From 8d39a453c515b18cd10079ec4b7e45f315f466da Mon Sep 17 00:00:00 2001
From: Keuin <keuinx@gmail.com>
Date: Sat, 16 Apr 2022 14:30:23 +0800
Subject: Add vec3::refract<Enable_TIR>, vec3::valid, float-point validity
 assertion in vec3 operations, and tests.

---
 vec.h | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 50 insertions(+), 5 deletions(-)

(limited to 'vec.h')

diff --git a/vec.h b/vec.h
index 78580a8..b233220 100644
--- a/vec.h
+++ b/vec.h
@@ -9,6 +9,7 @@
 #include <random>
 #include <ostream>
 #include <cassert>
+#include <algorithm>
 
 static inline bool eq(int a, int b) {
     return a == b;
@@ -102,11 +103,46 @@ struct vec3 {
         return *this * (1.0 / norm());
     }
 
+    // If all the three float-point scalars are finite real number.
+    bool valid() const {
+        return std::isfinite(x) && std::isfinite(y) && std::isfinite(z);
+    }
+
     // Get the reflected vector. Current vector is the normal vector (length should be 1), v is the incoming vector.
     vec3 reflect(const vec3 &v) const {
         assert(fabs(mod2() - 1.0) < 1e-8);
         return v - (2.0 * dot(v)) * (*this);
     }
+
+    // Get the refracted vector. Current vector is the normal vector (length should be 1),
+    // r1 is the incoming vector, ri_inv is the relative refraction index n2/n1,
+    // where n2 is the destination media's refraction index, and n1 is the source media's refraction index.
+    // TIR (Total Internal Reflection) is optionally enabled by macro TIR_OR and TIR_OFF.
+    // If TIR happens, the ray will be reflected.
+    template<bool Enable_TIR>
+    vec3 refract(const vec3 &r1, double ri_inv) const {
+        assert(fabs(mod2() - 1.0) < 1e-12);
+        assert(fabs(r1.mod2() - 1.0) < 1e-12);
+        assert(ri_inv > 0);
+        assert(dot(r1) < 0); // normal vector must be on the same side
+        const auto &n = *this; // normal vector
+        const auto m_cos1 = std::max(dot(r1), (T) (-1)); // cos(a1), a1 is the incoming angle
+//        assert(m_cos1 <= 0); // incoming angle must smaller than 90deg
+        const auto c = ri_inv; // c = nx * r`x + ny * r`y
+        auto d = 1 - c * c * (1 - m_cos1 * m_cos1);
+        if (d < 0) {
+            // TODO test TIR
+            if (Enable_TIR) {
+                // ri_inv < sin(a1), cannot refract, must reflect (Total Internal Reflection)
+                return reflect(r1);
+            } else {
+                d = -d; // abs, just make the sqrt has a real solution
+            }
+        }
+        const auto n2 = (r1 - dot(r1) * n) * c - sqrt(d) * n;
+        assert(n2.valid());
+        return n2;
+    }
 };
 
 // print to ostream
@@ -115,33 +151,42 @@ inline std::ostream &operator<<(std::ostream &out, const vec3<T> &vec) {
     return out << "vec3[x=" << vec.x << ", y=" << vec.y << ", z=" << vec.z << ']';
 }
 
-// product vec3 by a scalar
+// product vec3 by a scalar, with fp assertions
 template<
         typename T,
         typename S,
         typename = typename std::enable_if<std::is_arithmetic<S>::value, S>::type
 >
 inline vec3<T> operator*(const vec3<T> &vec, const S &b) {
+    if (std::is_floating_point<S>::value) {
+        assert(std::isfinite(b));;
+    }
     return vec3<T>{.x=(T) (vec.x * b), .y=(T) (vec.y * b), .z=(T) (vec.z * b)};
 }
 
-// product vec3 by a scalar
+// product vec3 by a scalar, with fp assertions
 template<
         typename T,
-        typename S,
-        typename = typename std::enable_if<std::is_arithmetic<S>::value, S>::type
+        typename S
 >
 inline vec3<T> operator*(const S &b, const vec3<T> &vec) {
+    if (std::is_floating_point<S>::value) {
+        assert(std::isfinite(b));
+    }
     return vec3<T>{.x=(T) (vec.x * b), .y=(T) (vec.y * b), .z=(T) (vec.z * b)};
 }
 
-// product vec3 by the inversion of a scalar (div by a scalar)
+// product vec3 by the inversion of a scalar (div by a scalar), with fp assertions
 template<
         typename T,
         typename S,
         typename = typename std::enable_if<std::is_arithmetic<S>::value, S>::type
 >
 inline vec3<T> operator/(const vec3<T> &vec, const S &b) {
+    if (std::is_floating_point<S>::value) {
+        assert(std::isfinite(b));
+        assert(b != 0);
+    }
     return vec3<T>{.x=(T) (vec.x / b), .y=(T) (vec.y / b), .z=(T) (vec.z / b)};
 }
 
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