From 24df6bc5bc476e0249cf81452cab48b756088fd2 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Fri, 6 May 2022 18:38:02 +0200
Subject: [PATCH]  Implement plane-plane intersection

---
 .../fj-kernel/src/algorithms/intersection.rs  | 81 +++++++++++++++++++
 crates/fj-kernel/src/algorithms/mod.rs        |  2 +
 2 files changed, 83 insertions(+)
 create mode 100644 crates/fj-kernel/src/algorithms/intersection.rs

diff --git a/crates/fj-kernel/src/algorithms/intersection.rs b/crates/fj-kernel/src/algorithms/intersection.rs
new file mode 100644
index 0000000000..01f3e54fd2
--- /dev/null
+++ b/crates/fj-kernel/src/algorithms/intersection.rs
@@ -0,0 +1,81 @@
+//! Intersection algorithms
+
+use fj_math::{Point, Scalar, Vector};
+
+use crate::geometry::{Curve, Line, Surface};
+
+/// Test intersection between two surfaces
+pub fn surface(a: &Surface, b: &Surface) -> Option<Curve> {
+    // Algorithm from Real-Time Collision Detection by Christer Ericson. See
+    // section 5.4.4, Intersection of Two Planes.
+
+    let (a_normal, a_distance) = extract_plane(a);
+    let (b_normal, b_distance) = extract_plane(b);
+
+    let direction = a_normal.cross(&b_normal);
+
+    let denom = direction.dot(&direction);
+    if denom == Scalar::ZERO {
+        // Comparing `denom` against zero looks fishy. It's probably better to
+        // compare it against an epsilon value, but I don't know how large that
+        // epsilon should be.
+        //
+        // I'll just leave it like that, until we had the opportunity to collect
+        // some experience with this code.
+        // - @hannobraun
+        return None;
+    }
+
+    let origin = (b_normal * a_distance - a_normal * b_distance)
+        .cross(&direction)
+        / denom;
+    let origin = Point { coords: origin };
+
+    Some(Curve::Line(Line { origin, direction }))
+}
+
+/// Extract a plane in constant-normal form from a `Surface`
+///
+/// Panics, if the given `Surface` is not a plane.
+fn extract_plane(surface: &Surface) -> (Vector<3>, Scalar) {
+    let Surface::SweptCurve(surface) = surface;
+    let line = match surface.curve {
+        Curve::Line(line) => line,
+        _ => todo!("Only plane-plane intersection is currently supported."),
+    };
+
+    // Convert plane from parametric form to three-point form.
+    let a = line.origin;
+    let b = line.origin + line.direction;
+    let c = line.origin + surface.path;
+
+    // Convert plane from three-point form to constant-normal form. See
+    // Real-Time Collision Detection by Christer Ericson, section 3.6, Planes
+    // and Halfspaces.
+    let normal = (b - a).cross(&(c - a)).normalize();
+    let distance = normal.dot(&a.coords);
+
+    (normal, distance)
+}
+
+#[cfg(test)]
+mod tests {
+    use fj_math::Transform;
+
+    use crate::geometry::{Curve, Surface};
+
+    use super::surface;
+
+    #[test]
+    fn plane_plane() {
+        let xy = Surface::xy_plane();
+        let xz = Surface::xz_plane();
+
+        assert_eq!(surface(&xy, &xy), None);
+        assert_eq!(
+            surface(&xy, &xy.transform(&Transform::translation([0., 0., 1.]))),
+            None,
+        );
+        assert_eq!(surface(&xy, &xz), Some(Curve::x_axis()));
+    }
+}
diff --git a/crates/fj-kernel/src/algorithms/mod.rs b/crates/fj-kernel/src/algorithms/mod.rs
index e99371a59b..58df2b1b57 100644
--- a/crates/fj-kernel/src/algorithms/mod.rs
+++ b/crates/fj-kernel/src/algorithms/mod.rs
@@ -7,6 +7,8 @@ mod approx;
 mod sweep;
 mod triangulation;
 
+pub mod intersection;
+
 pub use self::{
     approx::{CycleApprox, FaceApprox, Tolerance},
     sweep::sweep_shape,