image: Eliminate self-intersections for the pixman traps compositor

As pixman uses an accumulation mask, it oversamples neighbouring edges
within a cell. We can reduce the impact of this by eliminating
overlapping triangles/trapezoids from being passed into pixman.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/cairo-image-compositor.c b/src/cairo-image-compositor.c
index 51ffc34bb..6ff0f09c0 100644
--- a/src/cairo-image-compositor.c
+++ b/src/cairo-image-compositor.c
@@ -649,11 +649,18 @@ composite_traps (void			*_dst,
 {
     cairo_image_surface_t *dst = (cairo_image_surface_t *) _dst;
     cairo_image_source_t *src = (cairo_image_source_t *) abstract_src;
+    cairo_int_status_t status;
     pixman_image_t *mask;
     pixman_format_code_t format;
 
     TRACE ((stderr, "%s\n", __FUNCTION__));
 
+    /* pixman doesn't eliminate self-intersecting trapezoids/edges */
+    status = _cairo_bentley_ottmann_tessellate_traps (traps,
+						      CAIRO_FILL_RULE_WINDING);
+    if (status != CAIRO_INT_STATUS_SUCCESS)
+	    return status;
+
     /* Special case adding trapezoids onto a mask surface; we want to avoid
      * creating an intermediate temporary mask unnecessarily.
      *
@@ -738,6 +745,31 @@ composite_tristrip (void			*_dst,
     if (strip->num_points < 3)
 	return CAIRO_STATUS_SUCCESS;
 
+    if (1) { /* pixman doesn't eliminate self-intersecting triangles/edges */
+	    cairo_int_status_t status;
+	    cairo_traps_t traps;
+	    int n;
+
+	    _cairo_traps_init (&traps);
+	    for (n = 0; n < strip->num_points; n++) {
+		    cairo_point_t p[4];
+
+		    p[0] = strip->points[0];
+		    p[1] = strip->points[1];
+		    p[2] = strip->points[2];
+		    p[3] = strip->points[0];
+
+		    _cairo_traps_tessellate_convex_quad (&traps, p);
+	    }
+	    status = composite_traps (_dst, op, abstract_src,
+				      src_x, src_y,
+				      dst_x, dst_y,
+				      extents, antialias, &traps);
+	    _cairo_traps_fini (&traps);
+
+	    return status;
+    }
+
     format = antialias == CAIRO_ANTIALIAS_NONE ? PIXMAN_a1 : PIXMAN_a8;
     if (dst->pixman_format == format &&
 	(abstract_src == NULL ||