From 47fcd8a11e777c9c19075a8203d79b95a2a2c762 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Tue, 23 Sep 2025 15:36:08 +0100
Subject: [PATCH] Create obj module for scene objects

---
 CMakeLists.txt |  1 +
 demo.c         | 36 ++++++++++--------------------------
 include/obj.h  | 35 +++++++++++++++++++++++++++++++++++
 include/ray.h  | 10 ++++++++++
 src/obj.c      | 29 +++++++++++++++++++++++++++++
 5 files changed, 85 insertions(+), 26 deletions(-)
 create mode 100644 include/obj.h
 create mode 100644 include/ray.h
 create mode 100644 src/obj.c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bd5e3ab..e061c73 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,6 +10,7 @@ endmacro()
 
 add_library(batomorph
   src/ff.c
+  src/obj.c
   src/vec3.c
 )
 configure_target(batomorph)
diff --git a/demo.c b/demo.c
index 4eb2bac..af02ceb 100644
--- a/demo.c
+++ b/demo.c
@@ -1,4 +1,6 @@
 #include "ff.h"
+#include "obj.h"
+#include "ray.h"
 #include "vec3.h"
 
 #include <limits.h>
@@ -14,9 +16,7 @@
 
 #define FOCLEN 1.0
 
-typedef struct {
-	vec3_t orig, dir;
-} ray_t;
+#define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))
 
 typedef struct {
 	vec3_t centre;
@@ -34,35 +34,19 @@ static const vec3_t vp_v = { 0, -VP_H, 0 };
 static const vec3_t pix_x_step = { VP_W / W, 0, 0 };
 static const vec3_t pix_y_step = { 0, -VP_H / H, 0 };
 
-static const sphere_t obj = {
-	.centre = { 1.0, 0.0, -3.0 },
-	.rad = 1.0,
+static const obj_t scene[] = {
+	SPHERE(1.0, 0.0, -3.0, 1.0),
+	SPHERE(-2.0, 0.0, -5.0, 1.0),
 };
 
 static pix_t pix[W * H];
 
-static double intersect(sphere_t sphere, ray_t ray)
-{
-	const vec3_t disp = vec3_sub(sphere.centre, ray.orig);
-
-	const double a = vec3_dot(ray.dir, ray.dir);
-	const double b = -2.0 * vec3_dot(ray.dir, disp);
-	const double c = vec3_dot(disp, disp) - sphere.rad * sphere.rad;
-
-	const double discriminant = b * b - 4 * a * c;
-	if (discriminant < 0)
-		return -1.0;
-	else
-		return (-b - sqrt(discriminant)) / (2.0 * a);
-}
-
 static vec3_t raycol(ray_t ray)
 {
-	const double t = intersect(obj, ray);
-	if (t > 0.0) {
-		const vec3_t p = vec3_add(ray.orig, vec3_scale(ray.dir, t));
-		const vec3_t normal = vec3_unit(vec3_sub(p, obj.centre));
-		return vec3_scale(vec3_add(normal, white), 0.5);
+	hit_t hit;
+	for (unsigned i = 0; i < NELEMS(scene); ++i) {
+		if (scene[i].intersect(scene[i].params, ray, &hit))
+			return vec3_scale(vec3_add(hit.normal, white), 0.5);
 	}
 
 	const double a = (ray.dir.y + 1.0) / 2.0;
diff --git a/include/obj.h b/include/obj.h
new file mode 100644
index 0000000..5835d7d
--- /dev/null
+++ b/include/obj.h
@@ -0,0 +1,35 @@
+#ifndef OBJ_H
+#define OBJ_H
+
+#include "ray.h"
+
+#define SPHERE(x, y, z, r) \
+	{ \
+		.intersect = intersect_sphere, \
+		.params = { \
+			.sphere = { .centre = { x, y, z }, .radius = r }, \
+		}, \
+	}
+
+typedef struct {
+	vec3_t point, normal;
+	double t;
+} hit_t;
+
+typedef struct {
+	vec3_t centre;
+	double radius;
+} sphere_params_t;
+
+typedef union {
+	sphere_params_t sphere;
+} obj_params_t;
+
+typedef struct {
+	bool (*intersect)(obj_params_t params, ray_t ray, hit_t *hit_out);
+	obj_params_t params;
+} obj_t;
+
+bool intersect_sphere(obj_params_t params, ray_t ray, hit_t *hit_out);
+
+#endif
diff --git a/include/ray.h b/include/ray.h
new file mode 100644
index 0000000..775e774
--- /dev/null
+++ b/include/ray.h
@@ -0,0 +1,10 @@
+#ifndef RAY_H
+#define RAY_H
+
+#include "vec3.h"
+
+typedef struct {
+	vec3_t orig, dir;
+} ray_t;
+
+#endif
diff --git a/src/obj.c b/src/obj.c
new file mode 100644
index 0000000..520c58d
--- /dev/null
+++ b/src/obj.c
@@ -0,0 +1,29 @@
+#include "obj.h"
+
+#include <math.h>
+
+bool intersect_sphere(obj_params_t params, ray_t ray, hit_t *hit_out)
+{
+	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 c = vec3_dot(disp, disp) - r * r;
+
+	const double discriminant = b * b - 4 * a * c;
+	if (discriminant < 0)
+		return false;
+
+	const double t = (-b - sqrt(discriminant)) / (2.0 * a);
+	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;
+	}
+	return true;
+}