From 6eb4da9e067175c7025ee1bf1c20074d457cb2bc Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cobrien@witekio.com>
Date: Tue, 23 Sep 2025 15:35:26 +0100
Subject: [PATCH] Add front/back face detection

---
 demo.c        |  3 ++-
 include/obj.h | 11 +++++++++--
 src/obj.c     | 24 +++++++++++++++---------
 3 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/demo.c b/demo.c
index 88d104e..74e45a5 100644
--- a/demo.c
+++ b/demo.c
@@ -4,6 +4,7 @@
 #include "ray.h"
 #include "vec3.h"
 
+#include <float.h>
 #include <limits.h>
 #include <math.h>
 #include <stdio.h>
@@ -34,7 +35,7 @@ static vec3_t raycol(ray_t ray)
 {
 	hit_t hit;
 	for (unsigned i = 0; i < NELEMS(scene); ++i) {
-		if (scene[i].intersect(scene[i].params, ray, &hit))
+		if (scene[i].intersect(scene[i].params, ray, &hit, 0.0, DBL_MAX))
 			return vec3_scale(vec3_add(hit.normal, white), 0.5);
 	}
 
diff --git a/include/obj.h b/include/obj.h
index 5835d7d..f766634 100644
--- a/include/obj.h
+++ b/include/obj.h
@@ -14,6 +14,7 @@
 typedef struct {
 	vec3_t point, normal;
 	double t;
+	bool front;
 } hit_t;
 
 typedef struct {
@@ -25,11 +26,17 @@ typedef union {
 	sphere_params_t sphere;
 } obj_params_t;
 
+typedef bool intersect_fn_t(
+    obj_params_t params, ray_t ray, hit_t *hit_out, double t_min,
+    double t_max);
+
 typedef struct {
-	bool (*intersect)(obj_params_t params, ray_t ray, hit_t *hit_out);
+	intersect_fn_t *intersect;
 	obj_params_t params;
 } obj_t;
 
-bool intersect_sphere(obj_params_t params, ray_t ray, hit_t *hit_out);
+bool intersect_sphere(
+    obj_params_t params, ray_t ray, hit_t *hit_out, double t_min,
+    double t_max);
 
 #endif
diff --git a/src/obj.c b/src/obj.c
index 520c58d..0b5bdec 100644
--- a/src/obj.c
+++ b/src/obj.c
@@ -2,28 +2,34 @@
 
 #include <math.h>
 
-bool intersect_sphere(obj_params_t params, ray_t ray, hit_t *hit_out)
+bool intersect_sphere(
+    obj_params_t params, ray_t ray, hit_t *hit_out, double t_min,
+    double t_max)
 {
 	const vec3_t centre = params.sphere.centre;
 	const double r = params.sphere.radius;
 	const vec3_t disp = vec3_sub(centre, ray.orig);
 
 	const double a = vec3_dot(ray.dir, ray.dir);
-	const double b = -2.0 * vec3_dot(ray.dir, disp);
+	const double h = vec3_dot(ray.dir, disp);
 	const double c = vec3_dot(disp, disp) - r * r;
 
-	const double discriminant = b * b - 4 * a * c;
+	const double discriminant = h * h - a * c;
 	if (discriminant < 0)
 		return false;
 
-	const double t = (-b - sqrt(discriminant)) / (2.0 * a);
+	const double sqrtd = sqrt(discriminant);
+	const double low = (h - sqrtd) / a;
+	const double t = low > t_min ? low : (h + sqrtd) / a;
+	if (t < t_min || t_max < t)
+		return false;
+
 	const vec3_t point = vec3_add(ray.orig, vec3_scale(ray.dir, t));
 	const vec3_t normal = vec3_unit(vec3_sub(point, centre));
 
-	if (hit_out != nullptr) {
-		hit_out->point = point;
-		hit_out->normal = normal;
-		hit_out->t = t;
-	}
+	hit_out->point = point;
+	hit_out->normal = normal;
+	hit_out->t = t;
+	hit_out->front = vec3_dot(ray.dir, normal) < 0.0;
 	return true;
 }