stroke,traps: Emit join without loss of precision

As the target renderers operate at a different sample resolution then we
use internally for coordinate representation, there is always a potential
for discrepancies in the line gradients when passing around trapezoids.
To overcome this, the protocol specification of trapezoids uses the full
lines and vertical range as opposed to vertices and so long as we always
use the same lines for conjoint trapezoids, they remain abutting in the
rasteriser.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84115
Testcase: bug-84115
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 40 insertions, 15 deletions

diff --git a/src/cairo-path-stroke-traps.c b/src/cairo-path-stroke-traps.c
index 8b6e30f6c..520a3e5b9 100644
--- a/src/cairo-path-stroke-traps.c
+++ b/src/cairo-path-stroke-traps.c
@@ -249,9 +249,11 @@ join (struct stroker *stroker,
 	     in->dev_slope.y * out->dev_slope.y) < stroker->spline_cusp_tolerance)
 	{
 	    int start, stop;
-	    cairo_point_t tri[3];
+	    cairo_point_t tri[3], edges[4];
 	    cairo_pen_t *pen = &stroker->pen;
 
+	    edges[0] = in->cw;
+	    edges[1] = in->ccw;
 	    tri[0] = in->point;
 	    tri[1] = *inpt;
 	    if (clockwise) {
@@ -261,8 +263,13 @@ join (struct stroker *stroker,
 		while (start != stop) {
 		    tri[2] = in->point;
 		    translate_point (&tri[2], &pen->vertices[start].point);
-		    _cairo_traps_tessellate_triangle (stroker->traps, tri);
+		    edges[2] = in->point;
+		    edges[3] = tri[2];
+		    _cairo_traps_tessellate_triangle_with_edges (stroker->traps,
+								 tri, edges);
 		    tri[1] = tri[2];
+		    edges[0] = edges[2];
+		    edges[1] = edges[3];
 
 		    if (start-- == 0)
 			start += pen->num_vertices;
@@ -274,17 +281,29 @@ join (struct stroker *stroker,
 		while (start != stop) {
 		    tri[2] = in->point;
 		    translate_point (&tri[2], &pen->vertices[start].point);
-		    _cairo_traps_tessellate_triangle (stroker->traps, tri);
+		    edges[2] = in->point;
+		    edges[3] = tri[2];
+		    _cairo_traps_tessellate_triangle_with_edges (stroker->traps,
+								 tri, edges);
 		    tri[1] = tri[2];
+		    edges[0] = edges[2];
+		    edges[1] = edges[3];
 
 		    if (++start == pen->num_vertices)
 			start = 0;
 		}
 	    }
 	    tri[2] = *outpt;
-	    _cairo_traps_tessellate_triangle (stroker->traps, tri);
-	    break;
+	    edges[2] = out->cw;
+	    edges[3] = out->ccw;
+	    _cairo_traps_tessellate_triangle_with_edges (stroker->traps,
+							 tri, edges);
+	} else {
+	    cairo_point_t t[] = { in->point, *inpt, *outpt };
+	    cairo_point_t e[] = { in->cw, in->ccw, out->cw, out->ccw };
+	    _cairo_traps_tessellate_triangle_with_edges (stroker->traps, t, e);
 	}
+	break;
 
     case CAIRO_LINE_JOIN_MITER:
     default: {
@@ -442,12 +461,9 @@ join (struct stroker *stroker,
     }
 
     case CAIRO_LINE_JOIN_BEVEL: {
-	cairo_point_t tri[3];
-	tri[0] = in->point;
-	tri[1] = *inpt;
-	tri[2] = *outpt;
-
-	_cairo_traps_tessellate_triangle (stroker->traps, tri);
+	cairo_point_t t[] = { in->point, *inpt, *outpt };
+	cairo_point_t e[] = { in->cw, in->ccw, out->cw, out->ccw };
+	_cairo_traps_tessellate_triangle_with_edges (stroker->traps, t, e);
 	break;
     }
     }
@@ -460,7 +476,7 @@ add_cap (struct stroker *stroker, cairo_stroke_face_t *f)
     case CAIRO_LINE_CAP_ROUND: {
 	int start, stop;
 	cairo_slope_t in_slope, out_slope;
-	cairo_point_t tri[3];
+	cairo_point_t tri[3], edges[4];
 	cairo_pen_t *pen = &stroker->pen;
 
 	in_slope = f->dev_vector;
@@ -468,19 +484,29 @@ add_cap (struct stroker *stroker, cairo_stroke_face_t *f)
 	out_slope.dy = -in_slope.dy;
 	_cairo_pen_find_active_cw_vertices (pen, &in_slope, &out_slope,
 					    &start, &stop);
+	edges[0] = f->cw;
+	edges[1] = f->ccw;
 	tri[0] = f->point;
 	tri[1] = f->cw;
 	while (start != stop) {
 	    tri[2] = f->point;
 	    translate_point (&tri[2], &pen->vertices[start].point);
-	    _cairo_traps_tessellate_triangle (stroker->traps, tri);
+	    edges[2] = f->point;
+	    edges[3] = tri[2];
+	    _cairo_traps_tessellate_triangle_with_edges (stroker->traps,
+							 tri, edges);
 
 	    tri[1] = tri[2];
+	    edges[0] = edges[2];
+	    edges[1] = edges[3];
 	    if (++start == pen->num_vertices)
 		start = 0;
 	}
 	tri[2] = f->ccw;
-	_cairo_traps_tessellate_triangle (stroker->traps, tri);
+	edges[2] = f->cw;
+	edges[3] = f->ccw;
+	_cairo_traps_tessellate_triangle_with_edges (stroker->traps,
+						     tri, edges);
 	break;
     }
 
@@ -932,7 +958,6 @@ spline_to (void *closure,
 	cairo_point_t rectangle[4];
 
 	compute_face (&stroker->current_face.point, tangent, stroker, &face);
-
 	join (stroker, &stroker->current_face, &face);
 
 	rectangle[0] = face.cw;