From d248e30d365d5e401d5cd372aa415fb2f9a39326 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Mon, 24 Jun 2024 22:12:44 +0100
Subject: Chapter 10

---
 Makefile    |  2 +-
 camera.go   |  2 +-
 hittable.go |  3 +++
 material.go | 44 ++++++++++++++++++++++++++++++++++++++++++++
 ray.go      |  8 ++++++--
 rtiaw.go    | 11 +++++++++--
 vec3.go     |  9 +++++++++
 7 files changed, 73 insertions(+), 6 deletions(-)
 create mode 100644 material.go

diff --git a/Makefile b/Makefile
index 7a62fa2..5a16559 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@ out.ppm : rtiaw
 	./$< > $@
 
 rtiaw: $(wildcard *.go)
-	go build
+	go build -o $@ $^
 
 .phony: view clean
 
diff --git a/camera.go b/camera.go
index b42e9e1..f33d98a 100644
--- a/camera.go
+++ b/camera.go
@@ -74,7 +74,7 @@ func (cam Camera) GetRay(row uint, col uint) Ray {
   pixel_sample := cam.Pixel00.
                   Add(cam.PDU.Mul(SplatVec3(float64(col) + offset.X))).
                   Add(cam.PDV.Mul(SplatVec3(float64(row) + offset.Y)));
-  return Ray{cam.Centre, pixel_sample.Sub(cam.Centre)};
+  return Ray{cam.Centre, pixel_sample.Sub(cam.Centre), true};
 }
 
 func unit_square() Vec3 {
diff --git a/hittable.go b/hittable.go
index 5ab11cc..42599ab 100644
--- a/hittable.go
+++ b/hittable.go
@@ -11,6 +11,7 @@ type HitRecord struct {
   P Vec3
   N Vec3
   T float64
+  Mat Material
 }
 
 func (rec *HitRecord) SetFaceNormal(r Ray, out_n Vec3) {
@@ -32,6 +33,7 @@ type Hittable interface {
 type Sphere struct {
   Origin Vec3
   Radius float64
+  Mat Material
 }
 
 func (s Sphere) Hit(r Ray, ray_t Interval) (rec HitRecord) {
@@ -58,6 +60,7 @@ func (s Sphere) Hit(r Ray, ray_t Interval) (rec HitRecord) {
   rec.P = r.At(rec.T);
   out_n := rec.P.Sub(s.Origin).Div(SplatVec3(s.Radius));
   rec.SetFaceNormal(r, out_n);
+  rec.Mat = s.Mat;
   rec.Valid = true;
   return;
 }
diff --git a/material.go b/material.go
new file mode 100644
index 0000000..66ea256
--- /dev/null
+++ b/material.go
@@ -0,0 +1,44 @@
+package main
+
+//Material
+
+type Material interface {
+  Attenuation() Vec3
+  Scatter(r Ray, hit HitRecord) Ray
+}
+
+//Lambertian
+
+type Lambertian struct {
+  Albedo Vec3
+}
+
+func (l Lambertian) Attenuation() Vec3 {
+  return l.Albedo;
+}
+
+func (l Lambertian) Scatter(r Ray, hit HitRecord) Ray {
+  scat_dir := hit.N.Add(RandomVec3OnUnitSphere());
+  if scat_dir.NearZero() {
+    scat_dir = hit.N;
+  }
+  return Ray{hit.P, scat_dir, true};
+}
+
+//Metal
+
+type Metal struct {
+  Albedo Vec3
+  Fuzz float64
+}
+
+func (m Metal) Attenuation() Vec3 {
+  return m.Albedo;
+}
+
+func (m Metal) Scatter(r Ray, hit HitRecord) Ray {
+  refl := r.Direction.Reflect(hit.N).Unit().
+          Add(SplatVec3(m.Fuzz).Mul(RandomVec3OnUnitSphere()));
+  valid := refl.Dot(hit.N) > 0;
+  return Ray{hit.P, refl, valid};
+}
diff --git a/ray.go b/ray.go
index 18c8a39..2d7f29c 100644
--- a/ray.go
+++ b/ray.go
@@ -7,6 +7,7 @@ import (
 type Ray struct {
   Origin Vec3
   Direction Vec3
+  Valid bool
 }
 
 func (r Ray) At(t float64) Vec3 {
@@ -20,8 +21,11 @@ func (r Ray) Colour(world Hittable, md uint) Vec3 {
 
   hit := world.Hit(r, Interval{0.001, maths.Inf(1)})
   if hit.Valid {
-    dir := hit.N.Add(RandomVec3OnUnitSphere());
-    return Ray{hit.P, dir}.Colour(world, md - 1).Div(SplatVec3(10));
+    scat := hit.Mat.Scatter(r, hit);
+    if scat.Valid {
+      return scat.Colour(world, md - 1).Mul(hit.Mat.Attenuation());
+    }
+    return SplatVec3(0);
   }
 
   unit_dir := r.Direction.Unit();
diff --git a/rtiaw.go b/rtiaw.go
index 99bc31a..d95501d 100644
--- a/rtiaw.go
+++ b/rtiaw.go
@@ -9,9 +9,16 @@ import (
 func main() {
   start := time.Now();
 
+  mat_ground := Lambertian{Vec3{0.8, 0.8, 0}};
+  mat_centre := Lambertian{Vec3{0.1, 0.2, 0.5}};
+  mat_left   := Metal{Vec3{0.8, 0.8, 0.8}, 0.3};
+  mat_right  := Metal{Vec3{0.8, 0.6, 0.2}, 1.0};
+
   var world Hittables;
-  world.Add(Sphere{Vec3{0,    0  , -1},   0.5});
-  world.Add(Sphere{Vec3{0, -100.5, -1}, 100  });
+  world.Add(Sphere{Vec3{ 0,   -100.5, -1  }, 100  , mat_ground});
+  world.Add(Sphere{Vec3{ 0,      0  , -1.2},   0.5, mat_centre});
+  world.Add(Sphere{Vec3{-1.0,    0  , -1  },   0.5, mat_left});
+  world.Add(Sphere{Vec3{ 1.0,    0  , -1  },   0.5, mat_right});
 
   cam := NewCamera(400, 16.0/9.0, 100, 50);
   cam.Render(world);
diff --git a/vec3.go b/vec3.go
index bc16a07..a540b7c 100644
--- a/vec3.go
+++ b/vec3.go
@@ -37,6 +37,11 @@ func (v Vec3) Mag() float64 {
   return maths.Sqrt(v.MagSqr());
 }
 
+func (v Vec3) NearZero() bool {
+  s := 1e-8;
+  return v.X < s && v.Y < s && v.Z < s;
+}
+
 func (v Vec3) Unit() Vec3 {
   m := v.Mag();
   return Vec3{v.X / m, v.Y / m, v.Z / m};
@@ -86,3 +91,7 @@ func RandomVec3OnUnitSphere() (v Vec3) {
 
   return;
 }
+
+func (v Vec3) Reflect(n Vec3) Vec3 {
+  return v.Sub(SplatVec3(2 * v.Dot(n)).Mul(n));
+}
-- 
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