Use hit_sphere to draw red circle.

zigrtiow/src/main.zig

@@ -10,7 +10,19 @@ const info = std.log.info;
 const write_color = color.write_color;
 const Ray = ray.Ray;
 
+fn hit_sphere(center: Point3, radius: f32, r: Ray) bool {
+    const oc = r.origin().sub(center);
+    const a = Vec3.dot(r.direction(), r.direction());
+    const b = Vec3.dot(oc, r.direction()) * 2;
+    const c = Vec3.dot(oc, oc) - radius * radius;
+    const discriminant = b * b - 4 * a * c;
+    return discriminant > 0;
+}
+
 fn ray_color(r: Ray) Color {
+    if (hit_sphere(Point3.init(0, 0, -1), 0.5, r)) {
+        return Color.init(1, 0, 0);
+    }
     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));

zigrtiow/src/vec.zig

@@ -32,6 +32,10 @@ pub const Vec3 = struct {
     pub fn unit(vec: Vec3) Vec3 {
         return vec.scale(1 / vec.length());
     }
+    pub fn dot(u: Vec3, v: Vec3) f32 {
+        const t = u.v * v.v;
+        return t[0] + t[1] + t[2];
+    }
 };
 pub const Color = Vec3;
 pub const Point3 = Vec3;