From 8bc5e347cc5d769335a9b953dc45ddd32acb4136 Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Sun, 31 Jul 2022 16:34:26 -0700
Subject: [PATCH] Use ray casting to draw "blue sky" image.

---
 zigrtiow/src/main.zig | 45 ++++++++++++++++++++++++++++++++++---------
 zigrtiow/src/ray.zig  | 33 +++++++++++++++++++++++++++++++
 zigrtiow/src/vec.zig  | 18 +++++++++++++++++
 3 files changed, 87 insertions(+), 9 deletions(-)
 create mode 100644 zigrtiow/src/ray.zig

diff --git a/zigrtiow/src/main.zig b/zigrtiow/src/main.zig
index b5e5ad6..b27e58a 100644
--- a/zigrtiow/src/main.zig
+++ b/zigrtiow/src/main.zig
@@ -1,13 +1,36 @@
 const std = @import("std");
-const info = std.log.info;
+const color = @import("./color.zig");
+const ray = @import("./ray.zig");
 const vec = @import("./vec.zig");
-const Vec3 = vec.Vec3;
+
 const Color = vec.Color;
-const write_color = @import("./color.zig").write_color;
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+const info = std.log.info;
+const write_color = color.write_color;
+const Ray = ray.Ray;
+
+fn ray_color(r: Ray) Color {
+    var unit_direction = r.direction().unit();
+    var t = 0.5 * (unit_direction.y() + 1);
+    return Color.init(1, 1, 1).scale(1 - t).add(Color.init(0.5, 0.7, 1.0).scale(t));
+}
 
 pub fn main() anyerror!void {
-    const image_width: isize = 256;
-    const image_height: isize = 256;
+    // Image
+    const aspect_ratio: f32 = 16.0 / 9.0;
+    const image_width = 400;
+    const image_height = @floatToInt(isize, @intToFloat(f32, image_width) / aspect_ratio);
+
+    // Camera
+    const viewport_height = 2;
+    const viewport_width = aspect_ratio * viewport_height;
+    const focal_length = 1;
+
+    const origin = Point3.init(0, 0, 0);
+    const horizontal = Vec3.init(viewport_width, 0, 0);
+    const vertical = Vec3.init(0, viewport_height, 0);
+    const lower_left_corner = origin.sub(horizontal.scale(0.5)).sub(vertical.scale(0.5)).sub(Vec3.init(0, 0, focal_length));
 
     const stdout = std.io.getStdOut();
 
@@ -16,16 +39,20 @@ pub fn main() anyerror!void {
 
     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) {
-            const pixel_color = Color.init(@intToFloat(f32, i) / @intToFloat(f32, image_width - 1), @intToFloat(f32, j) / @intToFloat(f32, image_height - 1), 0.25);
+            const u = @intToFloat(f32, i) / @intToFloat(f32, image_width - 1);
+            const v = @intToFloat(f32, j) / @intToFloat(f32, image_height - 1);
+            const r = Ray.init(origin, lower_left_corner.add(horizontal.scale(u)).add(vertical.scale(v)).sub(origin));
+            const pixel_color = ray_color(r);
             try write_color(pixel_color);
         }
     }
 }
 
-test "basic test" {
-    try std.testing.expectEqual(10, 3 + 7);
+test {
+    // Run tests in imported source.
+    std.testing.refAllDecls(@This());
 }
diff --git a/zigrtiow/src/ray.zig b/zigrtiow/src/ray.zig
new file mode 100644
index 0000000..b989e88
--- /dev/null
+++ b/zigrtiow/src/ray.zig
@@ -0,0 +1,33 @@
+const std = @import("std");
+const vec = @import("./vec.zig");
+
+const Vec3 = vec.Vec3;
+const Point3 = vec.Point3;
+
+pub const Ray = struct {
+    orig: Vec3,
+    dir: Vec3,
+
+    pub fn init(orig: Point3, dir: Vec3) Ray {
+        return Ray{
+            .orig = orig,
+            .dir = dir,
+        };
+    }
+    pub fn origin(ray: Ray) Point3 {
+        return ray.orig;
+    }
+    pub fn direction(ray: Ray) Vec3 {
+        return ray.dir;
+    }
+    pub fn at(ray: Ray, t: f32) Point3 {
+        return ray.orig.add(ray.dir.scale(t));
+    }
+};
+
+test "ray_at" {
+    const want = Point3.init(0, 0, 1.5);
+    const r = Ray.init(Point3.init(0, 0, 0), Vec3.init(0, 0, 1));
+    const got = r.at(1.5);
+    try std.testing.expectEqual(want, got);
+}
diff --git a/zigrtiow/src/vec.zig b/zigrtiow/src/vec.zig
index 30c4858..2a67544 100644
--- a/zigrtiow/src/vec.zig
+++ b/zigrtiow/src/vec.zig
@@ -14,6 +14,24 @@ pub const Vec3 = struct {
     pub fn z(vec: Vec3) f32 {
         return vec.v[2];
     }
+    pub fn length(vec: Vec3) f32 {
+        return @sqrt(vec.length_squared());
+    }
+    fn length_squared(vec: Vec3) f32 {
+        return vec.v[0] * vec.v[0] + vec.v[1] * vec.v[1] + vec.v[2] * vec.v[2];
+    }
+    pub fn add(lhs: Vec3, rhs: Vec3) Vec3 {
+        return Vec3{ .v = lhs.v + rhs.v };
+    }
+    pub fn sub(lhs: Vec3, rhs: Vec3) Vec3 {
+        return Vec3{ .v = lhs.v - rhs.v };
+    }
+    pub fn scale(vec: Vec3, t: f32) Vec3 {
+        return Vec3{ .v = vec.v * @splat(3, t) };
+    }
+    pub fn unit(vec: Vec3) Vec3 {
+        return vec.scale(1 / vec.length());
+    }
 };
 pub const Color = Vec3;
 pub const Point3 = Vec3;