, int blendmode, byte * restrict hp, int alpha)
 {
 	int k;
 	int n1 = n - 1;
 	while (w--)
 	{
-		int ha = *hp++; /* ha = shape_alpha */
+		int ha = fz_mul255(*hp++, alpha); /* ha = shape_alpha */
 		/* If ha == 0 then leave everything unchanged */
 		if (ha != 0)
 		{
@@ -374,8 +374,9 @@ fz_blend_separable_isolated(byte * restrict bp, byte * restrict sp, int n, int w
 				case FZ_BLEND_DIFFERENCE: rc = fz_difference_byte(bc, sc); break;
 				case FZ_BLEND_EXCLUSION: rc = fz_exclusion_byte(bc, sc); break;
 				}
-
 				rc = fz_mul255(255 - ha, bc) + fz_mul255(255 - ba, sc) + fz_mul255(baha, rc);
+				if (rc < 0) rc = 0;
+				if (rc > 255) rc = 255;
 				bp[k] = fz_mul255(rc, ra);
 			}
 		}
@@ -385,12 +386,12 @@ fz_blend_separable_isolated(byte * restrict bp, byte * restrict sp, int n, int w
 	}
 }
 
-void
-fz_blend_nonseparable_isolated(byte * restrict bp, byte * restrict sp, int w, int blendmode, byte * restrict hp)
+static void
+fz_blend_nonseparable_nonisolated(byte * restrict bp, byte * restrict sp, int w, int blendmode, byte * restrict hp, int alpha)
 {
 	while (w--)
 	{
-		int ha = *hp++;
+		int ha = fz_mul255(*hp++, alpha);
 		if (ha != 0)
 		{
 			int sa = sp[3];
@@ -455,7 +456,7 @@ fz_blend_pixmap(fz_pixmap *dst, fz_pixmap *src, int alpha, int blendmode, int is
 	int x, y, w, h, n;
 
 	/* TODO: fix this hack! */
-	if (alpha < 255)
+	if (isolated && alpha < 255)
 	{
 		sp = src->samples;
 		n = src->w * src->h * src->n;
@@ -487,9 +488,9 @@ fz_blend_pixmap(fz_pixmap *dst, fz_pixmap *src, int alpha, int blendmode, int is
 		while (h--)
 		{
 			if (n == 4 && blendmode >= FZ_BLEND_HUE)
-				fz_blend_nonseparable_isolated(dp, sp, w, blendmode, hp);
+				fz_blend_nonseparable_nonisolated(dp, sp, w, blendmode, hp, alpha);
 			else
-				fz_blend_separable_isolated(dp, sp, n, w, blendmode, hp);
+				fz_blend_separable_nonisolated(dp, sp, n, w, blendmode, hp, alpha);
 			sp += src->w * n;
 			dp += dst->w * n;
 			hp += shape->w;
diff --git a/draw/draw_device.c b/draw/draw_device.c
index 22cdc720..8042ce84 100644
--- a/draw/draw_device.c
+++ b/draw/draw_device.c
@@ -43,6 +43,7 @@ static void fz_knockout_begin(void *user)
 	fz_draw_device *dev = user;
 	fz_bbox bbox;
 	fz_pixmap *dest, *shape;
+	int isolated = dev->blendmode & FZ_BLEND_ISOLATED;
 
 	if ((dev->blendmode & FZ_BLEND_KNOCKOUT) == 0)
 		return;
@@ -57,10 +58,9 @@ static void fz_knockout_begin(void *user)
 	bbox = fz_intersect_bbox(bbox, dev->scissor);
 	dest = fz_new_pixmap_with_rect(dev->dest->colorspace, bbox);
 
-	if (dev->blendmode & FZ_BLEND_ISOLATED)
+	if (isolated)
 	{
 		fz_clear_pixmap(dest);
-		shape = dev->shape;
 	}
 	else
 	{
@@ -69,9 +69,19 @@ static void fz_knockout_begin(void *user)
 		do {
 			prev = dev->stack[--i].dest;
 		} while (prev == NULL);
+		fz_copy_pixmap_rect(dest, prev, bbox);
+	}
+
+	if (dev->blendmode == 0 && isolated)
+	{
+		/* We can render direct to any existing shape plane. If there
+		 * isn't one, we don't need to make one. */
+		shape = dev->shape;
+	}
+	else
+	{
 		shape = fz_new_pixmap_with_rect(NULL, bbox);
 		fz_clear_pixmap(shape);
-		fz_copy_pixmap_rect(dest, prev, bbox);
 	}
 	dev->stack[dev->top].blendmode = dev->blendmode;
 	dev->stack[dev->top].scissor = dev->scissor;
@@ -1149,13 +1159,22 @@ fz_draw_begin_group(void *user, fz_rect rect, int isolated, int knockout, int bl
 	if (isolated)
 	{
 		fz_clear_pixmap(dest);
+	}
+	else
+	{
+		fz_copy_pixmap_rect(dest, dev->dest, bbox);
+	}
+
+	if (blendmode == 0 && alpha == 1.0 && isolated)
+	{
+		/* We can render direct to any existing shape plane. If there
+		 * isn't one, we don't need to make one. */
 		shape = dev->shape;
 	}
 	else
 	{
 		shape = fz_new_pixmap_with_rect(NULL, bbox);
 		fz_clear_pixmap(shape);
-		fz_copy_pixmap_rect(dest, dev->dest, bbox);
 	}
 
 	dev->stack[dev->top].alpha = alpha;
@@ -1202,7 +1221,7 @@ fz_draw_end_group(void *user)
 		{
 			if (dev->shape)
 			{
-				fz_paint_pixmap(dev->shape, shape, 255);
+				fz_paint_pixmap(dev->shape, shape, alpha * 255);
 			}
 			fz_drop_pixmap(shape);
 		}