From a97b38299d5f8297d15077160df5439d274bd571 Mon Sep 17 00:00:00 2001
From: Keuin <keuinx@gmail.com>
Date: Tue, 12 Apr 2022 11:01:32 +0800
Subject: Make object::hit provides hit time t. Visualize normal vector at hit
 point. Add pixel<T>::from_normalized(const vec3d&) for visualizing normal
 vector.

---
 bitmap.h                |  7 ++++++-
 main_simple_scanner.cpp | 31 +++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/bitmap.h b/bitmap.h
index 31b9053..09f3711 100644
--- a/bitmap.h
+++ b/bitmap.h
@@ -31,10 +31,15 @@ struct pixel {
      * Create a pixel with given depth, from normalized color values.
      * For example: for 8bit pixel, with (1, 0.5, 0.25), we get: (255, 127, 63).
      */
-    static pixel<T> from_normalized(double r, double g, double b) {
+    static inline pixel<T> from_normalized(double r, double g, double b) {
         const auto mod = (1ULL << (sizeof(T) * 8U)) - 1ULL;
         return pixel<T>{.r = (T) (mod * r), .g = (T) (mod * g), .b = (T) (mod * b)};
     }
+
+    // v3d must be a normalized vector
+    static inline pixel<T> from_normalized(const vec3d &v3d) {
+        return from_normalized(v3d.x / 2.0 + 0.5, v3d.y / 2.0 + 0.5, v3d.z / 2.0 + 0.5);
+    }
 };
 
 // Mix two colors a and b. Returns a*u + b*v
diff --git a/main_simple_scanner.cpp b/main_simple_scanner.cpp
index 7279e12..e57ef03 100644
--- a/main_simple_scanner.cpp
+++ b/main_simple_scanner.cpp
@@ -17,8 +17,11 @@
 
 class object {
 public:
-    // Will the given ray hit.
-    virtual bool hit(const ray3d &r) const = 0;
+    // Will the given ray hit. Returns time t if hits.
+    virtual bool hit(const ray3d &r, double &t) const = 0;
+
+    // Given a point on the surface, returns the normalized normal vector on that point.
+    virtual vec3d normal_vector(const vec3d &where) const = 0;
 
     // object color, currently not parameterized
     virtual pixel8b color() const = 0;
@@ -38,7 +41,12 @@ public:
 
     ~sphere() override = default;
 
-    bool hit(const ray3d &r) const override {
+    vec3d normal_vector(const vec3d &where) const override {
+        // We assume the point is on surface, speeding up the normalization
+        return (where - center) / radius;
+    }
+
+    bool hit(const ray3d &r, double &t) const override {
         // Ray: {Source, Direction, time}
         // Sphere: {Center, radius}
         // sphere hit formula: |Source + Direction * time - Center| = radius
@@ -51,7 +59,13 @@ public:
         const auto rel_src = r.source() - center; // relative position of ray source
         // C = (S - C) dot (S - C) - radius^2
         const double c = dot(rel_src, rel_src) - radius * radius;
-        return b * b - 4 * a * c >= 0;
+        const auto delta = b * b - 4 * a * c;
+        const auto hit = delta >= 0;
+        if (hit) {
+            // the smaller root is the first point the ray hits, so return that one
+            t = (-b - sqrt(delta)) / (2.0 * a);
+        }
+        return hit;
     }
 
     pixel8b color() const override {
@@ -67,9 +81,14 @@ class viewport {
     // Given a ray from the camera, generate a color the camera seen on the viewport.
     pixel8b color(const ray3d &r) {
         // Detect hits
+        double hit_t;
         for (const auto &obj: objects) {
-            if (obj->hit(r)) {
-                return obj->color();
+            if (obj->hit(r, hit_t)) {
+                // normal vector on hit point
+                const auto nv = obj->normal_vector(r.at(hit_t));
+//                return obj->color();
+                // visualize normal vector at hit point
+                return pixel8b::from_normalized(nv);
             }
         }
 
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