From 0d367470313533ed2e170762e24dbabc60c79714 Mon Sep 17 00:00:00 2001
From: Stephen Seo <seo.disparate@gmail.com>
Date: Mon, 7 Aug 2023 21:38:39 +0900
Subject: [PATCH] Add helper fn to calc. ray/plane intersection

---
 .gitignore        |  1 +
 src/3d_helpers.cc | 17 +++++++++++++++++
 src/3d_helpers.h  |  6 ++++++
 src/test/test.cc  | 25 +++++++++++++++++++++++++
 4 files changed, 49 insertions(+)

diff --git a/.gitignore b/.gitignore
index cf39983..984437f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 /objdir/
 compile_commands.json
 /.cache/
+/test
diff --git a/src/3d_helpers.cc b/src/3d_helpers.cc
index f1209bc..ec4f3ea 100644
--- a/src/3d_helpers.cc
+++ b/src/3d_helpers.cc
@@ -143,3 +143,20 @@ bool ray_to_xz_plane(const Ray &ray, float &x_out, float &z_out) {
 
   return true;
 }
+
+std::optional<Vector3> ray_to_plane(const Ray &ray, const Ray &plane) {
+  // Ray dir and plane normal.
+  float rd_pn = Vector3DotProduct(ray.direction, plane.direction);
+  if (rd_pn == 0.0F) {
+    return std::nullopt;
+  }
+
+  float amount = (Vector3DotProduct(plane.position, plane.direction) -
+                  Vector3DotProduct(ray.position, plane.direction)) /
+                 rd_pn;
+
+  // Amount * ray_dir + ray_pos == plane intersection.
+  return Vector3{ray.position.x + ray.direction.x * amount,
+                 ray.position.y + ray.direction.y * amount,
+                 ray.position.z + ray.direction.z * amount};
+}
diff --git a/src/3d_helpers.h b/src/3d_helpers.h
index 8e1b3e1..88b0021 100644
--- a/src/3d_helpers.h
+++ b/src/3d_helpers.h
@@ -1,6 +1,9 @@
 #ifndef JUMPARTIFACT_DOT_COM_DEMO_0_3D_HELPERS_H_
 #define JUMPARTIFACT_DOT_COM_DEMO_0_3D_HELPERS_H_
 
+// standard library includes
+#include <optional>
+
 // third party includes
 #include <raylib.h>
 
@@ -21,6 +24,9 @@ extern Matrix operator*(const Matrix &a, const Matrix &b);
 /// Returns true if intersects with xz plane.
 extern bool ray_to_xz_plane(const Ray &ray, float &x_out, float &z_out);
 
+/// plane.direction is plane normal, plane.position is position on plane.
+extern std::optional<Vector3> ray_to_plane(const Ray &ray, const Ray &plane);
+
 // Unimplemented as this function isn't really needed and it exposes some
 // weirdness regarding column-major matrices.
 // extern Vector4 operator*(const Matrix &m, const Vector4 &v);
diff --git a/src/test/test.cc b/src/test/test.cc
index 53e87c6..bf5536b 100644
--- a/src/test/test.cc
+++ b/src/test/test.cc
@@ -230,6 +230,31 @@ int main() {
     ASSERT_FLOAT_EQUALS(m.m15, 1.0F);
   }
 
+  {
+    // ray_to_plane tests.
+    auto result =
+        ray_to_plane(Ray{Vector3{0.0F, 0.0F, 1.0F}, Vector3{0.0F, 1.0F, 0.0F}},
+                     Ray{Vector3{0.0F, 2.0F, 0.0F}, Vector3{0.0F, 1.0F, 0.0F}});
+    ASSERT_TRUE(result.has_value());
+    ASSERT_FLOAT_EQUALS(0.0F, result.value().x);
+    ASSERT_FLOAT_EQUALS(2.0F, result.value().y);
+    ASSERT_FLOAT_EQUALS(1.0F, result.value().z);
+
+    result =
+        ray_to_plane(Ray{Vector3{7.0F, 2.0F, 1.0F}, Vector3{0.0F, 1.0F, 0.0F}},
+                     Ray{Vector3{0.0F, 5.0F, 0.0F}, Vector3{0.0F, 1.0F, 0.0F}});
+    ASSERT_TRUE(result.has_value());
+    ASSERT_FLOAT_EQUALS(7.0F, result.value().x);
+    ASSERT_FLOAT_EQUALS(5.0F, result.value().y);
+    ASSERT_FLOAT_EQUALS(1.0F, result.value().z);
+
+    // Parallel to plane test.
+    result =
+        ray_to_plane(Ray{Vector3{7.0F, 2.0F, 1.0F}, Vector3{0.0F, 0.0F, 1.0F}},
+                     Ray{Vector3{0.0F, 5.0F, 0.0F}, Vector3{0.0F, 1.0F, 0.0F}});
+    ASSERT_FALSE(result.has_value());
+  }
+
   std::cout << "Finished tests.\n";
   return 0;
 }
-- 
2.49.0