Bug 697241: Fix blending through clips.

Non rectangular clips are currently handled by rendering to a
'isolated' background, and then plotting that through a mask.

This runs into problems when the rendering needs to use non
standard blend modes that need to access the background
colors.

Instead, copy the background to the new pixmap, render to that
then plot that through the mask.

This simplifies the painting code, because we now never have
mismatched source and destination alphas.

3 files changed, 131 insertions, 497 deletions

diff --git a/include/mupdf/fitz/math.h b/include/mupdf/fitz/math.h
index dc29e3fb..b8954505 100644
--- a/include/mupdf/fitz/math.h
+++ b/include/mupdf/fitz/math.h
@@ -19,11 +19,6 @@ static inline int fz_mul255(int a, int b)
  * to give a single value in the same range as A was. */
 #define FZ_COMBINE(A,B) (((A)*(B))>>8)
 
-/* Combine values A (in the 0..255 range) and B (in the 0..256 range),
- * then reverse it within that range to give a single value in the
- * 0..256 range. */
-#define FZ_REVERSE_COMBINE(A,B) ((0xFF00 - (A)*(B))>>8)
-
 /* Combine values A and C (in the same (any) range) and B and D (in the
  * 0..256 range), to give a single value in the same range as A and C were. */
 #define FZ_COMBINE2(A,B,C,D) (((A) * (B) + (C) * (D))>>8)
diff --git a/source/fitz/draw-device.c b/source/fitz/draw-device.c
index 1947115c..51d394f6 100644
--- a/source/fitz/draw-device.c
+++ b/source/fitz/draw-device.c
@@ -590,15 +590,8 @@ fz_draw_clip_path(fz_context *ctx, fz_device *devp, const fz_path *path, int eve
 	{
 		state[1].mask = fz_new_pixmap_with_bbox(ctx, NULL, &bbox, 1);
 		fz_clear_pixmap(ctx, state[1].mask);
-		/* When there is no alpha in the current destination (state[0].dest->alpha == 0)
-		 * we have a choice. We can either create the new destination WITH alpha, or
-		 * we can copy the old pixmap contents in. We opt for the latter here, but
-		 * may want to revisit this decision in future. */
 		state[1].dest = fz_new_pixmap_with_bbox(ctx, model, &bbox, state[0].dest->alpha);
-		if (state[0].dest->alpha)
-			fz_clear_pixmap(ctx, state[1].dest);
-		else
-			fz_copy_pixmap_rect(ctx, state[1].dest, state[0].dest, &bbox);
+		fz_copy_pixmap_rect(ctx, state[1].dest, state[0].dest, &bbox);
 		if (state[1].shape)
 		{
 			state[1].shape = fz_new_pixmap_with_bbox(ctx, NULL, &bbox, 1);
@@ -1895,10 +1888,7 @@ fz_draw_end_mask(fz_context *ctx, fz_device *devp)
 		/* create new dest scratch buffer */
 		fz_pixmap_bbox(ctx, temp, &bbox);
 		dest = fz_new_pixmap_with_bbox(ctx, state->dest->colorspace, &bbox, state->dest->alpha);
-		if (state->dest->alpha)
-			fz_clear_pixmap(ctx, dest);
-		else
-			fz_copy_pixmap_rect(ctx, dest, state->dest, &bbox);
+		fz_copy_pixmap_rect(ctx, dest, state->dest, &bbox);
 
 		/* push soft mask as clip mask */
 		state[1].dest = dest;
diff --git a/source/fitz/draw-paint.c b/source/fitz/draw-paint.c
index ea2a57e6..2ec7d7d9 100644
--- a/source/fitz/draw-paint.c
+++ b/source/fitz/draw-paint.c
@@ -840,71 +840,31 @@ fz_get_span_color_painter(int n, int da, const byte * restrict color)
 
 /* FIXME: There is potential for SWAR optimisation here */
 static inline void
-template_span_with_mask_1_general(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int w)
+template_span_with_mask_1_general(byte * restrict dp, const byte * restrict sp, int a, const byte * restrict mp, int w)
 {
 	do
 	{
-		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		if (ma == 0 || (sa && sp[1] == 0))
+		if (ma == 0 || (a && sp[1] == 0))
 		{
-			dp += 1 + da;
-			sp += 1 + sa;
+			dp += 1 + a;
+			sp += 1 + a;
 		}
 		else if (ma == 256)
 		{
-			masa = (sa ? 255 - sp[1] : 0);
-			if (masa == 0)
-			{
+			*dp++ = *sp++;
+			if (a)
 				*dp++ = *sp++;
-				if (da)
-					*dp++ = (sa ? *sp++ : 255);
-			}
-			else
-			{
-				masa = FZ_EXPAND(masa);
-				*dp = *sp + FZ_COMBINE(*dp, masa);
-				sp++; dp++;
-				if (da)
-				{
-					*dp = (sa ? *sp : 255) + FZ_COMBINE(*dp, masa);
-					dp++;
-				}
-				if (sa)
-					sp++;
-			}
 		}
 		else
 		{
-			if (sa)
-			{
-				if (sp[1] == 0)
-				{
-					sp += 2;
-					dp += 1+da;
-					continue;
-				}
-				masa = FZ_REVERSE_COMBINE(sp[1], ma);
-				masa = FZ_EXPAND(masa);
-				*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-				sp++; dp++;
-				if (da)
-				{
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					dp++;
-				}
-				sp++;
-			}
-			else
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+			if (a)
 			{
 				*dp = FZ_BLEND(*sp, *dp, ma);
 				sp++; dp++;
-				if (da)
-				{
-					*dp = FZ_BLEND(255, *dp, ma);
-					dp++;
-				}
 			}
 		}
 	}
@@ -912,119 +872,51 @@ template_span_with_mask_1_general(byte * restrict dp, int da, const byte * restr
 }
 
 static inline void
-template_span_with_mask_3_general(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int w)
+template_span_with_mask_3_general(byte * restrict dp, const byte * restrict sp, int a, const byte * restrict mp, int w)
 {
 	do
 	{
-		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		if (ma == 0 || (sa && sp[3] == 0))
+		if (ma == 0 || (a && sp[3] == 0))
 		{
-			dp += 3 + da;
-			sp += 3 + sa;
+			dp += 3 + a;
+			sp += 3 + a;
 		}
 		else if (ma == 256)
 		{
-			masa = (sa ? 255 - sp[3] : 0);
-			if (masa == 0)
+			if (a)
 			{
-				if (da && sa)
-				{
-					*(int32_t *)dp = *(int32_t *)sp;
-					sp += 4; dp += 4;
-				}
-				else
-				{
-					*dp++ = *sp++;
-					*dp++ = *sp++;
-					*dp++ = *sp++;
-					if (da)
-						*dp++ = (sa ? *sp : 255);
-					if (sa)
-						sp++;
-				}
+				*(int32_t *)dp = *(int32_t *)sp;
+				sp += 4; dp += 4;
 			}
 			else
 			{
-				masa = FZ_EXPAND(masa);
-				if (da && sa)
-				{
-					const uint32_t mask = 0x00ff00ff;
-					uint32_t d0 = *(uint32_t *)dp;
-					uint32_t d1 = d0>>8;
-					uint32_t s0 = *(uint32_t *)sp;
-					uint32_t s1 = s0>>8;
-					sp += 4;
-					d0 &= mask;
-					d1 &= mask;
-					s0 &= mask;
-					s1 &= mask;
-					s0 += (d0*masa)>>8;
-					s1 += (d1*masa)>>8;
-					s0 &= mask;
-					s1 &= mask;
-					s0 |= s1<<8;
-					*(uint32_t *)dp = s0;
-					dp += 4;
-				}
-				else
-				{
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					if (da)
-					{
-						*dp = (sa ? *sp : 255) + FZ_COMBINE(*dp, masa);
-						dp++;
-					}
-					if (sa)
-						sp++;
-				}
+				*dp++ = *sp++;
+				*dp++ = *sp++;
+				*dp++ = *sp++;
 			}
 		}
-		else if (sa)
+		else if (a)
 		{
-			if (sp[3] == 0)
-			{
-				sp += 4;
-				dp += 3+da;
-			}
-			else
-			{
-				masa = FZ_REVERSE_COMBINE(sp[3], ma);
-				masa = FZ_EXPAND(masa);
-				if (da)
-				{
-					const uint32_t mask = 0x00ff00ff;
-					uint32_t d0 = *(uint32_t *)dp;
-					uint32_t d1 = d0>>8;
-					uint32_t s0 = *(uint32_t *)sp;
-					uint32_t s1 = s0>>8;
-					sp += 4;
-					d0 &= mask;
-					d1 &= mask;
-					s0 &= mask;
-					s1 &= mask;
-					d0 = ((s0 * ma + d0 * masa)>>8) & mask;
-					d1 = (s1 * ma + d1 * masa) & ~mask;
-					d0 |= d1;
-					*(uint32_t *)dp = d0;
-					dp += 4;
-				}
-				else
-				{
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp+=2; dp++;
-				}
-			}
+			const uint32_t mask = 0x00ff00ff;
+			uint32_t d0 = *(uint32_t *)dp;
+			uint32_t d1 = d0>>8;
+			uint32_t s0 = *(uint32_t *)sp;
+			uint32_t s1 = s0>>8;
+			d0 &= mask;
+			d1 &= mask;
+			s0 &= mask;
+			s1 &= mask;
+			d0 = (((d0<<8) + (s0-d0)*ma)>>8) & mask;
+			d1 = ((d1<<8) + (s1-d1)*ma) & ~mask;
+			d0 |= d1;
+			assert((d0>>24) >= (d0 & 0xff));
+			assert((d0>>24) >= ((d0>>8) & 0xff));
+			assert((d0>>24) >= ((d0>>16) & 0xff));
+			*(uint32_t *)dp = d0;
+			sp += 4;
+			dp += 4;
 		}
 		else
 		{
@@ -1034,244 +926,102 @@ template_span_with_mask_3_general(byte * restrict dp, int da, const byte * restr
 			sp++; dp++;
 			*dp = FZ_BLEND(*sp, *dp, ma);
 			sp++; dp++;
-			if (da)
-			{
-				*dp = FZ_BLEND(255, *dp, ma);
-				dp++;
-			}
 		}
 	}
 	while (--w);
 }
 
 static inline void
-template_span_with_mask_4_general(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int w)
+template_span_with_mask_4_general(byte * restrict dp, const byte * restrict sp, int a, const byte * restrict mp, int w)
 {
 	do
 	{
-		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		if (ma == 0 || (sa && sp[4] == 0))
+		if (ma == 0 || (a && sp[4] == 0))
 		{
-			dp += 4 + da;
-			sp += 4 + sa;
+			dp += 4 + a;
+			sp += 4 + a;
 		}
 		else if (ma == 256)
 		{
-			masa = (sa ? 255 - sp[4] : 0);
-			if (masa == 0)
+			if (!a)
 			{
-				if (!da && !sa)
-				{
-					*(uint32_t *)dp = *(uint32_t *)sp;
-					dp += 4;
-					sp += 4;
-				}
-				else
-				{
-					*dp++ = *sp++;
-					*dp++ = *sp++;
-					*dp++ = *sp++;
-					*dp++ = *sp++;
-					if (da)
-						*dp++ = (sa ? *sp : 255);
-					if (sa)
-						sp++;
-				}
+				*(uint32_t *)dp = *(uint32_t *)sp;
+				dp += 4;
+				sp += 4;
 			}
 			else
 			{
-				masa = FZ_EXPAND(masa);
-				if (!da && !sa)
-				{
-					const uint32_t mask = 0x00ff00ff;
-					uint32_t d0 = *(uint32_t *)dp;
-					uint32_t d1 = d0>>8;
-					uint32_t s1 = *(uint32_t *)sp;
-					uint32_t s0 = s1<<8;
-					sp += 4;
-					d0 &= mask;
-					d1 &= mask;
-					s0 &= ~mask;
-					s1 &= ~mask;
-					d0 = ((s0 + d0 * masa)>>8) & mask;
-					d1 =  (s1 + d1 * masa) & ~mask;
-					d0 |= d1;
-					*(uint32_t *)dp = d0;
-					dp += 4;
-				}
-				else
-				{
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-					if (da)
-					{
-						*dp = (sa ? *sp : 255) + FZ_COMBINE(*dp, masa);
-						dp++;
-					}
-					if (sa)
-						sp++;
-				}
+				*dp++ = *sp++;
+				*dp++ = *sp++;
+				*dp++ = *sp++;
+				*dp++ = *sp++;
+				*dp++ = *sp++;
 			}
 		}
+		else if (a)
+		{
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+			*dp = FZ_BLEND(*sp, *dp, ma);
+			sp++; dp++;
+		}
 		else
 		{
-			if (sa)
-			{
-				if (sp[4] == 0)
-				{
-					sp += 5;
-					dp += 4+da;
-				}
-				else
-				{
-					masa = FZ_REVERSE_COMBINE(sp[4], ma);
-					masa = FZ_EXPAND(masa);
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-					if (da)
-					{
-						*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-						dp++;
-					}
-					sp++;
-				}
-			}
-			else if (!da)
-			{
-				const uint32_t mask = 0x00ff00ff;
-				uint32_t d0 = *(uint32_t *)dp;
-				uint32_t d1 = d0 & ~mask;
-				uint32_t s0 = *(uint32_t *)sp;
-				uint32_t s1 = s0>>8;
-				sp += 4;
-				d0 &= mask;
-				s0 &= mask;
-				s1 &= mask;
-				s0 -= d0;
-				s1 -= d1>>8;
-				d0 = (((d0<<8) + s0 * ma)>>8) & mask;
-				d1 =  (d1 + s1 * ma) & ~mask;
-				d0 |= d1;
-				*(uint32_t *)dp = d0;
-				dp += 4;
-			}
-			else
-			{
-				*dp = FZ_BLEND(*sp, *dp, ma);
-				sp++; dp++;
-				*dp = FZ_BLEND(*sp, *dp, ma);
-				sp++; dp++;
-				*dp = FZ_BLEND(*sp, *dp, ma);
-				sp++; dp++;
-				*dp = FZ_BLEND(*sp, *dp, ma);
-				sp++; dp++;
-				if (da)
-				{
-					*dp = FZ_BLEND(255, *dp, ma);
-					dp++;
-				}
-			}
+			const uint32_t mask = 0x00ff00ff;
+			uint32_t d0 = *(uint32_t *)dp;
+			uint32_t d1 = d0>>8;
+			uint32_t s0 = *(uint32_t *)sp;
+			uint32_t s1 = s0>>8;
+			sp += 4;
+			d0 &= mask;
+			d1 &= mask;
+			s0 &= mask;
+			s1 &= mask;
+			d0 = (((d0<<8) + (s0-d0)*ma)>>8) & mask;
+			d1 = ((d1<<8) + (s1-d1)*ma) & ~mask;
+			d0 |= d1;
+			*(uint32_t *)dp = d0;
+			dp += 4;
 		}
 	}
 	while (--w);
 }
 
 static inline void
-template_span_with_mask_N_general(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+template_span_with_mask_N_general(byte * restrict dp, const byte * restrict sp, int a, const byte * restrict mp, int n, int w)
 {
 	do
 	{
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		if (ma == 0 || (sa && sp[n] == 0))
+		if (ma == 0 || (a && sp[n] == 0))
 		{
-			dp += n + da;
-			sp += n + sa;
+			dp += n + a;
+			sp += n + a;
 		}
 		else if (ma == 256)
 		{
-			int k = n;
-			int masa = (sa ? 255 - sp[n] : 0);
-			if (masa == 0)
+			int k = n+a;
+			while (k--)
 			{
-				while (k--)
-				{
-					*dp++ = *sp++;
-				}
-				if (da)
-					*dp++ = (sa ? *sp : 255);
-				if (sa)
-					sp++;
-			}
-			else
-			{
-				masa = FZ_EXPAND(masa);
-				while (k--)
-				{
-					*dp = *sp + FZ_COMBINE(*dp, masa);
-					sp++; dp++;
-				}
-				if (da)
-				{
-					*dp = (sa ? *sp : 255) + FZ_COMBINE(*dp, masa);
-					dp++;
-				}
-				if (sa)
-					sp++;
+				*dp++ = *sp++;
 			}
 		}
 		else
 		{
-			int k = n;
-			if (sa)
+			int k = n+a;
+			while (k--)
 			{
-				int masa;
-				if (sp[n] == 0)
-				{
-					sp += n+1;
-					dp += n+da;
-					continue;
-				}
-				masa = FZ_REVERSE_COMBINE(sp[n], ma);
-				masa = FZ_EXPAND(masa);
-				while (k--)
-				{
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					sp++; dp++;
-				}
-				if (da)
-				{
-					*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-					dp++;
-				}
-				sp++;
-			}
-			else
-			{
-				while (k--)
-				{
-					*dp = FZ_BLEND(*sp, *dp, ma);
-					sp++; dp++;
-				}
-				if (da)
-				{
-					*dp = FZ_BLEND(255, *dp, ma);
-					dp++;
-				}
+				*dp = FZ_BLEND(*sp, *dp, ma);
+				sp++; dp++;
 			}
 		}
 	}
@@ -1279,214 +1029,110 @@ template_span_with_mask_N_general(byte * restrict dp, int da, const byte * restr
 }
 
 static void
-paint_span_with_mask_0_da_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_N_general(dp, 1, sp, 1, mp, 0, w);
-}
-
-static void
-paint_span_with_mask_0_da(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_0_a(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_N_general(dp, 1, sp, 0, mp, 0, w);
+	template_span_with_mask_N_general(dp, sp, 1, mp, 0, w);
 }
 
 static void
-paint_span_with_mask_1_da_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_1_a(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_1_general(dp, 1, sp, 1, mp, w);
+	template_span_with_mask_1_general(dp, sp, 1, mp, w);
 }
 
 static void
-paint_span_with_mask_1(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_1(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_1_general(dp, 0, sp, 0, mp, w);
+	template_span_with_mask_1_general(dp, sp, 0, mp, w);
 }
 
-#if FZ_PLOTTERS_G
-static void
-paint_span_with_mask_1_da(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_1_general(dp, 1, sp, 0, mp, w);
-}
-
-static void
-paint_span_with_mask_1_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_1_general(dp, 0, sp, 1, mp, w);
-}
-#endif /* FZ_PLOTTERS_G */
-
 #if FZ_PLOTTERS_RGB
 static void
-paint_span_with_mask_3_da_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_3_general(dp, 1, sp, 1, mp, w);
-}
-
-static void
-paint_span_with_mask_3_da(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_3_general(dp, 1, sp, 0, mp, w);
-}
-
-static void
-paint_span_with_mask_3_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_3_a(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_3_general(dp, 0, sp, 1, mp, w);
+	template_span_with_mask_3_general(dp, sp, 1, mp, w);
 }
 
 static void
-paint_span_with_mask_3(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_3(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_3_general(dp, 0, sp, 0, mp, w);
+	template_span_with_mask_3_general(dp, sp, 0, mp, w);
 }
 #endif /* FZ_PLOTTERS_RGB */
 
 #if FZ_PLOTTERS_CMYK
 static void
-paint_span_with_mask_4_da_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_4_general(dp, 1, sp, 1, mp, w);
-}
-
-static void
-paint_span_with_mask_4_da(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_4_general(dp, 1, sp, 0, mp, w);
-}
-
-static void
-paint_span_with_mask_4_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_4_a(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_4_general(dp, 0, sp, 1, mp, w);
+	template_span_with_mask_4_general(dp, sp, 1, mp, w);
 }
 
 static void
-paint_span_with_mask_4(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_4(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_4_general(dp, 0, sp, 0, mp, w);
+	template_span_with_mask_4_general(dp, sp, 0, mp, w);
 }
 #endif /* FZ_PLOTTERS_CMYK */
 
 #if FZ_PLOTTERS_N
 static void
-paint_span_with_mask_N_da_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_N_general(dp, 1, sp, 1, mp, n, w);
-}
-
-static void
-paint_span_with_mask_N_da(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
-{
-	TRACK_FN();
-	template_span_with_mask_N_general(dp, 1, sp, 0, mp, n, w);
-}
-
-static void
-paint_span_with_mask_N_sa(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_N_a(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_N_general(dp, 0, sp, 1, mp, n, w);
+	template_span_with_mask_N_general(dp, sp, 1, mp, n, w);
 }
 
 static void
-paint_span_with_mask_N(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w)
+paint_span_with_mask_N(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a)
 {
 	TRACK_FN();
-	template_span_with_mask_N_general(dp, 0, sp, 0, mp, n, w);
+	template_span_with_mask_N_general(dp, sp, 0, mp, n, w);
 }
 #endif /* FZ_PLOTTERS_N */
 
-typedef void (fz_span_mask_painter_t)(byte * restrict dp, int da, const byte * restrict sp, int sa, const byte * restrict mp, int n, int w);
+typedef void (fz_span_mask_painter_t)(byte * restrict dp, const byte * restrict sp, const byte * restrict mp, int w, int n, int a);
 
 static fz_span_mask_painter_t *
-fz_get_span_mask_painter(int da, int sa, int n)
+fz_get_span_mask_painter(int a, int n)
 {
 	switch(n)
 	{
 		case 0:
-			if (!da)
-				return NULL;
-			if (sa)
-				return paint_span_with_mask_0_da_sa;
-			else
-				return paint_span_with_mask_0_da;
+			/* assert(a); */
+			return paint_span_with_mask_0_a;
 		case 1:
-#if FZ_PLOTTERS_G
-			if (da)
-				if (sa)
-					return paint_span_with_mask_1_da_sa;
-				else
-					return paint_span_with_mask_1_da;
+			if (a)
+				return paint_span_with_mask_1_a;
 			else
-				if (sa)
-					return paint_span_with_mask_1_sa;
-				else
-					return paint_span_with_mask_1;
-#else
-			if (da && sa)
-					return paint_span_with_mask_1_da_sa;
-			if (!da & !sa)
-					return paint_span_with_mask_1;
-			goto fallback;
-#endif /* FZ_PLOTTERS_G */
+				return paint_span_with_mask_1;
 #if FZ_PLOTTERS_RGB
 		case 3:
-			if (da)
-				if (sa)
-					return paint_span_with_mask_3_da_sa;
-				else
-					return paint_span_with_mask_3_da;
+			if (a)
+				return paint_span_with_mask_3_a;
 			else
-				if (sa)
-					return paint_span_with_mask_3_sa;
-				else
-					return paint_span_with_mask_3;
+				return paint_span_with_mask_3;
 #endif /* FZ_PLOTTERS_RGB */
 #if FZ_PLOTTERS_CMYK
 		case 4:
-			if (da)
-				if (sa)
-					return paint_span_with_mask_4_da_sa;
-				else
-					return paint_span_with_mask_4_da;
+			if (a)
+				return paint_span_with_mask_4_a;
 			else
-				if (sa)
-					return paint_span_with_mask_4_sa;
-				else
-					return paint_span_with_mask_4;
+				return paint_span_with_mask_4;
 #endif /* FZ_PLOTTERS_CMYK */
 		default:
 		{
-#if !FZ_PLOTTERS_G
-fallback:{}
-#endif /* !FZ_PLOTTERS_G */
 #if FZ_PLOTTERS_N
-			if (da)
-				if (sa)
-					return paint_span_with_mask_N_da_sa;
-				else
-					return paint_span_with_mask_N_da;
+			if (a)
+				return paint_span_with_mask_N_a;
 			else
-				if (sa)
-					return paint_span_with_mask_N_sa;
-				else
-					return paint_span_with_mask_N;
+				return paint_span_with_mask_N;
 #else
 			return NULL;
 #endif /* FZ_PLOTTERS_N */
@@ -2322,14 +1968,17 @@ fz_paint_pixmap_with_mask(fz_pixmap * restrict dst, const fz_pixmap * restrict s
 	dp = dst->samples + (unsigned int)((y - dst->y) * dst->stride + (x - dst->x) * dst->n);
 	da = dst->alpha;
 
+	/* sa == da, or something has gone very wrong! */
+	assert(sa == da);
+
 	n -= sa;
-	fn = fz_get_span_mask_painter(da, sa, n);
+	fn = fz_get_span_mask_painter(da, n);
 	if (fn == NULL)
 		return;
 
 	while (h--)
 	{
-		(*fn)(dp, da, sp, sa, mp, n, w);
+		(*fn)(dp, sp, mp, w, n, sa);
 		sp += src->stride;
 		dp += dst->stride;
 		mp += msk->stride;