From 41ad372253b7104f4934e4cec02a96ef219a7a65 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Tue, 28 Jul 2020 12:56:05 -0700
Subject: [PATCH] Clip line segments before tiling them.

This reduces pathological behavior from very large off-screen segments.

Part of #416.
---
 content/src/clip.rs     | 76 ++++++++++++++++++++++++++++++++++++++++-
 renderer/src/builder.rs |  2 +-
 renderer/src/tiler.rs   | 10 ++++++
 3 files changed, 86 insertions(+), 2 deletions(-)

diff --git a/content/src/clip.rs b/content/src/clip.rs
index b92abe52..34fee613 100644
--- a/content/src/clip.rs
+++ b/content/src/clip.rs
@@ -18,7 +18,7 @@ use arrayvec::ArrayVec;
 use pathfinder_geometry::line_segment::LineSegment2F;
 use pathfinder_geometry::rect::RectF;
 use pathfinder_geometry::util::lerp;
-use pathfinder_geometry::vector::{Vector2F, Vector4F};
+use pathfinder_geometry::vector::{Vector2F, Vector4F, vec2f};
 use smallvec::SmallVec;
 use std::fmt::Debug;
 use std::mem;
@@ -490,6 +490,80 @@ pub(crate) fn rect_is_inside_polygon(rect: RectF, polygon_points: &[Vector2F]) -
     true
 }
 
+/// Clips a line segment to an axis-aligned rectangle using Cohen-Sutherland clipping.
+pub fn clip_line_segment_to_rect(mut line_segment: LineSegment2F, rect: RectF)
+                                 -> Option<LineSegment2F> {
+    let mut outcode_from = compute_outcode(line_segment.from(), rect);
+    let mut outcode_to = compute_outcode(line_segment.to(), rect);
+
+    loop {
+        if outcode_from.is_empty() && outcode_to.is_empty() {
+            return Some(line_segment);
+        }
+        if !(outcode_from & outcode_to).is_empty() {
+            return None;
+        }
+
+        let clip_from = outcode_from.bits() > outcode_to.bits();
+        let (mut point, outcode) = if clip_from {
+            (line_segment.from(), outcode_from)
+        } else {
+            (line_segment.to(), outcode_to)
+        };
+
+        if outcode.contains(Outcode::LEFT) {
+            point = vec2f(rect.min_x(),
+                          lerp(line_segment.from_y(),
+                               line_segment.to_y(),
+                               (line_segment.min_x() - line_segment.from_x()) /
+                                (line_segment.max_x() - line_segment.min_x())));
+        } else if outcode.contains(Outcode::RIGHT) {
+            point = vec2f(rect.max_x(),
+                          lerp(line_segment.from_y(),
+                               line_segment.to_y(),
+                               (line_segment.max_x() - line_segment.from_x()) /
+                                (line_segment.max_x() - line_segment.min_x())));
+        } else if outcode.contains(Outcode::TOP) {
+            point = vec2f(lerp(line_segment.from_x(),
+                               line_segment.to_x(),
+                               (line_segment.min_y() - line_segment.from_y()) /
+                                (line_segment.max_y() - line_segment.min_y())),
+                          rect.min_y());
+        } else if outcode.contains(Outcode::LEFT) {
+            point = vec2f(lerp(line_segment.from_x(),
+                               line_segment.to_x(),
+                               (line_segment.max_y() - line_segment.from_y()) /
+                                (line_segment.max_y() - line_segment.min_y())),
+                          rect.min_y());
+        }
+
+        if clip_from {
+            line_segment.set_from(point);
+            outcode_from = compute_outcode(point, rect);
+        } else {
+            line_segment.set_to(point);
+            outcode_to = compute_outcode(point, rect);
+        }
+    }
+
+    fn compute_outcode(point: Vector2F, rect: RectF) -> Outcode {
+        let mut outcode = Outcode::empty();
+        if point.x() < rect.min_x() {
+            outcode.insert(Outcode::LEFT);
+        }
+        if point.y() < rect.min_y() {
+            outcode.insert(Outcode::TOP);
+        }
+        if point.x() > rect.max_x() {
+            outcode.insert(Outcode::RIGHT);
+        }
+        if point.y() > rect.max_y() {
+            outcode.insert(Outcode::BOTTOM);
+        }
+        outcode
+    }
+}
+
 bitflags! {
     struct Outcode: u8 {
         const LEFT   = 0x01;
diff --git a/renderer/src/builder.rs b/renderer/src/builder.rs
index 904d4f97..0a391cc2 100644
--- a/renderer/src/builder.rs
+++ b/renderer/src/builder.rs
@@ -50,7 +50,7 @@ const CURVE_IS_CUBIC:     u32 = 0x40000000;
 const MAX_CLIP_BATCHES: u32 = 32;
 
 pub(crate) struct SceneBuilder<'a, 'b, 'c, 'd> {
-    scene: &'a mut Scene,
+    pub(crate) scene: &'a mut Scene,
     built_options: &'b PreparedBuildOptions,
     next_alpha_tile_indices: [AtomicUsize; ALPHA_TILE_LEVEL_COUNT],
     pub(crate) sink: &'c mut SceneSink<'d>,
diff --git a/renderer/src/tiler.rs b/renderer/src/tiler.rs
index f2a088a2..f7a021d0 100644
--- a/renderer/src/tiler.rs
+++ b/renderer/src/tiler.rs
@@ -17,6 +17,7 @@ use crate::gpu_data::AlphaTileId;
 use crate::options::PrepareMode;
 use crate::scene::{ClipPathId, PathId};
 use crate::tiles::{TILE_HEIGHT, TILE_WIDTH, TilingPathInfo};
+use pathfinder_content::clip;
 use pathfinder_content::fill::FillRule;
 use pathfinder_content::outline::{ContourIterFlags, Outline};
 use pathfinder_content::segment::Segment;
@@ -24,6 +25,7 @@ use pathfinder_geometry::line_segment::LineSegment2F;
 use pathfinder_geometry::rect::RectF;
 use pathfinder_geometry::vector::{Vector2F, Vector2I, vec2f, vec2i};
 use pathfinder_simd::default::{F32x2, U32x2};
+use std::f32::NEG_INFINITY;
 
 const FLATTENING_TOLERANCE: f32 = 0.25;
 
@@ -189,6 +191,14 @@ fn process_segment(segment: &Segment,
 fn process_line_segment(line_segment: LineSegment2F,
                         scene_builder: &SceneBuilder,
                         object_builder: &mut ObjectBuilder) {
+    let view_box = scene_builder.scene.view_box();
+    let clip_box = RectF::from_points(vec2f(view_box.min_x(), NEG_INFINITY),
+                                      view_box.lower_right());
+    let line_segment = match clip::clip_line_segment_to_rect(line_segment, clip_box) {
+        None => return,
+        Some(line_segment) => line_segment,
+    };
+
     let tile_size = vec2f(TILE_WIDTH as f32, TILE_HEIGHT as f32);
     let tile_size_recip = Vector2F::splat(1.0) / tile_size;