add hittable spheres

in_one_weekend/src/hittable.zig

@@ -0,0 +1,21 @@
+const Point3 = @import("vec3.zig").Point3;
+const Vec3 = @import("vec3.zig").Vec3;
+const Ray = @import("ray.zig").Ray;
+
+pub const HitRecord = struct {
+    p: Point3,
+    normal: Vec3,
+    t: f32,
+    front_face: bool,
+
+    pub fn setFaceNormal(hit: *HitRecord, ray: Ray, outward_normal: Vec3) void {
+        const front_face = Vec3.dot(ray.direction, outward_normal) < 0;
+        hit.*.front_face = front_face;
+        if (front_face) {
+            hit.*.normal = outward_normal;
+        } else {
+            const origin = Vec3{};
+            hit.*.normal = origin.sub(outward_normal);
+        }
+    }
+};

in_one_weekend/src/main.zig

@@ -6,6 +6,8 @@ const Vec3 = vec3.Vec3;
 const Color = vec3.Color;
 const Point3 = vec3.Point3;
 const Ray = @import("ray.zig").Ray;
+const Sphere = @import("sphere.zig").Sphere;
+const World = @import("world.zig").World;
 
 // From: https://github.com/Nelarius/weekend-raytracer-zig/blob/master/src/main.zig
 // See https://github.com/zig-lang/zig/issues/565
@@ -29,14 +31,12 @@ fn hitSphere(center: Point3, radius: f32, ray: Ray) f32 {
     }
 }
 
-fn rayColor(ray: Ray) Color {
-    var t = hitSphere(Point3{ .x = 0, .y = 0, .z = -1 }, 0.5, ray);
-    if (t > 0) {
-        const normal = vec3.unitVector(ray.at(t).sub(Vec3{ .z = -1 }));
-        return normal.add(Vec3{ .x = 1, .y = 1, .z = 1}).div(2);
+fn rayColor(ray: Ray, world: World) Color {
+    if (world.hit(ray, 0, 99999)) |hit| {
+        return hit.normal.add(Vec3{ .x = 1, .y = 1, .z = 1}).div(2);
     }
     const unitDirection = vec3.unitVector(ray.direction);
-    t = 0.5 * (unitDirection.y + 1.0);
+    const t = 0.5 * (unitDirection.y + 1.0);
     const white = Color{ .x = 1.0, .y = 1.0, .z = 1.0 };
     const blue = Color{ .x = 0.5, .y = 0.7, .z = 1.0 };
     return white.mul(1.0 - t).add(blue.mul(t));
@@ -54,6 +54,13 @@ pub fn main() anyerror!void {
     const imageWidth = 600;
     const imageHeight = @floatToInt(usize, imageWidth / aspectRatio);
 
+    // World
+    const spheres = [_]Sphere{
+        Sphere{ .center = Point3{ .x = 0, .y = 0, .z = -1 }, .radius = 0.5 },
+        Sphere{ .center = Point3{ .x = 0, .y = -100.5, .z = -1 }, .radius = 100 },
+     };
+    const world = World{ .spheres = spheres[0..spheres.len] };
+
     // Camera
     const viewportHeight = 2.0;
     const viewportWidth = viewportHeight * aspectRatio;
@@ -95,7 +102,7 @@ pub fn main() anyerror!void {
                     .origin = origin,
                     .direction = lowerLeftCorner.add(horizontal.mul(u)).add(vertical.mul(v)).sub(origin),
                 };
-                const pixelColor = rayColor(r);
+                const pixelColor = rayColor(r, world);
                 sdl.setSurfacePixel(surface, i, j, pixelColor);
                 i += 1;
             }

in_one_weekend/src/sphere.zig

@@ -0,0 +1,45 @@
+const math = @import("std").math;
+
+const Point3 = @import("vec3.zig").Point3;
+const Vec3 = @import("vec3.zig").Vec3;
+const HitRecord = @import("hittable.zig").HitRecord;
+const Ray = @import("ray.zig").Ray;
+
+pub const Sphere = struct {
+    center: Point3,
+    radius: f32,
+
+    pub fn hit(self: Sphere, ray: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        const oc = ray.origin.sub(self.center);
+        const a = ray.direction.length_squared();
+        const half_b = Vec3.dot(oc, ray.direction);
+        const c = oc.length_squared() - self.radius * self.radius;
+
+        const discriminant = half_b * half_b - a * c;
+        if (discriminant < 0) {
+            return null;
+        }
+
+        const sqrtd = math.sqrt(discriminant);
+
+        // Find the nearest root that lies in the acceptable range.
+        var root = (-half_b - sqrtd) / a;
+        if (root < t_min or t_max < root) {
+            root = (-half_b + sqrtd) / a;
+            if (root < t_min or t_max < root) {
+                return null;
+            }
+        }
+
+        const rayAtRoot = ray.at(root);
+        var hitLocal = HitRecord{
+            .t = root,
+            .p = rayAtRoot,
+            .normal = rayAtRoot.sub(self.center).div(self.radius),
+            .front_face = false,
+        };
+        const outwardNormal = hitLocal.p.sub(self.center).div(self.radius);
+        HitRecord.setFaceNormal(&hitLocal, ray, outwardNormal);
+        return hitLocal;
+    }
+};

in_one_weekend/src/world.zig

@@ -0,0 +1,21 @@
+const Sphere = @import("sphere.zig").Sphere;
+const HitRecord = @import("hittable.zig").HitRecord;
+const Ray = @import("ray.zig").Ray;
+
+pub const World = struct {
+    spheres: []const Sphere,
+
+    pub fn hit(self: World, ray: Ray, t_min: f32, t_max: f32) ?HitRecord {
+        var hitRecord: ?HitRecord = null;
+        var closest = t_max;
+
+        for (self.spheres) |sphere| {
+            if (sphere.hit(ray, t_min, closest)) |localHit| {
+                closest = localHit.t;
+                hitRecord = localHit;
+            }
+        }
+
+        return hitRecord;
+    }
+};