From 94b0f8355ea5cc1da39393d1f9a535116522c7ab Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Thu, 4 Aug 2022 21:43:47 -0700
Subject: [PATCH] zigrtiow: shoot child rays for diffuse shading.

---
 zigrtiow/src/main.zig | 27 ++++++++++++++-------------
 zigrtiow/src/vec.zig  | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/zigrtiow/src/main.zig b/zigrtiow/src/main.zig
index 257abac..23a461d 100644
--- a/zigrtiow/src/main.zig
+++ b/zigrtiow/src/main.zig
@@ -15,18 +15,18 @@ const Point3 = vec.Point3;
 const Ray = ray.Ray;
 const Sphere = sphere.Sphere;
 const Vec3 = vec.Vec3;
+const random_in_unit_sphere = vec.random_in_unit_sphere;
 const info = std.log.info;
 const write_color = color.write_color;
 
-fn ray_color(r: Ray, world: Hittable) Color {
+fn ray_color(r: Ray, world: Hittable, depth: isize) Color {
+    // If we've exceeded the ray bounce limit, no more light is gathered.
+    if (depth <= 0) return Color.init(0, 0, 0);
+
     var hit = world.hit(r, 0, std.math.inf(f32));
     if (hit) |rec| {
-        const n = rec.normal;
-        return Color.init(
-            n.x() + 1,
-            n.y() + 1,
-            n.z() + 1,
-        ).scale(0.5);
+        const target = rec.p.add(rec.normal.add(random_in_unit_sphere()));
+        return ray_color(Ray.init(rec.p, target.sub(rec.p)), world, depth - 1);
     }
     var unit_direction = r.direction().unit();
     const t = 0.5 * (unit_direction.y() + 1);
@@ -39,11 +39,13 @@ pub fn main() anyerror!void {
     const image_width = 400;
     const image_height = @floatToInt(isize, @intToFloat(f32, image_width) / aspect_ratio);
     const samples_per_pixel = 100;
+    const max_depth = 50;
 
     // World
-    var world = HittableList.init();
-    try world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, 0, -1), 0.5) });
-    try world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, -100.5, -1), 100) });
+    var tmp_world = HittableList.init();
+    try tmp_world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, 0, -1), 0.5) });
+    try tmp_world.add(Hittable{ .sphere = Sphere.init(Point3.init(0, -100.5, -1), 100) });
+    const world = Hittable{ .hittable_list = tmp_world };
 
     // Camera
     const cam = Camera.init();
@@ -58,7 +60,7 @@ pub fn main() anyerror!void {
     const rand = prng.random();
     var j: isize = image_height - 1;
     while (j >= 0) : (j -= 1) {
-        //info("Scanlines remaining: {d}", .{j});
+        info("Scanlines remaining: {d}", .{j});
 
         var i: isize = 0;
         while (i < image_width) : (i += 1) {
@@ -68,8 +70,7 @@ pub fn main() anyerror!void {
                 const u = (@intToFloat(f32, i) + rand.float(f32)) / @intToFloat(f32, image_width - 1);
                 const v = (@intToFloat(f32, j) + rand.float(f32)) / @intToFloat(f32, image_height - 1);
                 const r = cam.get_ray(u, v);
-                //Ray.init(origin, lower_left_corner.add(horizontal.scale(u)).add(vertical.scale(v)).sub(origin));
-                pixel_color = pixel_color.add(ray_color(r, Hittable{ .hittable_list = world }));
+                pixel_color = pixel_color.add(ray_color(r, world, max_depth));
             }
             try write_color(pixel_color, samples_per_pixel);
         }
diff --git a/zigrtiow/src/vec.zig b/zigrtiow/src/vec.zig
index 0d785f8..332c7f4 100644
--- a/zigrtiow/src/vec.zig
+++ b/zigrtiow/src/vec.zig
@@ -1,5 +1,13 @@
+const std = @import("std");
+
+const info = std.log.info;
+
+var prng = std.rand.DefaultPrng.init(0);
+const rand = prng.random();
+
 pub const Vec3 = struct {
     v: @Vector(3, f32),
+
     pub fn init(v0: f32, v1: f32, v2: f32) Vec3 {
         return Vec3{
             .v = .{ v0, v1, v2 },
@@ -36,6 +44,23 @@ pub const Vec3 = struct {
         const t = u.v * v.v;
         return t[0] + t[1] + t[2];
     }
+    /// Return value in unit cube from -1, 1
+    pub fn random() Vec3 {
+        return Vec3.init(
+            rand.float(f32) * 2 - 0.5,
+            rand.float(f32) * 2 - 0.5,
+            rand.float(f32) * 2 - 0.5,
+        );
+    }
 };
+
+pub fn random_in_unit_sphere() Vec3 {
+    while (true) {
+        const p = Vec3.random();
+        if (p.length_squared() > 1) continue;
+        return p;
+    }
+}
+
 pub const Color = Vec3;
 pub const Point3 = Vec3;