Bug 683243. Dither 16 bit gradients. r=BenWa
☠☠ backed out by 8919db908bed ☠ ☠
authorJeff Muizelaar <jmuizelaar@mozilla.com>
Wed, 04 Jul 2012 11:42:29 -0400
changeset 99148 0c1f34eb5b9304e636789e4a23819cf2a8ca69d5
parent 99147 0f987ac28e8c0ba440e1bd9c1280a0bbe44834e8
child 99149 23c13d1dd6069d0643ec80a0ebe17936433dd099
push id11846
push userjmuizelaar@mozilla.com
push dateFri, 13 Jul 2012 03:42:19 +0000
treeherdermozilla-inbound@0c1f34eb5b93 [default view] [failures only]
perfherder[talos] [build metrics] [platform microbench] (compared to previous push)
reviewersBenWa
bugs683243
milestone16.0a1
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nightly linux64
nightly mac
nightly win32
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last release without
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Bug 683243. Dither 16 bit gradients. r=BenWa This does a 2x2 ordered dither in the same way that Skia does. One of the things I'm currently unhappy with is that it duplicates the gradient walker code for 16 bits. We could turn it into a large macro that does the appropriate things for 16 bit and 32 bit versions, but that's not particularly appealing.
gfx/cairo/README
gfx/cairo/libpixman/src/pixman-dither.h
gfx/cairo/libpixman/src/pixman-linear-gradient.c
gfx/cairo/libpixman/src/pixman-radial-gradient.c
gfx/cairo/pixman-dither.patch
--- a/gfx/cairo/README
+++ b/gfx/cairo/README
@@ -195,11 +195,13 @@ pixman-export.patch: use cairo_public fo
 pixman-limits.patch: include limits.h for SIZE_MAX
 
 pixman-lowres-interp.patch: Use lower quality interpolation for more speed.
 
 pixman-bilinear-fastpath.patch: Bilinear fast paths for non-neon
 
 pixman-16-bit-pipeline.patch: 16 bit pipeline for dithering
 
+pixman-dither.patch: Add dithering of 16 bit gradients
+
 ==== disable printing patch ====
 
 disable-printing.patch:  allows us to use NS_PRINTING to disable printing.
new file mode 100644
--- /dev/null
+++ b/gfx/cairo/libpixman/src/pixman-dither.h
@@ -0,0 +1,51 @@
+#define R16_BITS     5
+#define G16_BITS     6
+#define B16_BITS     5
+
+#define R16_SHIFT    (B16_BITS + G16_BITS)
+#define G16_SHIFT    (B16_BITS)
+#define B16_SHIFT    0
+
+#define MASK 0xff
+#define ONE_HALF 0x80
+
+#define A_SHIFT 8 * 3
+#define R_SHIFT 8 * 2
+#define G_SHIFT 8
+#define A_MASK 0xff000000
+#define R_MASK 0xff0000
+#define G_MASK 0xff00
+
+#define RB_MASK 0xff00ff
+#define AG_MASK 0xff00ff00
+#define RB_ONE_HALF 0x800080
+#define RB_MASK_PLUS_ONE 0x10000100
+
+#define ALPHA_8(x) ((x) >> A_SHIFT)
+#define RED_8(x) (((x) >> R_SHIFT) & MASK)
+#define GREEN_8(x) (((x) >> G_SHIFT) & MASK)
+#define BLUE_8(x) ((x) & MASK)
+
+// This uses the same dithering technique that Skia does.
+// It is essentially preturbing the lower bit based on the
+// high bit
+static inline uint16_t dither_32_to_16(uint32_t c)
+{
+    uint8_t b = BLUE_8(c);
+    uint8_t g = GREEN_8(c);
+    uint8_t r = RED_8(c);
+    r = ((r << 1) - ((r >> (8 - R16_BITS) << (8 - R16_BITS)) | (r >> R16_BITS))) >> (8 - R16_BITS);
+    g = ((g << 1) - ((g >> (8 - G16_BITS) << (8 - G16_BITS)) | (g >> G16_BITS))) >> (8 - G16_BITS);
+    b = ((b << 1) - ((b >> (8 - B16_BITS) << (8 - B16_BITS)) | (b >> B16_BITS))) >> (8 - B16_BITS);
+    return ((r << R16_SHIFT) | (g << G16_SHIFT) | (b << B16_SHIFT));
+}
+
+static inline uint16_t dither_8888_to_0565(uint32_t color, pixman_bool_t toggle)
+{
+    // alternate between a preturbed truncation and a regular truncation
+    if (toggle) {
+	return dither_32_to_16(color);
+    } else {
+	return CONVERT_8888_TO_0565(color);
+    }
+}
--- a/gfx/cairo/libpixman/src/pixman-linear-gradient.c
+++ b/gfx/cairo/libpixman/src/pixman-linear-gradient.c
@@ -26,16 +26,18 @@
  */
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif
 #include <stdlib.h>
 #include "pixman-private.h"
 
+#include "pixman-dither.h"
+
 static pixman_bool_t
 linear_gradient_is_horizontal (pixman_image_t *image,
 			       int             x,
 			       int             y,
 			       int             width,
 			       int             height)
 {
     linear_gradient_t *linear = (linear_gradient_t *)image;
@@ -222,25 +224,28 @@ linear_get_scanline_narrow (pixman_iter_
     return iter->buffer;
 }
 
 static uint16_t convert_8888_to_0565(uint32_t color)
 {
     return CONVERT_8888_TO_0565(color);
 }
 
+
+
 static uint32_t *
 linear_get_scanline_16 (pixman_iter_t  *iter,
 			const uint32_t *mask)
 {
     pixman_image_t *image  = iter->image;
     int             x      = iter->x;
     int             y      = iter->y;
     int             width  = iter->width;
     uint16_t *      buffer = (uint16_t*)iter->buffer;
+    pixman_bool_t   toggle = ((x ^ y) & 1);
 
     pixman_vector_t v, unit;
     pixman_fixed_32_32_t l;
     pixman_fixed_48_16_t dx, dy;
     gradient_t *gradient = (gradient_t *)image;
     linear_gradient_t *linear = (linear_gradient_t *)image;
     uint16_t *end = buffer + width;
     pixman_gradient_walker_t walker;
@@ -294,34 +299,47 @@ linear_get_scanline_16 (pixman_iter_t  *
 	    t = ((dx * v.vector[0] + dy * v.vector[1]) - 
 		 (dx * linear->p1.x + dy * linear->p1.y) * v2) * invden;
 	    inc = (dx * unit.vector[0] + dy * unit.vector[1]) * invden;
 	}
 	next_inc = 0;
 
 	if (((pixman_fixed_32_32_t )(inc * width)) == 0)
 	{
-	    register uint16_t color;
+	    register uint32_t color;
+	    uint16_t dither_diff;
+	    uint16_t color16;
+	    uint16_t color16b;
 
-	    color = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t));
-	    while (buffer < end)
-		*buffer++ = color;
+	    color = _pixman_gradient_walker_pixel (&walker, t);
+	    color16 = dither_8888_to_0565(color, toggle);
+	    color16b = dither_8888_to_0565(color, toggle^1);
+	    // compute the difference
+	    dither_diff =  color16 ^ color16b;
+	    while (buffer < end) {
+		*buffer++ = color16;
+		// use dither_diff to toggle between color16 and color16b
+		color16 ^= dither_diff;
+		toggle ^= 1;
+	    }
 	}
 	else
 	{
 	    int i;
 
 	    i = 0;
 	    while (buffer < end)
 	    {
 		if (!mask || *mask++)
 		{
-		    *buffer = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker,
-										  t + next_inc));
+		    *buffer = dither_8888_to_0565(_pixman_gradient_walker_pixel (&walker,
+										 t + next_inc),
+						  toggle);
 		}
+		toggle ^= 1;
 		i++;
 		next_inc = inc * i;
 		buffer++;
 	    }
 	}
     }
     else
     {
@@ -340,18 +358,20 @@ linear_get_scanline_16 (pixman_iter_t  *
 
 		    invden = pixman_fixed_1 * (double) pixman_fixed_1 /
 			(l * (double) v.vector[2]);
 		    v2 = v.vector[2] * (1. / pixman_fixed_1);
 		    t = ((dx * v.vector[0] + dy * v.vector[1]) - 
 			 (dx * linear->p1.x + dy * linear->p1.y) * v2) * invden;
 		}
 
-		*buffer = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t));
+		*buffer = dither_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t),
+					      toggle);
 	    }
+	    toggle ^= 1;
 
 	    ++buffer;
 
 	    v.vector[0] += unit.vector[0];
 	    v.vector[1] += unit.vector[1];
 	    v.vector[2] += unit.vector[2];
 	}
     }
@@ -369,17 +389,18 @@ linear_get_scanline_wide (pixman_iter_t 
     pixman_expand ((uint64_t *)buffer, buffer, PIXMAN_a8r8g8b8, iter->width);
 
     return buffer;
 }
 
 void
 _pixman_linear_gradient_iter_init (pixman_image_t *image, pixman_iter_t  *iter)
 {
-    if (linear_gradient_is_horizontal (
+    // XXX: we can't use this optimization when dithering
+    if (0 && linear_gradient_is_horizontal (
 	    iter->image, iter->x, iter->y, iter->width, iter->height))
     {
 	if (iter->flags & ITER_16)
 	    linear_get_scanline_16 (iter, NULL);
 	else if (iter->flags & ITER_NARROW)
 	    linear_get_scanline_narrow (iter, NULL);
 	else
 	    linear_get_scanline_wide (iter, NULL);
--- a/gfx/cairo/libpixman/src/pixman-radial-gradient.c
+++ b/gfx/cairo/libpixman/src/pixman-radial-gradient.c
@@ -29,16 +29,18 @@
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif
 #include <stdlib.h>
 #include <math.h>
 #include "pixman-private.h"
 
+#include "pixman-dither.h"
+
 static inline pixman_fixed_32_32_t
 dot (pixman_fixed_48_16_t x1,
      pixman_fixed_48_16_t y1,
      pixman_fixed_48_16_t z1,
      pixman_fixed_48_16_t x2,
      pixman_fixed_48_16_t y2,
      pixman_fixed_48_16_t z2)
 {
@@ -489,16 +491,17 @@ radial_get_scanline_16 (pixman_iter_t *i
      *   <=> for every p, the radiuses associated with the two t solutions
      *       have opposite sign
      */
     pixman_image_t *image = iter->image;
     int x = iter->x;
     int y = iter->y;
     int width = iter->width;
     uint16_t *buffer = iter->buffer;
+    pixman_bool_t toggle = ((x ^ y) & 1);
 
     gradient_t *gradient = (gradient_t *)image;
     radial_gradient_t *radial = (radial_gradient_t *)image;
     uint16_t *end = buffer + width;
     pixman_gradient_walker_t walker;
     pixman_vector_t v, unit;
 
     /* reference point is the center of the pixel */
@@ -575,25 +578,27 @@ radial_get_scanline_16 (pixman_iter_t *i
 		  unit.vector[0], unit.vector[1], 0);
 	ddc = 2 * dot (unit.vector[0], unit.vector[1], 0,
 		       unit.vector[0], unit.vector[1], 0);
 
 	while (buffer < end)
 	{
 	    if (!mask || *mask++)
 	    {
-		*buffer = convert_8888_to_0565(
+		*buffer = dither_8888_to_0565(
 			  radial_compute_color (radial->a, b, c,
 						radial->inva,
 						radial->delta.radius,
 						radial->mindr,
 						&walker,
-						image->common.repeat));
+						image->common.repeat),
+			  toggle);
 	    }
 
+	    toggle ^= 1;
 	    b += db;
 	    c += dc;
 	    dc += ddc;
 	    ++buffer;
 	}
     }
     else
     {
@@ -621,31 +626,33 @@ radial_get_scanline_16 (pixman_iter_t *i
 			      radial->delta.x, radial->delta.y,
 			      radial->delta.radius);
 		    /*  / pixman_fixed_1 / pixman_fixed_1 */
 
 		    c = fdot (pdx, pdy, -radial->c1.radius,
 			      pdx, pdy, radial->c1.radius);
 		    /*  / pixman_fixed_1 / pixman_fixed_1 */
 
-		    *buffer = convert_8888_to_0565 (
+		    *buffer = dither_8888_to_0565 (
 			      radial_compute_color (radial->a, b, c,
 						    radial->inva,
 						    radial->delta.radius,
 						    radial->mindr,
 						    &walker,
-						    image->common.repeat));
+						    image->common.repeat),
+			      toggle);
 		}
 		else
 		{
 		    *buffer = 0;
 		}
 	    }
 
 	    ++buffer;
+	    toggle ^= 1;
 
 	    v.vector[0] += unit.vector[0];
 	    v.vector[1] += unit.vector[1];
 	    v.vector[2] += unit.vector[2];
 	}
     }
 
     iter->y++;
new file mode 100644
--- /dev/null
+++ b/gfx/cairo/pixman-dither.patch
@@ -0,0 +1,310 @@
+diff --git a/gfx/cairo/libpixman/src/pixman-dither.h b/gfx/cairo/libpixman/src/pixman-dither.h
+new file mode 100644
+--- /dev/null
++++ b/gfx/cairo/libpixman/src/pixman-dither.h
+@@ -0,0 +1,51 @@
++#define R16_BITS     5
++#define G16_BITS     6
++#define B16_BITS     5
++
++#define R16_SHIFT    (B16_BITS + G16_BITS)
++#define G16_SHIFT    (B16_BITS)
++#define B16_SHIFT    0
++
++#define MASK 0xff
++#define ONE_HALF 0x80
++
++#define A_SHIFT 8 * 3
++#define R_SHIFT 8 * 2
++#define G_SHIFT 8
++#define A_MASK 0xff000000
++#define R_MASK 0xff0000
++#define G_MASK 0xff00
++
++#define RB_MASK 0xff00ff
++#define AG_MASK 0xff00ff00
++#define RB_ONE_HALF 0x800080
++#define RB_MASK_PLUS_ONE 0x10000100
++
++#define ALPHA_8(x) ((x) >> A_SHIFT)
++#define RED_8(x) (((x) >> R_SHIFT) & MASK)
++#define GREEN_8(x) (((x) >> G_SHIFT) & MASK)
++#define BLUE_8(x) ((x) & MASK)
++
++// This uses the same dithering technique that Skia does.
++// It is essentially preturbing the lower bit based on the
++// high bit
++static inline uint16_t dither_32_to_16(uint32_t c)
++{
++    uint8_t b = BLUE_8(c);
++    uint8_t g = GREEN_8(c);
++    uint8_t r = RED_8(c);
++    r = ((r << 1) - ((r >> (8 - R16_BITS) << (8 - R16_BITS)) | (r >> R16_BITS))) >> (8 - R16_BITS);
++    g = ((g << 1) - ((g >> (8 - G16_BITS) << (8 - G16_BITS)) | (g >> G16_BITS))) >> (8 - G16_BITS);
++    b = ((b << 1) - ((b >> (8 - B16_BITS) << (8 - B16_BITS)) | (b >> B16_BITS))) >> (8 - B16_BITS);
++    return ((r << R16_SHIFT) | (g << G16_SHIFT) | (b << B16_SHIFT));
++}
++
++static inline uint16_t dither_8888_to_0565(uint32_t color, pixman_bool_t toggle)
++{
++    // alternate between a preturbed truncation and a regular truncation
++    if (toggle) {
++	return dither_32_to_16(color);
++    } else {
++	return CONVERT_8888_TO_0565(color);
++    }
++}
+diff --git a/gfx/cairo/libpixman/src/pixman-linear-gradient.c b/gfx/cairo/libpixman/src/pixman-linear-gradient.c
+--- a/gfx/cairo/libpixman/src/pixman-linear-gradient.c
++++ b/gfx/cairo/libpixman/src/pixman-linear-gradient.c
+@@ -26,16 +26,18 @@
+  */
+ 
+ #ifdef HAVE_CONFIG_H
+ #include <config.h>
+ #endif
+ #include <stdlib.h>
+ #include "pixman-private.h"
+ 
++#include "pixman-dither.h"
++
+ static pixman_bool_t
+ linear_gradient_is_horizontal (pixman_image_t *image,
+ 			       int             x,
+ 			       int             y,
+ 			       int             width,
+ 			       int             height)
+ {
+     linear_gradient_t *linear = (linear_gradient_t *)image;
+@@ -222,25 +224,28 @@ linear_get_scanline_narrow (pixman_iter_
+     return iter->buffer;
+ }
+ 
+ static uint16_t convert_8888_to_0565(uint32_t color)
+ {
+     return CONVERT_8888_TO_0565(color);
+ }
+ 
++
++
+ static uint32_t *
+ linear_get_scanline_16 (pixman_iter_t  *iter,
+ 			const uint32_t *mask)
+ {
+     pixman_image_t *image  = iter->image;
+     int             x      = iter->x;
+     int             y      = iter->y;
+     int             width  = iter->width;
+     uint16_t *      buffer = (uint16_t*)iter->buffer;
++    pixman_bool_t   toggle = ((x ^ y) & 1);
+ 
+     pixman_vector_t v, unit;
+     pixman_fixed_32_32_t l;
+     pixman_fixed_48_16_t dx, dy;
+     gradient_t *gradient = (gradient_t *)image;
+     linear_gradient_t *linear = (linear_gradient_t *)image;
+     uint16_t *end = buffer + width;
+     pixman_gradient_walker_t walker;
+@@ -294,34 +299,47 @@ linear_get_scanline_16 (pixman_iter_t  *
+ 	    t = ((dx * v.vector[0] + dy * v.vector[1]) - 
+ 		 (dx * linear->p1.x + dy * linear->p1.y) * v2) * invden;
+ 	    inc = (dx * unit.vector[0] + dy * unit.vector[1]) * invden;
+ 	}
+ 	next_inc = 0;
+ 
+ 	if (((pixman_fixed_32_32_t )(inc * width)) == 0)
+ 	{
+-	    register uint16_t color;
++	    register uint32_t color;
++	    uint16_t dither_diff;
++	    uint16_t color16;
++	    uint16_t color16b;
+ 
+-	    color = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t));
+-	    while (buffer < end)
+-		*buffer++ = color;
++	    color = _pixman_gradient_walker_pixel (&walker, t);
++	    color16 = dither_8888_to_0565(color, toggle);
++	    color16b = dither_8888_to_0565(color, toggle^1);
++	    // compute the difference
++	    dither_diff =  color16 ^ color16b;
++	    while (buffer < end) {
++		*buffer++ = color16;
++		// use dither_diff to toggle between color16 and color16b
++		color16 ^= dither_diff;
++		toggle ^= 1;
++	    }
+ 	}
+ 	else
+ 	{
+ 	    int i;
+ 
+ 	    i = 0;
+ 	    while (buffer < end)
+ 	    {
+ 		if (!mask || *mask++)
+ 		{
+-		    *buffer = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker,
+-										  t + next_inc));
++		    *buffer = dither_8888_to_0565(_pixman_gradient_walker_pixel (&walker,
++										 t + next_inc),
++						  toggle);
+ 		}
++		toggle ^= 1;
+ 		i++;
+ 		next_inc = inc * i;
+ 		buffer++;
+ 	    }
+ 	}
+     }
+     else
+     {
+@@ -340,18 +358,20 @@ linear_get_scanline_16 (pixman_iter_t  *
+ 
+ 		    invden = pixman_fixed_1 * (double) pixman_fixed_1 /
+ 			(l * (double) v.vector[2]);
+ 		    v2 = v.vector[2] * (1. / pixman_fixed_1);
+ 		    t = ((dx * v.vector[0] + dy * v.vector[1]) - 
+ 			 (dx * linear->p1.x + dy * linear->p1.y) * v2) * invden;
+ 		}
+ 
+-		*buffer = convert_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t));
++		*buffer = dither_8888_to_0565(_pixman_gradient_walker_pixel (&walker, t),
++					      toggle);
+ 	    }
++	    toggle ^= 1;
+ 
+ 	    ++buffer;
+ 
+ 	    v.vector[0] += unit.vector[0];
+ 	    v.vector[1] += unit.vector[1];
+ 	    v.vector[2] += unit.vector[2];
+ 	}
+     }
+@@ -369,17 +389,18 @@ linear_get_scanline_wide (pixman_iter_t 
+     pixman_expand ((uint64_t *)buffer, buffer, PIXMAN_a8r8g8b8, iter->width);
+ 
+     return buffer;
+ }
+ 
+ void
+ _pixman_linear_gradient_iter_init (pixman_image_t *image, pixman_iter_t  *iter)
+ {
+-    if (linear_gradient_is_horizontal (
++    // XXX: we can't use this optimization when dithering
++    if (0 && linear_gradient_is_horizontal (
+ 	    iter->image, iter->x, iter->y, iter->width, iter->height))
+     {
+ 	if (iter->flags & ITER_16)
+ 	    linear_get_scanline_16 (iter, NULL);
+ 	else if (iter->flags & ITER_NARROW)
+ 	    linear_get_scanline_narrow (iter, NULL);
+ 	else
+ 	    linear_get_scanline_wide (iter, NULL);
+diff --git a/gfx/cairo/libpixman/src/pixman-radial-gradient.c b/gfx/cairo/libpixman/src/pixman-radial-gradient.c
+--- a/gfx/cairo/libpixman/src/pixman-radial-gradient.c
++++ b/gfx/cairo/libpixman/src/pixman-radial-gradient.c
+@@ -29,16 +29,18 @@
+ 
+ #ifdef HAVE_CONFIG_H
+ #include <config.h>
+ #endif
+ #include <stdlib.h>
+ #include <math.h>
+ #include "pixman-private.h"
+ 
++#include "pixman-dither.h"
++
+ static inline pixman_fixed_32_32_t
+ dot (pixman_fixed_48_16_t x1,
+      pixman_fixed_48_16_t y1,
+      pixman_fixed_48_16_t z1,
+      pixman_fixed_48_16_t x2,
+      pixman_fixed_48_16_t y2,
+      pixman_fixed_48_16_t z2)
+ {
+@@ -489,16 +491,17 @@ radial_get_scanline_16 (pixman_iter_t *i
+      *   <=> for every p, the radiuses associated with the two t solutions
+      *       have opposite sign
+      */
+     pixman_image_t *image = iter->image;
+     int x = iter->x;
+     int y = iter->y;
+     int width = iter->width;
+     uint16_t *buffer = iter->buffer;
++    pixman_bool_t toggle = ((x ^ y) & 1);
+ 
+     gradient_t *gradient = (gradient_t *)image;
+     radial_gradient_t *radial = (radial_gradient_t *)image;
+     uint16_t *end = buffer + width;
+     pixman_gradient_walker_t walker;
+     pixman_vector_t v, unit;
+ 
+     /* reference point is the center of the pixel */
+@@ -575,25 +578,27 @@ radial_get_scanline_16 (pixman_iter_t *i
+ 		  unit.vector[0], unit.vector[1], 0);
+ 	ddc = 2 * dot (unit.vector[0], unit.vector[1], 0,
+ 		       unit.vector[0], unit.vector[1], 0);
+ 
+ 	while (buffer < end)
+ 	{
+ 	    if (!mask || *mask++)
+ 	    {
+-		*buffer = convert_8888_to_0565(
++		*buffer = dither_8888_to_0565(
+ 			  radial_compute_color (radial->a, b, c,
+ 						radial->inva,
+ 						radial->delta.radius,
+ 						radial->mindr,
+ 						&walker,
+-						image->common.repeat));
++						image->common.repeat),
++			  toggle);
+ 	    }
+ 
++	    toggle ^= 1;
+ 	    b += db;
+ 	    c += dc;
+ 	    dc += ddc;
+ 	    ++buffer;
+ 	}
+     }
+     else
+     {
+@@ -621,31 +626,33 @@ radial_get_scanline_16 (pixman_iter_t *i
+ 			      radial->delta.x, radial->delta.y,
+ 			      radial->delta.radius);
+ 		    /*  / pixman_fixed_1 / pixman_fixed_1 */
+ 
+ 		    c = fdot (pdx, pdy, -radial->c1.radius,
+ 			      pdx, pdy, radial->c1.radius);
+ 		    /*  / pixman_fixed_1 / pixman_fixed_1 */
+ 
+-		    *buffer = convert_8888_to_0565 (
++		    *buffer = dither_8888_to_0565 (
+ 			      radial_compute_color (radial->a, b, c,
+ 						    radial->inva,
+ 						    radial->delta.radius,
+ 						    radial->mindr,
+ 						    &walker,
+-						    image->common.repeat));
++						    image->common.repeat),
++			      toggle);
+ 		}
+ 		else
+ 		{
+ 		    *buffer = 0;
+ 		}
+ 	    }
+ 
+ 	    ++buffer;
++	    toggle ^= 1;
+ 
+ 	    v.vector[0] += unit.vector[0];
+ 	    v.vector[1] += unit.vector[1];
+ 	    v.vector[2] += unit.vector[2];
+ 	}
+     }
+ 
+     iter->y++;
+