Merge branch 'trap-fixes'

3 files changed, 17 insertions, 13 deletions

diff --git a/pixman/pixman-edge-imp.h b/pixman/pixman-edge-imp.h
index 74720d10..a4698edd 100644
--- a/pixman/pixman-edge-imp.h
+++ b/pixman/pixman-edge-imp.h
@@ -49,10 +49,14 @@ RASTERIZE_EDGES (pixman_image_t  *image,
 	rx = r->x;
 #if N_BITS == 1
 	/* For the non-antialiased case, round the coordinates up, in effect
-	 * sampling the center of the pixel. (The AA case does a similar 
-	 * adjustment in RENDER_SAMPLES_X) */
-	lx += X_FRAC_FIRST(1);
-	rx += X_FRAC_FIRST(1);
+	 * sampling just slightly to the left of the pixel. This is so that
+	 * when the sample point lies exactly on the line, we round towards
+	 * north-west.
+	 *
+	 * (The AA case does a similar  adjustment in RENDER_SAMPLES_X)
+	 */
+	lx += X_FRAC_FIRST(1) - pixman_fixed_e;
+	rx += X_FRAC_FIRST(1) - pixman_fixed_e;
 #endif
 	/* clip X */
 	if (lx < 0)
diff --git a/pixman/pixman-private.h b/pixman/pixman-private.h
index 048d030e..7ed8aa7a 100644
--- a/pixman/pixman-private.h
+++ b/pixman/pixman-private.h
@@ -349,13 +349,13 @@ _pixman_gradient_walker_pixel (pixman_gradient_walker_t *walker,
 #define STEP_Y_SMALL(n) (pixman_fixed_1 / N_Y_FRAC (n))
 #define STEP_Y_BIG(n)   (pixman_fixed_1 - (N_Y_FRAC (n) - 1) * STEP_Y_SMALL (n))
 
-#define Y_FRAC_FIRST(n) (STEP_Y_SMALL (n) / 2)
+#define Y_FRAC_FIRST(n) (STEP_Y_BIG (n) / 2)
 #define Y_FRAC_LAST(n)  (Y_FRAC_FIRST (n) + (N_Y_FRAC (n) - 1) * STEP_Y_SMALL (n))
 
 #define STEP_X_SMALL(n) (pixman_fixed_1 / N_X_FRAC (n))
 #define STEP_X_BIG(n)   (pixman_fixed_1 - (N_X_FRAC (n) - 1) * STEP_X_SMALL (n))
 
-#define X_FRAC_FIRST(n) (STEP_X_SMALL (n) / 2)
+#define X_FRAC_FIRST(n) (STEP_X_BIG (n) / 2)
 #define X_FRAC_LAST(n)  (X_FRAC_FIRST (n) + (N_X_FRAC (n) - 1) * STEP_X_SMALL (n))
 
 #define RENDER_SAMPLES_X(x, n)						\
diff --git a/pixman/pixman-trap.c b/pixman/pixman-trap.c
index 962cbb39..8353992c 100644
--- a/pixman/pixman-trap.c
+++ b/pixman/pixman-trap.c
@@ -28,8 +28,8 @@
 #include "pixman-private.h"
 
 /*
- * Compute the smallest value no less than y which is on a
- * grid row
+ * Compute the smallest value greater than or equal to y which is on a
+ * grid row.
  */
 
 PIXMAN_EXPORT pixman_fixed_t
@@ -38,7 +38,7 @@ pixman_sample_ceil_y (pixman_fixed_t y, int n)
     pixman_fixed_t f = pixman_fixed_frac (y);
     pixman_fixed_t i = pixman_fixed_floor (y);
 
-    f = ((f + Y_FRAC_FIRST (n)) / STEP_Y_SMALL (n)) * STEP_Y_SMALL (n) +
+    f = DIV (f - Y_FRAC_FIRST (n) + (STEP_Y_SMALL (n) - pixman_fixed_e), STEP_Y_SMALL (n)) * STEP_Y_SMALL (n) +
 	Y_FRAC_FIRST (n);
     
     if (f > Y_FRAC_LAST (n))
@@ -57,8 +57,8 @@ pixman_sample_ceil_y (pixman_fixed_t y, int n)
 }
 
 /*
- * Compute the largest value no greater than y which is on a
- * grid row
+ * Compute the largest value strictly less than y which is on a
+ * grid row.
  */
 PIXMAN_EXPORT pixman_fixed_t
 pixman_sample_floor_y (pixman_fixed_t y,
@@ -67,7 +67,7 @@ pixman_sample_floor_y (pixman_fixed_t y,
     pixman_fixed_t f = pixman_fixed_frac (y);
     pixman_fixed_t i = pixman_fixed_floor (y);
 
-    f = DIV (f - Y_FRAC_FIRST (n), STEP_Y_SMALL (n)) * STEP_Y_SMALL (n) +
+    f = DIV (f - pixman_fixed_e - Y_FRAC_FIRST (n), STEP_Y_SMALL (n)) * STEP_Y_SMALL (n) +
 	Y_FRAC_FIRST (n);
 
     if (f < Y_FRAC_FIRST (n))
@@ -380,7 +380,7 @@ pixman_rasterize_trapezoid (pixman_image_t *          image,
     if (pixman_fixed_to_int (b) >= height)
 	b = pixman_int_to_fixed (height) - 1;
     b = pixman_sample_floor_y (b, bpp);
-
+    
     if (b >= t)
     {
 	/* initialize edge walkers */