polygon: Extend intersection edges to cover entire range

By simply swapping the continuation edges, we end up with a set of edges
that are defined over a shorter range than their extents. Whilst this is
numerically stable at our normal precision we start to encounter issues
when using a coarser grid during rasterisation as the derivative of the
edge becomes unstable.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44722
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/cairo-polygon-intersect.c b/src/cairo-polygon-intersect.c
index 573f801a7..0e5b0b972 100644
--- a/src/cairo-polygon-intersect.c
+++ b/src/cairo-polygon-intersect.c
@@ -1179,8 +1179,17 @@ edges_start_or_continue (cairo_bo_edge_t	*left,
 
     if (left->deferred.other != NULL) {
 	if (right != NULL && edges_colinear (left->deferred.other, right)) {
-	    /* continuation on right, so just swap edges */
-	    assert (left->deferred.other->deferred.other == NULL);
+	    cairo_bo_edge_t *old = left->deferred.other;
+
+	    /* continuation on right, extend right to cover both */
+	    assert (old->deferred.other == NULL);
+	    assert (old->edge.dir == right->edge.dir);
+	    assert (old->edge.line.p2.y > old->edge.line.p1.y);
+
+	    if (old->edge.line.p1.y < right->edge.line.p1.y)
+		right->edge.line.p1 = old->edge.line.p1;
+	    if (old->edge.line.p2.y > right->edge.line.p2.y)
+		right->edge.line.p2 = old->edge.line.p2;
 	    left->deferred.other = right;
 	    return;
 	}