From a7268a2a83d3a3c7bc7c7dee3f220ae3fe5fd707 Mon Sep 17 00:00:00 2001
From: K Tyl <me@ktyl.dev>
Date: Sat, 6 Jun 2020 03:54:05 +0100
Subject: [PATCH] Visualise surface normals

---
 src/main.cpp | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 5c5ff72..660c732 100755
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -8,12 +8,39 @@ const double ASPECT_RATIO = 16.0 / 9.0;
 const int WIDTH = 384;
 const int HEIGHT = static_cast<int>(WIDTH / ASPECT_RATIO);
 
+double hitSphere(const point3& centre, double radius, const ray& r)
+{
+    vec3 oc = r.origin() - centre;
+
+    auto a = r.direction().lengthSquared();
+    auto halfB = dot(oc, r.direction());
+    auto c = oc.lengthSquared() - radius*radius;
+
+    auto discriminant = halfB*halfB - a*c;
+
+    if (discriminant < 0)
+    {
+        return -1.0;
+    }
+    else
+    {
+        return (-halfB - sqrt(discriminant)) / a;
+    }
+}
+
 colour rayColour(const ray& r)
 {
-    vec3 unitDirection = unitVector(r.direction());
-    double t = 0.5 * (unitDirection.y() + 1.0);
+    auto t = hitSphere(point3(0,0,-1), 0.5, r);
+    if (t > 0.0)
+    {
+        vec3 N = unitVector(r.at(t) - vec3(0,0,-1));
+        return 0.5*colour(N.x()+1, N.y()+1, N.z()+1);
+    }
 
-    auto a = colour(1.0, 1.0, 1.0);
+    vec3 unitDirection = unitVector(r.direction());
+    t = 0.5 * (unitDirection.y() + 1.0);
+
+    auto a = colour(1.0, 0.5, 0.6);
     auto b = colour(0.0, 0.0, 0.0);
 
     return lerp(a, b, t);