From 4c2d0cb5ab9141fe6fdaab4e22485154763d59a0 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Wed, 20 Dec 2017 13:42:20 -0800
Subject: [PATCH] Use homogeneous coordinates to reduce floating point error
 when computing the intersection of self-intersecting line segments.

Partially addresses #57.
---
 partitioner/src/partitioner.rs | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/partitioner/src/partitioner.rs b/partitioner/src/partitioner.rs
index d79f84b8..32eec49b 100644
--- a/partitioner/src/partitioner.rs
+++ b/partitioner/src/partitioner.rs
@@ -342,9 +342,10 @@ impl<'a> Partitioner<'a> {
                     self.emit_b_quads_around_active_edge(upper_active_edge_index, crossing_point.x);
                     self.emit_b_quads_around_active_edge(lower_active_edge_index, crossing_point.x);
                 } else {
-                    debug!("warning: swapped active edges {} & {} without finding intersection",
-                           upper_active_edge_index,
-                           lower_active_edge_index);
+                    warn!("swapped active edges {} & {} without finding intersection; rendering \
+                           will probably be incorrect",
+                          upper_active_edge_index,
+                          lower_active_edge_index);
                 }
 
                 self.active_edges.swap(upper_active_edge_index as usize,
@@ -1009,7 +1010,8 @@ impl<'a> Partitioner<'a> {
                 let (lower_line, _) =
                     Line::new(lower_left_vertex_position,
                               lower_right_endpoint_position).subdivide_at_x(max_x);
-                upper_line.intersect_with_line(&lower_line)
+                // Use `intersect_at_infinity()` to reduce floating point error.
+                upper_line.intersect_at_infinity(&lower_line)
             }
 
             (upper_control_point_vertex_index, u32::MAX) => {