From 7e6ba7d445aca008987dfc6850e572109ad2d4ef Mon Sep 17 00:00:00 2001
From: Jean-Michel Gorius <jean-michel.gorius@protonmail.com>
Date: Fri, 11 Nov 2022 12:27:06 +0100
Subject: [PATCH] Add glass material

---
 main.c     | 15 ++++++++++-----
 material.c | 36 ++++++++++++++++++++++++++++++++++++
 material.h | 10 ++++++++++
 vec3.c     |  9 +++++++++
 vec3.h     |  1 +
 5 files changed, 66 insertions(+), 5 deletions(-)

diff --git a/main.c b/main.c
index 65d5aac..b6f1577 100644
--- a/main.c
+++ b/main.c
@@ -47,13 +47,11 @@ int main(void) {
   Lambertian material_ground = {.type = MATERIAL_LAMBERTIAN,
                                 .albedo = (Color){0.8, 0.8, 0.0}};
   Lambertian material_center = {.type = MATERIAL_LAMBERTIAN,
-                                .albedo = (Color){0.7, 0.3, 0.3}};
-  Metal material_left = {.type = MATERIAL_METAL,
-                         .albedo = (Color){0.8, 0.8, 0.8},
-                         .fuzziness = 0.3};
+                                .albedo = (Color){0.1, 0.2, 0.5}};
+  Dielectric material_left = {.type = MATERIAL_DIELECTRIC, .eta = 1.5};
   Metal material_right = {.type = MATERIAL_METAL,
                           .albedo = (Color){0.8, 0.6, 0.2},
-                          .fuzziness = 1.0};
+                          .fuzziness = 0.0};
 
   Sphere sphere_ground = {
       .type = HITTABLE_SPHERE,
@@ -73,6 +71,12 @@ int main(void) {
       .radius = 0.5,
       .material = (const Material *)&material_left,
   };
+  Sphere sphere_inside_left = {
+      .type = HITTABLE_SPHERE,
+      .center = (Point3){-1.0, 0.0, -1.0},
+      .radius = -0.4,
+      .material = (const Material *)&material_left,
+  };
   Sphere sphere_right = {
       .type = HITTABLE_SPHERE,
       .center = (Point3){1.0, 0.0, -1.0},
@@ -82,6 +86,7 @@ int main(void) {
   hittable_list_add(&world, (const Hittable *)&sphere_ground);
   hittable_list_add(&world, (const Hittable *)&sphere_center);
   hittable_list_add(&world, (const Hittable *)&sphere_left);
+  hittable_list_add(&world, (const Hittable *)&sphere_inside_left);
   hittable_list_add(&world, (const Hittable *)&sphere_right);
 
   /* Camera */
diff --git a/material.c b/material.c
index dd9c314..b329a19 100644
--- a/material.c
+++ b/material.c
@@ -1,8 +1,10 @@
 #include "material.h"
 #include "hittable.h"
+#include "utils.h"
 #include "vec3.h"
 
 #include <assert.h>
+#include <math.h>
 
 bool material_scatter(const Material *material, Ray r,
                       const struct HitRecord *record, Color *attenuation,
@@ -14,6 +16,9 @@ bool material_scatter(const Material *material, Ray r,
   case MATERIAL_METAL:
     return metal_scatter((const Metal *)material, r, record, attenuation,
                          scattered);
+  case MATERIAL_DIELECTRIC:
+    return dielectric_scatter((const Dielectric *)material, r, record,
+                              attenuation, scattered);
   }
   return false;
 }
@@ -45,3 +50,34 @@ bool metal_scatter(const Metal *metal, Ray r, const struct HitRecord *record,
   *attenuation = metal->albedo;
   return vec3_dot(scattered->direction, record->normal) > 0;
 }
+
+static double schlick_reflectance(double cosine, double eta) {
+  double r0 = (1 - eta) / (1 + eta);
+  r0 *= r0;
+  return r0 + (1 - r0) * pow(1 - cosine, 5);
+}
+
+bool dielectric_scatter(const Dielectric *dielectric, Ray r,
+                        const struct HitRecord *record, Color *attenuation,
+                        Ray *scattered) {
+  *attenuation = (Color){1.0, 1.0, 1.0};
+  double refraction_ratio =
+      record->front_face ? (1.0 / dielectric->eta) : dielectric->eta;
+
+  Vec3 unit_direction = vec3_normalize(r.direction);
+  double cos_theta =
+      fmin(vec3_dot(vec3_neg(unit_direction), record->normal), 1.0);
+  double sin_theta = sqrt(1.0 - cos_theta * cos_theta);
+
+  bool cannot_refract = refraction_ratio * sin_theta > 1.0;
+
+  Vec3 direction;
+  if (cannot_refract ||
+      schlick_reflectance(cos_theta, refraction_ratio) > random_double())
+    direction = vec3_reflect(unit_direction, record->normal);
+  else
+    direction = vec3_refract(unit_direction, record->normal, refraction_ratio);
+
+  *scattered = (Ray){record->p, direction};
+  return true;
+}
diff --git a/material.h b/material.h
index a194414..f9d3a78 100644
--- a/material.h
+++ b/material.h
@@ -11,6 +11,7 @@ struct HitRecord;
 typedef enum MaterialType {
   MATERIAL_LAMBERTIAN,
   MATERIAL_METAL,
+  MATERIAL_DIELECTRIC,
 } MaterialType;
 
 typedef struct Material {
@@ -39,4 +40,13 @@ typedef struct Metal {
 bool metal_scatter(const Metal *metal, Ray r, const struct HitRecord *record,
                    Color *attenuation, Ray *scattered);
 
+typedef struct Dielectric {
+  MaterialType type;
+  double eta;
+} Dielectric;
+
+bool dielectric_scatter(const Dielectric *dielectric, Ray r,
+                        const struct HitRecord *record, Color *attenuation,
+                        Ray *scattered);
+
 #endif /* INCLUDED_MATERIAL_H */
diff --git a/vec3.c b/vec3.c
index 8807e9f..318c4ec 100644
--- a/vec3.c
+++ b/vec3.c
@@ -73,3 +73,12 @@ bool vec3_is_near_zero(Vec3 v) {
 Vec3 vec3_reflect(Vec3 v, Vec3 n) {
   return vec3_sub(v, vec3_mul(2 * vec3_dot(v, n), n));
 }
+
+Vec3 vec3_refract(Vec3 uv, Vec3 n, double refraction_ratio) {
+  double cos_theta = fmin(vec3_dot(vec3_neg(uv), n), 1.0);
+  Vec3 r_out_perpendicular =
+      vec3_mul(refraction_ratio, vec3_add(uv, vec3_mul(cos_theta, n)));
+  Vec3 r_out_parallel =
+      vec3_mul(-sqrt(fabs(1.0 - vec3_length2(r_out_perpendicular))), n);
+  return vec3_add(r_out_perpendicular, r_out_parallel);
+}
diff --git a/vec3.h b/vec3.h
index 71add31..dcea3f0 100644
--- a/vec3.h
+++ b/vec3.h
@@ -29,5 +29,6 @@ Vec3 vec3_random_unit_vector(void);
 bool vec3_is_near_zero(Vec3 v);
 
 Vec3 vec3_reflect(Vec3 v, Vec3 n);
+Vec3 vec3_refract(Vec3 uv, Vec3 n, double refraction_ratio);
 
 #endif /* INCLUDED_VEC3_H */