holy shit it just kinda works

examples/select.rs

@@ -1,8 +1,9 @@
-#![feature(iter_next_chunk)]
+#![feature(iter_array_chunks)]
 
 /// Example to illustrate selecting objects in 3d space
 ///
 use bevy::{
+    math::Vec3A,
     prelude::*,
     render::mesh::MeshVertexAttribute,
     render::render_resource::{Extent3d, TextureDimension, TextureFormat},
@@ -40,32 +41,25 @@ fn startup(
     });
 
     commands.spawn(PbrBundle {
-        mesh: meshes.add(shape::Plane::default().into()),
+        mesh: meshes.add(shape::Cube::default().into()),
         material: debug_material.clone(),
-        transform: Transform::from_xyz(1.0, 1.0, 1.0),
+        transform: Transform::from_xyz(0.0, 0.0, 0.0),
         ..default()
     });
 
-    // commands.spawn(PbrBundle {
-    //     mesh: meshes.add(shape::Cube::default().into()),
-    //     material: debug_material.clone(),
-    //     transform: Transform::from_xyz(0.0, 0.0, 0.0),
-    //     ..default()
-    // });
-
-    // commands.spawn(PbrBundle {
-    //     mesh: meshes.add(shape::Torus::default().into()),
-    //     material: debug_material.clone(),
-    //     transform: Transform::from_xyz(3.0, 0.0, 0.0),
-    //     ..default()
-    // });
-
-    // commands.spawn(PbrBundle {
-    //     mesh: meshes.add(shape::Icosphere::default().try_into().unwrap()),
-    //     material: debug_material.clone(),
-    //     transform: Transform::from_xyz(0.0, 3.0, 0.0),
-    //     ..default()
-    // });
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(shape::Torus::default().into()),
+        material: debug_material.clone(),
+        transform: Transform::from_xyz(3.0, 0.0, 0.0),
+        ..default()
+    });
+
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(shape::Icosphere::default().try_into().unwrap()),
+        material: debug_material.clone(),
+        transform: Transform::from_xyz(0.0, 3.0, 0.0),
+        ..default()
+    });
 }
 
 /// Creates a colorful test pattern
@@ -117,26 +111,60 @@ fn select(
                         .for_each(|(entity, mesh, gt)| {
                             let hit = intersects(&ray, &gt, mesh);
                             if hit.is_some() {
-                                info!("We got a hit at {:?}", entity);
+                                info!("We got a hit on {:?} at {:?}", entity, hit);
                             }
                         });
                 }
             });
         }
     });
-    panic!("Lol");
 }
 
+#[derive(Debug)]
+struct Triangle {
+    v0: Vec3,
+    v1: Vec3,
+    v2: Vec3,
+}
+
+impl Triangle {
+    fn normal(&self) -> Vec3 {
+        (self.edge_a()).cross(self.edge_b())
+    }
+
+    fn edge_a(&self) -> Vec3 {
+        self.v1 - self.v0
+    }
+
+    fn edge_b(&self) -> Vec3 {
+        self.v2 - self.v0
+    }
+
+    fn edge0(&self) -> Vec3 {
+        self.v1 - self.v0
+    }
+
+    fn edge1(&self) -> Vec3 {
+        self.v2 - self.v1
+    }
+
+    fn edge2(&self) -> Vec3 {
+        self.v0 - self.v2
+    }
+}
+
+/// Heavily synthesized by these two things:
+/// * Textbook: https://www.scratchapixel.com/lessons/3d-basic-rendering/ray-tracing-rendering-a-triangle/ray-triangle-intersection-geometric-solution.html
+/// * Example: https://github.com/aevyrie/bevy_mod_raycast/blob/435d8ef100738797161ac3a9b910ea346a4ed6e6/src/raycast.rs#L43
 fn intersects(ray: &Ray, gt: &GlobalTransform, mesh: &Mesh) -> Option<f32> {
-    info!("Ray: {:#?}", ray);
     let attr = MeshVertexAttribute::new("Vertex_Position", 0, VertexFormat::Float32x3);
     if let Some(verts) = mesh.attribute(attr) {
         if let Some(idxs) = mesh.indices() {
             match verts {
                 VertexAttributeValues::Float32x3(vals) => {
-                    let mut itr = idxs.iter();
-                    while let Ok(points) = itr
-                        .next_chunk::<3>()
+                    idxs.iter()
+                        .array_chunks::<3>()
+                        // Convert arrays to vec3
                         .map(|[x, y, z]| {
                             [
                                 Vec3::from_array(vals[x]),
@@ -144,6 +172,7 @@ fn intersects(ray: &Ray, gt: &GlobalTransform, mesh: &Mesh) -> Option<f32> {
                                 Vec3::from_array(vals[z]),
                             ]
                         })
+                        // Transform each point by the global transform
                         .map(|[a, b, c]| {
                             [
                                 gt.transform_point(a),
@@ -151,17 +180,40 @@ fn intersects(ray: &Ray, gt: &GlobalTransform, mesh: &Mesh) -> Option<f32> {
                                 gt.transform_point(c),
                             ]
                         })
-                    {
-                        info!("Does ray intersect with points: {:?}", points);
-                        // OK so I have a ray with an origin and direction
-                        // And I have a triangle made of 3 points
-                        // How do I test if ray intersects with tri???
-                    }
+                        // Collect everything into a triangle for easy operations
+                        .map(|[v0, v1, v2]| Triangle { v0, v1, v2 })
+                        .filter_map(|triangle| {
+                            // Calculate the distance this ray hits the plane normal to the tri
+                            if let Some(d) = ray.intersect_plane(triangle.v0, triangle.normal()) {
+                                // Calculate the point on that plane which intersects
+                                let p = ray.get_point(d);
+                                // Inside out test
+                                let hit = {
+                                    // Determine if p is w/in edge0
+                                    let c0 = triangle.edge0().cross(p - triangle.v0);
+                                    // Determine if p is w/in edge1
+                                    let c1 = triangle.edge1().cross(p - triangle.v1);
+                                    // Determine if p is w/in edge2
+                                    let c2 = triangle.edge2().cross(p - triangle.v2);
+
+                                    // Check all three at once
+                                    triangle.normal().dot(c0) > 0.0
+                                        && triangle.normal().dot(c1) > 0.0
+                                        && triangle.normal().dot(c2) > 0.0
+                                };
+                                hit.then_some(d)
+                            } else {
+                                None
+                            }
+                        })
+                        .min_by(|a, b| a.total_cmp(b))
                 }
-                _ => warn!("Unrecognized indexes"),
+                _ => None,
             }
+        } else {
+            None
         }
+    } else {
+        None
     }
-
-    None
 }