From 157bf8d93e4b0469d43fbeb6e3c7d3c4e5f34391 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cobrien@witekio.com>
Date: Mon, 22 Sep 2025 16:13:55 +0100
Subject: [PATCH] Add fuzz parameter to reflective material

---
 demo.c             | 4 ++--
 include/material.h | 5 +++--
 src/material.c     | 9 +++++++--
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/demo.c b/demo.c
index 2d3e103..262efff 100644
--- a/demo.c
+++ b/demo.c
@@ -20,8 +20,8 @@ static const obj_t scene[] = {
 	SPHERE(-0.8, 0.0, -3.8, 1.0, LAMBERTIAN(1.0, 1.0, 1.0)),
 	SPHERE(-1.0, -0.8, -2.6, 0.2, LAMBERTIAN(0.2, 0.9, 0.3)),
 	SPHERE(0.3, -0.7, -1.8, 0.3, LAMBERTIAN(0.9, 0.6, 0.2)),
-	SPHERE(-1.6, -0.8, -2.0, 0.2, REFLECTIVE(1.0, 0.9, 0.4)),
-	SPHERE(-6.0, 5.0, -5.0, 6.0, REFLECTIVE(0.9, 0.9, 0.9)),
+	SPHERE(-1.6, -0.8, -2.0, 0.2, REFLECTIVE(1.0, 0.9, 0.4, 0.0)),
+	SPHERE(-6.0, 5.0, -5.0, 6.0, REFLECTIVE(0.9, 0.9, 0.9, 0.05)),
 	SPHERE(-0.7, -0.75, -1.5, 0.25, DIELECTRIC(1.5)),
 	SPHERE(0.0, -1001.0, 0.0, 1000.0, LAMBERTIAN(0.3, 0.3, 0.3)),
 };
diff --git a/include/material.h b/include/material.h
index 6c49cc4..ec2bb41 100644
--- a/include/material.h
+++ b/include/material.h
@@ -10,10 +10,10 @@
 		.params = { .lambertian = { .albedo = { r, g, b } } }, \
 	}
 
-#define REFLECTIVE(r, g, b) \
+#define REFLECTIVE(r, g, b, f) \
 	{ \
 		.scatter = scatter_reflective, \
-		.params = { .reflective = { .tint = { r, g, b } } }, \
+		.params = { .reflective = { .tint = { r, g, b }, .fuzz = f } }, \
 	}
 
 #define DIELECTRIC(e) \
@@ -34,6 +34,7 @@ typedef struct {
 
 typedef struct {
 	vec3_t tint;
+	double fuzz;
 } reflective_params_t;
 
 typedef struct {
diff --git a/src/material.c b/src/material.c
index 87321fe..bed8db1 100644
--- a/src/material.c
+++ b/src/material.c
@@ -34,10 +34,15 @@ bool scatter_reflective(
     material_params_t params, hit_t hit, rng_t *rng, ray_t *ray,
     vec3_t *atten_out)
 {
-	(void)rng;
+	vec3_t reflected = reflect(ray->dir, hit.normal);
+	if (params.reflective.fuzz != 0.0) {
+		const double weight = params.reflective.fuzz * rng_canon(rng);
+		const vec3_t scatter = vec3_scale(rng_vec3(rng), weight);
+		reflected = vec3_unit(vec3_add(reflected, scatter));
+	}
 
 	ray->orig = hit.point;
-	ray->dir = reflect(ray->dir, hit.normal);
+	ray->dir = reflected;
 	*atten_out = params.reflective.tint;
 	return true;
 }