From f04c000caebfc2449f6a2cd7a583274d0571fd9a Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Sat, 9 Feb 2019 19:24:30 -0800
Subject: [PATCH] Add a quick check to skip clipping when an outline is
 entirely inside or outside the clip area

---
 geometry/src/clip.rs    | 64 +++++++++++++++++++++++++++++++++++++++++
 geometry/src/outline.rs | 15 +++++++++-
 renderer/src/scene.rs   | 18 ++++++++----
 3 files changed, 91 insertions(+), 6 deletions(-)

diff --git a/geometry/src/clip.rs b/geometry/src/clip.rs
index fab35255..013ee1a3 100644
--- a/geometry/src/clip.rs
+++ b/geometry/src/clip.rs
@@ -541,3 +541,67 @@ impl Edge3D {
         prev.lerp(next, alpha as f32)
     }
 }
+
+/// Coarse collision detection
+
+// Separating axis theorem. Requires that the polygon be convex.
+pub(crate) fn rect_is_outside_polygon(rect: RectF32, polygon_points: &[Point2DF32]) -> bool {
+    let mut outcode = Outcode::all();
+    for point in polygon_points {
+        if point.x() > rect.min_x() { outcode.remove(Outcode::LEFT);   }
+        if point.x() < rect.max_x() { outcode.remove(Outcode::RIGHT);  }
+        if point.y() > rect.min_y() { outcode.remove(Outcode::TOP);    }
+        if point.y() < rect.max_y() { outcode.remove(Outcode::BOTTOM); }
+    }
+    if !outcode.is_empty() {
+        return true
+    }
+
+    // FIXME(pcwalton): Check winding!
+    let rect_points = [rect.origin(), rect.upper_right(), rect.lower_left(), rect.lower_right()];
+    for (next_point_index, &next) in polygon_points.iter().enumerate() {
+        let prev_point_index = if next_point_index == 0 {
+            polygon_points.len() - 1
+        } else {
+            next_point_index - 1
+        };
+        let prev = polygon_points[prev_point_index];
+        let polygon_edge_vector = next - prev;
+        if rect_points.iter().all(|&rect_point| polygon_edge_vector.det(rect_point - prev) < 0.0) {
+            return true
+        }
+    }
+
+    false
+}
+
+// Edge equation method. Requires that the polygon be convex.
+pub(crate) fn rect_is_inside_polygon(rect: RectF32, polygon_points: &[Point2DF32]) -> bool {
+    // FIXME(pcwalton): Check winding!
+    let rect_points = [rect.origin(), rect.upper_right(), rect.lower_left(), rect.lower_right()];
+    for (next_point_index, &next) in polygon_points.iter().enumerate() {
+        let prev_point_index = if next_point_index == 0 {
+            polygon_points.len() - 1
+        } else {
+            next_point_index - 1
+        };
+        let prev = polygon_points[prev_point_index];
+        let polygon_edge_vector = next - prev;
+        for &rect_point in &rect_points {
+            if polygon_edge_vector.det(rect_point - prev) < 0.0 {
+                return false;
+            }
+        }
+    }
+
+    true
+}
+
+bitflags! {
+    struct Outcode: u8 {
+        const LEFT   = 0x01;
+        const RIGHT  = 0x02;
+        const TOP    = 0x04;
+        const BOTTOM = 0x08;
+    }
+}
diff --git a/geometry/src/outline.rs b/geometry/src/outline.rs
index 95ffbb21..41caee30 100644
--- a/geometry/src/outline.rs
+++ b/geometry/src/outline.rs
@@ -15,7 +15,7 @@ use crate::basic::point::Point2DF32;
 use crate::basic::rect::RectF32;
 use crate::basic::transform2d::Transform2DF32;
 use crate::basic::transform3d::Perspective;
-use crate::clip::{ContourPolygonClipper, ContourRectClipper};
+use crate::clip::{self, ContourPolygonClipper, ContourRectClipper};
 use crate::dilation::ContourDilator;
 use crate::orientation::Orientation;
 use crate::segment::{Segment, SegmentFlags, SegmentKind};
@@ -138,7 +138,20 @@ impl Outline {
         self.bounds = self.bounds.intersection(view_box).unwrap_or_else(|| RectF32::default());
     }
 
+    pub fn is_outside_polygon(&self, clip_polygon: &[Point2DF32]) -> bool {
+        clip::rect_is_outside_polygon(self.bounds, clip_polygon)
+    }
+
+    fn is_inside_polygon(&self, clip_polygon: &[Point2DF32]) -> bool {
+        clip::rect_is_inside_polygon(self.bounds, clip_polygon)
+    }
+
     pub fn clip_against_polygon(&mut self, clip_polygon: &[Point2DF32]) {
+        // Quick check.
+        if self.is_inside_polygon(clip_polygon) {
+            return;
+        }
+
         let mut new_bounds = None;
         for contour in mem::replace(&mut self.contours, vec![]) {
             let contour = ContourPolygonClipper::new(clip_polygon, contour).clip();
diff --git a/renderer/src/scene.rs b/renderer/src/scene.rs
index 7c2c67e9..98402e56 100644
--- a/renderer/src/scene.rs
+++ b/renderer/src/scene.rs
@@ -104,19 +104,27 @@ impl Scene {
             .collect()
     }
 
-    fn apply_render_options(&self, outline: &Outline, options: &PreparedRenderOptions) -> Outline {
-        // FIXME(pcwalton): Don't clone?
-        let mut outline = (*outline).clone();
+    fn apply_render_options(&self, original_outline: &Outline, options: &PreparedRenderOptions)
+                            -> Outline {
+        let mut outline;
         match options.transform {
             PreparedRenderTransform::Perspective { ref perspective, ref clip_polygon } => {
-                outline.clip_against_polygon(clip_polygon);
-                outline.apply_perspective(perspective);
+                if original_outline.is_outside_polygon(clip_polygon) {
+                    outline = Outline::new();
+                } else {
+                    outline = (*original_outline).clone();
+                    outline.clip_against_polygon(clip_polygon);
+                    outline.apply_perspective(perspective);
+                }
             }
             PreparedRenderTransform::Transform2D(ref transform) => {
+                // TODO(pcwalton): Short circuit.
+                outline = (*original_outline).clone();
                 outline.transform(transform);
                 outline.clip_against_rect(self.view_box);
             }
             PreparedRenderTransform::None => {
+                outline = (*original_outline).clone();
                 outline.clip_against_rect(self.view_box);
             }
         }