stroke: Fix large line widths for fallback stroke shaper

Fix the test case line-width-tolerance for the fallback stroke shaper.
Instead of drawing quads between spline points, draw triangle based on
the actual spline edges. This roughly follows the approach of the
tristrip and polygonal shapers.

1 files changed, 34 insertions, 0 deletions

diff --git a/src/cairo-fixed-private.h b/src/cairo-fixed-private.h
index b6cc6be7d..352ca39d0 100644
--- a/src/cairo-fixed-private.h
+++ b/src/cairo-fixed-private.h
@@ -352,6 +352,40 @@ _cairo_edge_compute_intersection_x_for_y (const cairo_point_t *p1,
     return x;
 }
 
+/* Intersect two segments based on the algorithm described at
+ * http://paulbourke.net/geometry/pointlineplane/. This implementation
+ * uses floating point math. */
+static inline cairo_bool_t
+_slow_segment_intersection (const cairo_point_t *seg1_p1,
+			    const cairo_point_t *seg1_p2,
+			    const cairo_point_t *seg2_p1,
+			    const cairo_point_t *seg2_p2,
+			    cairo_point_t *intersection)
+{
+    double denominator, u_a, u_b;
+    double seg1_dx, seg1_dy, seg2_dx, seg2_dy, seg_start_dx, seg_start_dy;
+
+    seg1_dx = _cairo_fixed_to_double (seg1_p2->x - seg1_p1->x);
+    seg1_dy = _cairo_fixed_to_double (seg1_p2->y - seg1_p1->y);
+    seg2_dx = _cairo_fixed_to_double (seg2_p2->x - seg2_p1->x);
+    seg2_dy = _cairo_fixed_to_double (seg2_p2->y - seg2_p1->y);
+    denominator = (seg2_dy * seg1_dx) - (seg2_dx * seg1_dy);
+    if (denominator == 0)
+	return FALSE;
+
+    seg_start_dx = _cairo_fixed_to_double (seg1_p1->x - seg2_p1->x);
+    seg_start_dy = _cairo_fixed_to_double (seg1_p1->y - seg2_p1->y);
+    u_a = ((seg2_dx * seg_start_dy) - (seg2_dy * seg_start_dx)) / denominator;
+    u_b = ((seg1_dx * seg_start_dy) - (seg1_dy * seg_start_dx)) / denominator;
+
+    if (u_a <= 0 || u_a >= 1 || u_b <= 0 || u_b >= 1)
+	return FALSE;
+
+    intersection->x = seg1_p1->x + _cairo_fixed_from_double ((u_a * seg1_dx));
+    intersection->y = seg1_p1->y + _cairo_fixed_from_double ((u_a * seg1_dy));
+    return TRUE;
+}
+
 #else
 # error Please define multiplication and other operands for your fixed-point type size
 #endif