Merge pull request #158 from orange-cpp/feature/quaternions

added files

include/omath/linear_algebra/quaternion.hpp

@@ -0,0 +1,219 @@
+//
+// Created by vlad on 3/1/2026.
+//
+#pragma once
+
+#include "omath/linear_algebra/mat.hpp"
+#include "omath/linear_algebra/vector3.hpp"
+#include <array>
+#include <cmath>
+#include <format>
+
+namespace omath
+{
+    template<class Type>
+    requires std::is_arithmetic_v<Type>
+    class Quaternion
+    {
+    public:
+        using ContainedType = Type;
+
+        Type x = static_cast<Type>(0);
+        Type y = static_cast<Type>(0);
+        Type z = static_cast<Type>(0);
+        Type w = static_cast<Type>(1);  // identity quaternion
+
+        constexpr Quaternion() noexcept = default;
+
+        constexpr Quaternion(const Type& x, const Type& y, const Type& z, const Type& w) noexcept
+            : x(x), y(y), z(z), w(w)
+        {
+        }
+
+        // Factory: build from a normalized axis and an angle in radians
+        [[nodiscard]]
+        static Quaternion from_axis_angle(const Vector3<Type>& axis, const Type& angle_rad) noexcept
+        {
+            const Type half = angle_rad / static_cast<Type>(2);
+            const Type s = std::sin(half);
+            return {axis.x * s, axis.y * s, axis.z * s, std::cos(half)};
+        }
+
+        [[nodiscard]] constexpr bool operator==(const Quaternion& other) const noexcept
+        {
+            return x == other.x && y == other.y && z == other.z && w == other.w;
+        }
+
+        [[nodiscard]] constexpr bool operator!=(const Quaternion& other) const noexcept
+        {
+            return !(*this == other);
+        }
+
+        // Hamilton product: this * other
+        [[nodiscard]] constexpr Quaternion operator*(const Quaternion& other) const noexcept
+        {
+            return {
+                w * other.x + x * other.w + y * other.z - z * other.y,
+                w * other.y - x * other.z + y * other.w + z * other.x,
+                w * other.z + x * other.y - y * other.x + z * other.w,
+                w * other.w - x * other.x - y * other.y - z * other.z,
+            };
+        }
+
+        constexpr Quaternion& operator*=(const Quaternion& other) noexcept
+        {
+            return *this = *this * other;
+        }
+
+        [[nodiscard]] constexpr Quaternion operator*(const Type& scalar) const noexcept
+        {
+            return {x * scalar, y * scalar, z * scalar, w * scalar};
+        }
+
+        constexpr Quaternion& operator*=(const Type& scalar) noexcept
+        {
+            x *= scalar;
+            y *= scalar;
+            z *= scalar;
+            w *= scalar;
+            return *this;
+        }
+
+        [[nodiscard]] constexpr Quaternion operator+(const Quaternion& other) const noexcept
+        {
+            return {x + other.x, y + other.y, z + other.z, w + other.w};
+        }
+
+        constexpr Quaternion& operator+=(const Quaternion& other) noexcept
+        {
+            x += other.x;
+            y += other.y;
+            z += other.z;
+            w += other.w;
+            return *this;
+        }
+
+        [[nodiscard]] constexpr Quaternion operator-() const noexcept
+        {
+            return {-x, -y, -z, -w};
+        }
+
+        // Conjugate: negates the vector part (x, y, z)
+        [[nodiscard]] constexpr Quaternion conjugate() const noexcept
+        {
+            return {-x, -y, -z, w};
+        }
+
+        [[nodiscard]] constexpr Type dot(const Quaternion& other) const noexcept
+        {
+            return x * other.x + y * other.y + z * other.z + w * other.w;
+        }
+
+        [[nodiscard]] constexpr Type length_sqr() const noexcept
+        {
+            return x * x + y * y + z * z + w * w;
+        }
+
+#ifndef _MSC_VER
+        [[nodiscard]] constexpr Type length() const noexcept
+        {
+            return std::sqrt(length_sqr());
+        }
+
+        [[nodiscard]] constexpr Quaternion normalized() const noexcept
+        {
+            const Type len = length();
+            return len != static_cast<Type>(0) ? *this * (static_cast<Type>(1) / len) : *this;
+        }
+#else
+        [[nodiscard]] Type length() const noexcept
+        {
+            return std::sqrt(length_sqr());
+        }
+
+        [[nodiscard]] Quaternion normalized() const noexcept
+        {
+            const Type len = length();
+            return len != static_cast<Type>(0) ? *this * (static_cast<Type>(1) / len) : *this;
+        }
+#endif
+
+        // Inverse: q* / |q|^2  (for unit quaternions inverse == conjugate)
+        [[nodiscard]] constexpr Quaternion inverse() const noexcept
+        {
+            return conjugate() * (static_cast<Type>(1) / length_sqr());
+        }
+
+        // Rotate a 3D vector: v' = q * pure(v) * q^-1
+        // Computed via Rodrigues' formula to avoid full quaternion product overhead
+        [[nodiscard]] constexpr Vector3<Type> rotate(const Vector3<Type>& v) const noexcept
+        {
+            const Vector3<Type> q_vec{x, y, z};
+            const Vector3<Type> cross = q_vec.cross(v);
+            return v + cross * (static_cast<Type>(2) * w) + q_vec.cross(cross) * static_cast<Type>(2);
+        }
+
+        // 3x3 rotation matrix from this (unit) quaternion
+        [[nodiscard]] constexpr Mat<3, 3, Type> to_rotation_matrix3() const noexcept
+        {
+            const Type xx = x * x, yy = y * y, zz = z * z;
+            const Type xy = x * y, xz = x * z, yz = y * z;
+            const Type wx = w * x, wy = w * y, wz = w * z;
+            const Type one = static_cast<Type>(1);
+            const Type two = static_cast<Type>(2);
+
+            return {
+                {one - two * (yy + zz), two * (xy - wz),       two * (xz + wy)      },
+                {two * (xy + wz),       one - two * (xx + zz), two * (yz - wx)      },
+                {two * (xz - wy),       two * (yz + wx),       one - two * (xx + yy)},
+            };
+        }
+
+        // 4x4 rotation matrix (with homogeneous row/column)
+        [[nodiscard]] constexpr Mat<4, 4, Type> to_rotation_matrix4() const noexcept
+        {
+            const Type xx = x * x, yy = y * y, zz = z * z;
+            const Type xy = x * y, xz = x * z, yz = y * z;
+            const Type wx = w * x, wy = w * y, wz = w * z;
+            const Type one = static_cast<Type>(1);
+            const Type two = static_cast<Type>(2);
+            const Type zero = static_cast<Type>(0);
+
+            return {
+                {one - two * (yy + zz), two * (xy - wz),       two * (xz + wy),       zero},
+                {two * (xy + wz),       one - two * (xx + zz), two * (yz - wx),       zero},
+                {two * (xz - wy),       two * (yz + wx),       one - two * (xx + yy), zero},
+                {zero,                  zero,                   zero,                  one },
+            };
+        }
+
+        [[nodiscard]] constexpr std::array<Type, 4> as_array() const noexcept
+        {
+            return {x, y, z, w};
+        }
+    };
+} // namespace omath
+
+template<class Type>
+struct std::formatter<omath::Quaternion<Type>> // NOLINT(*-dcl58-cpp)
+{
+    [[nodiscard]]
+    static constexpr auto parse(std::format_parse_context& ctx)
+    {
+        return ctx.begin();
+    }
+
+    template<class FormatContext>
+    [[nodiscard]]
+    static auto format(const omath::Quaternion<Type>& q, FormatContext& ctx)
+    {
+        if constexpr (std::is_same_v<typename FormatContext::char_type, char>)
+            return std::format_to(ctx.out(), "[{}, {}, {}, {}]", q.x, q.y, q.z, q.w);
+
+        if constexpr (std::is_same_v<typename FormatContext::char_type, wchar_t>)
+            return std::format_to(ctx.out(), L"[{}, {}, {}, {}]", q.x, q.y, q.z, q.w);
+
+        if constexpr (std::is_same_v<typename FormatContext::char_type, char8_t>)
+            return std::format_to(ctx.out(), u8"[{}, {}, {}, {}]", q.x, q.y, q.z, q.w);
+    }
+};

include/omath/omath.hpp

@@ -17,6 +17,9 @@
 // Matrix classes
 #include "omath/linear_algebra/mat.hpp"
 
+// Quaternion
+#include "omath/linear_algebra/quaternion.hpp"
+
 // Color functionality
 #include "omath/utility/color.hpp"
 

tests/general/unit_test_quaternion.cpp

@@ -0,0 +1,402 @@
+//
+// Created by vlad on 3/1/2026.
+//
+#include <omath/linear_algebra/quaternion.hpp>
+#include <cmath>
+#include <gtest/gtest.h>
+#include <numbers>
+
+using namespace omath;
+
+static constexpr float kEps = 1e-5f;
+
+// ── Helpers ──────────────────────────────────────────────────────────────────
+
+static void expect_quat_near(const Quaternion<float>& a, const Quaternion<float>& b, float eps = kEps)
+{
+    EXPECT_NEAR(a.x, b.x, eps);
+    EXPECT_NEAR(a.y, b.y, eps);
+    EXPECT_NEAR(a.z, b.z, eps);
+    EXPECT_NEAR(a.w, b.w, eps);
+}
+
+static void expect_vec3_near(const Vector3<float>& a, const Vector3<float>& b, float eps = kEps)
+{
+    EXPECT_NEAR(a.x, b.x, eps);
+    EXPECT_NEAR(a.y, b.y, eps);
+    EXPECT_NEAR(a.z, b.z, eps);
+}
+
+// ── Constructors ─────────────────────────────────────────────────────────────
+
+TEST(Quaternion, DefaultConstructorIsIdentity)
+{
+    constexpr Quaternion<float> q;
+    EXPECT_FLOAT_EQ(q.x, 0.f);
+    EXPECT_FLOAT_EQ(q.y, 0.f);
+    EXPECT_FLOAT_EQ(q.z, 0.f);
+    EXPECT_FLOAT_EQ(q.w, 1.f);
+}
+
+TEST(Quaternion, ValueConstructor)
+{
+    constexpr Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    EXPECT_FLOAT_EQ(q.x, 1.f);
+    EXPECT_FLOAT_EQ(q.y, 2.f);
+    EXPECT_FLOAT_EQ(q.z, 3.f);
+    EXPECT_FLOAT_EQ(q.w, 4.f);
+}
+
+TEST(Quaternion, DoubleInstantiation)
+{
+    constexpr Quaternion<double> q{0.0, 0.0, 0.0, 1.0};
+    EXPECT_DOUBLE_EQ(q.w, 1.0);
+}
+
+// ── Equality ─────────────────────────────────────────────────────────────────
+
+TEST(Quaternion, EqualityOperators)
+{
+    constexpr Quaternion<float> a{1.f, 2.f, 3.f, 4.f};
+    constexpr Quaternion<float> b{1.f, 2.f, 3.f, 4.f};
+    constexpr Quaternion<float> c{1.f, 2.f, 3.f, 5.f};
+
+    EXPECT_TRUE(a == b);
+    EXPECT_FALSE(a == c);
+    EXPECT_FALSE(a != b);
+    EXPECT_TRUE(a != c);
+}
+
+// ── Arithmetic ───────────────────────────────────────────────────────────────
+
+TEST(Quaternion, ScalarMultiply)
+{
+    constexpr Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    constexpr auto r = q * 2.f;
+    EXPECT_FLOAT_EQ(r.x, 2.f);
+    EXPECT_FLOAT_EQ(r.y, 4.f);
+    EXPECT_FLOAT_EQ(r.z, 6.f);
+    EXPECT_FLOAT_EQ(r.w, 8.f);
+}
+
+TEST(Quaternion, ScalarMultiplyAssign)
+{
+    Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    q *= 3.f;
+    EXPECT_FLOAT_EQ(q.x, 3.f);
+    EXPECT_FLOAT_EQ(q.y, 6.f);
+    EXPECT_FLOAT_EQ(q.z, 9.f);
+    EXPECT_FLOAT_EQ(q.w, 12.f);
+}
+
+TEST(Quaternion, Addition)
+{
+    constexpr Quaternion<float> a{1.f, 2.f, 3.f, 4.f};
+    constexpr Quaternion<float> b{4.f, 3.f, 2.f, 1.f};
+    constexpr auto r = a + b;
+    EXPECT_FLOAT_EQ(r.x, 5.f);
+    EXPECT_FLOAT_EQ(r.y, 5.f);
+    EXPECT_FLOAT_EQ(r.z, 5.f);
+    EXPECT_FLOAT_EQ(r.w, 5.f);
+}
+
+TEST(Quaternion, AdditionAssign)
+{
+    Quaternion<float> a{1.f, 0.f, 0.f, 0.f};
+    const Quaternion<float> b{0.f, 1.f, 0.f, 0.f};
+    a += b;
+    EXPECT_FLOAT_EQ(a.x, 1.f);
+    EXPECT_FLOAT_EQ(a.y, 1.f);
+}
+
+TEST(Quaternion, UnaryNegation)
+{
+    constexpr Quaternion<float> q{1.f, -2.f, 3.f, -4.f};
+    constexpr auto r = -q;
+    EXPECT_FLOAT_EQ(r.x, -1.f);
+    EXPECT_FLOAT_EQ(r.y,  2.f);
+    EXPECT_FLOAT_EQ(r.z, -3.f);
+    EXPECT_FLOAT_EQ(r.w,  4.f);
+}
+
+// ── Hamilton product ──────────────────────────────────────────────────────────
+
+TEST(Quaternion, MultiplyByIdentityIsNoop)
+{
+    constexpr Quaternion<float> identity;
+    constexpr Quaternion<float> q{0.5f, 0.5f, 0.5f, 0.5f};
+    expect_quat_near(q * identity, q);
+    expect_quat_near(identity * q, q);
+}
+
+TEST(Quaternion, MultiplyAssign)
+{
+    constexpr Quaternion<float> identity;
+    Quaternion<float> q{0.5f, 0.5f, 0.5f, 0.5f};
+    q *= identity;
+    expect_quat_near(q, {0.5f, 0.5f, 0.5f, 0.5f});
+}
+
+TEST(Quaternion, MultiplyKnownResult)
+{
+    // i * j = k  →  (1,0,0,0) * (0,1,0,0) = (0,0,1,0)
+    constexpr Quaternion<float> i{1.f, 0.f, 0.f, 0.f};
+    constexpr Quaternion<float> j{0.f, 1.f, 0.f, 0.f};
+    constexpr auto k = i * j;
+    EXPECT_FLOAT_EQ(k.x, 0.f);
+    EXPECT_FLOAT_EQ(k.y, 0.f);
+    EXPECT_FLOAT_EQ(k.z, 1.f);
+    EXPECT_FLOAT_EQ(k.w, 0.f);
+}
+
+TEST(Quaternion, MultiplyByInverseGivesIdentity)
+{
+    const Quaternion<float> q = Quaternion<float>::from_axis_angle({0.f, 0.f, 1.f},
+                                                                     std::numbers::pi_v<float> / 3.f);
+    const auto result = q * q.inverse();
+    expect_quat_near(result, Quaternion<float>{});
+}
+
+// ── Conjugate ────────────────────────────────────────────────────────────────
+
+TEST(Quaternion, Conjugate)
+{
+    constexpr Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    constexpr auto c = q.conjugate();
+    EXPECT_FLOAT_EQ(c.x, -1.f);
+    EXPECT_FLOAT_EQ(c.y, -2.f);
+    EXPECT_FLOAT_EQ(c.z, -3.f);
+    EXPECT_FLOAT_EQ(c.w,  4.f);
+}
+
+TEST(Quaternion, ConjugateOfIdentityIsIdentity)
+{
+    constexpr Quaternion<float> id;
+    constexpr auto c = id.conjugate();
+    EXPECT_FLOAT_EQ(c.x, 0.f);
+    EXPECT_FLOAT_EQ(c.y, 0.f);
+    EXPECT_FLOAT_EQ(c.z, 0.f);
+    EXPECT_FLOAT_EQ(c.w, 1.f);
+}
+
+// ── Dot / length ─────────────────────────────────────────────────────────────
+
+TEST(Quaternion, Dot)
+{
+    constexpr Quaternion<float> a{1.f, 0.f, 0.f, 0.f};
+    constexpr Quaternion<float> b{0.f, 1.f, 0.f, 0.f};
+    EXPECT_FLOAT_EQ(a.dot(b), 0.f);
+    EXPECT_FLOAT_EQ(a.dot(a), 1.f);
+}
+
+TEST(Quaternion, LengthSqrIdentity)
+{
+    constexpr Quaternion<float> id;
+    EXPECT_FLOAT_EQ(id.length_sqr(), 1.f);
+}
+
+TEST(Quaternion, LengthSqrGeneral)
+{
+    constexpr Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    EXPECT_FLOAT_EQ(q.length_sqr(), 30.f);
+}
+
+TEST(Quaternion, LengthIdentity)
+{
+    const Quaternion<float> id;
+    EXPECT_NEAR(id.length(), 1.f, kEps);
+}
+
+TEST(Quaternion, Normalized)
+{
+    const Quaternion<float> q{1.f, 1.f, 1.f, 1.f};
+    const auto n = q.normalized();
+    EXPECT_NEAR(n.length(), 1.f, kEps);
+    EXPECT_NEAR(n.x, 0.5f, kEps);
+    EXPECT_NEAR(n.y, 0.5f, kEps);
+    EXPECT_NEAR(n.z, 0.5f, kEps);
+    EXPECT_NEAR(n.w, 0.5f, kEps);
+}
+
+TEST(Quaternion, NormalizedOfZeroLengthReturnsSelf)
+{
+    // length_sqr = 0 would be UB, but zero-vector part + zero w is degenerate;
+    // we just verify the guard branch (divides by zero) doesn't crash by
+    // keeping length > 0 via the default constructor path.
+    const Quaternion<float> unit;
+    const auto n = unit.normalized();
+    expect_quat_near(n, unit);
+}
+
+// ── Inverse ───────────────────────────────────────────────────────────────────
+
+TEST(Quaternion, InverseOfUnitIsConjugate)
+{
+    const Quaternion<float> q = Quaternion<float>::from_axis_angle({1.f, 0.f, 0.f},
+                                                                     std::numbers::pi_v<float> / 4.f);
+    const auto inv = q.inverse();
+    const auto conj = q.conjugate();
+    expect_quat_near(inv, conj);
+}
+
+// ── from_axis_angle ──────────────────────────────────────────────────────────
+
+TEST(Quaternion, FromAxisAngleZeroAngleIsIdentity)
+{
+    const auto q = Quaternion<float>::from_axis_angle({1.f, 0.f, 0.f}, 0.f);
+    EXPECT_NEAR(q.x, 0.f, kEps);
+    EXPECT_NEAR(q.y, 0.f, kEps);
+    EXPECT_NEAR(q.z, 0.f, kEps);
+    EXPECT_NEAR(q.w, 1.f, kEps);
+}
+
+TEST(Quaternion, FromAxisAngle90DegZ)
+{
+    const float half_pi = std::numbers::pi_v<float> / 2.f;
+    const auto q = Quaternion<float>::from_axis_angle({0.f, 0.f, 1.f}, half_pi);
+    const float s = std::sin(half_pi / 2.f);
+    const float c = std::cos(half_pi / 2.f);
+    EXPECT_NEAR(q.x, 0.f, kEps);
+    EXPECT_NEAR(q.y, 0.f, kEps);
+    EXPECT_NEAR(q.z, s,   kEps);
+    EXPECT_NEAR(q.w, c,   kEps);
+}
+
+// ── rotate ───────────────────────────────────────────────────────────────────
+
+TEST(Quaternion, RotateByIdentityIsNoop)
+{
+    constexpr Quaternion<float> id;
+    constexpr Vector3<float> v{1.f, 2.f, 3.f};
+    const auto r = id.rotate(v);
+    expect_vec3_near(r, v);
+}
+
+TEST(Quaternion, Rotate90DegAroundZ)
+{
+    // Rotating (1,0,0) by 90° around Z should give (0,1,0)
+    const auto q = Quaternion<float>::from_axis_angle({0.f, 0.f, 1.f}, std::numbers::pi_v<float> / 2.f);
+    const auto r = q.rotate({1.f, 0.f, 0.f});
+    expect_vec3_near(r, {0.f, 1.f, 0.f});
+}
+
+TEST(Quaternion, Rotate180DegAroundY)
+{
+    // Rotating (1,0,0) by 180° around Y should give (-1,0,0)
+    const auto q = Quaternion<float>::from_axis_angle({0.f, 1.f, 0.f}, std::numbers::pi_v<float>);
+    const auto r = q.rotate({1.f, 0.f, 0.f});
+    expect_vec3_near(r, {-1.f, 0.f, 0.f});
+}
+
+TEST(Quaternion, Rotate90DegAroundX)
+{
+    // Rotating (0,1,0) by 90° around X should give (0,0,1)
+    const auto q = Quaternion<float>::from_axis_angle({1.f, 0.f, 0.f}, std::numbers::pi_v<float> / 2.f);
+    const auto r = q.rotate({0.f, 1.f, 0.f});
+    expect_vec3_near(r, {0.f, 0.f, 1.f});
+}
+
+// ── to_rotation_matrix3 ───────────────────────────────────────────────────────
+
+TEST(Quaternion, RotationMatrix3FromIdentityIsIdentityMatrix)
+{
+    constexpr Quaternion<float> id;
+    constexpr auto m = id.to_rotation_matrix3();
+    for (size_t i = 0; i < 3; ++i)
+        for (size_t j = 0; j < 3; ++j)
+            EXPECT_NEAR(m.at(i, j), i == j ? 1.f : 0.f, kEps);
+}
+
+TEST(Quaternion, RotationMatrix3From90DegZ)
+{
+    //  Expected:  | 0 -1  0 |
+    //             | 1  0  0 |
+    //             | 0  0  1 |
+    const auto q = Quaternion<float>::from_axis_angle({0.f, 0.f, 1.f}, std::numbers::pi_v<float> / 2.f);
+    const auto m = q.to_rotation_matrix3();
+    EXPECT_NEAR(m.at(0, 0),  0.f, kEps);
+    EXPECT_NEAR(m.at(0, 1), -1.f, kEps);
+    EXPECT_NEAR(m.at(0, 2),  0.f, kEps);
+    EXPECT_NEAR(m.at(1, 0),  1.f, kEps);
+    EXPECT_NEAR(m.at(1, 1),  0.f, kEps);
+    EXPECT_NEAR(m.at(1, 2),  0.f, kEps);
+    EXPECT_NEAR(m.at(2, 0),  0.f, kEps);
+    EXPECT_NEAR(m.at(2, 1),  0.f, kEps);
+    EXPECT_NEAR(m.at(2, 2),  1.f, kEps);
+}
+
+TEST(Quaternion, RotationMatrix3ConsistentWithRotate)
+{
+    // Matrix-vector multiply must agree with the rotate() method
+    const auto q = Quaternion<float>::from_axis_angle({1.f, 1.f, 0.f}, std::numbers::pi_v<float> / 3.f);
+    const Vector3<float> v{2.f, -1.f, 0.5f};
+
+    const auto rotated = q.rotate(v);
+    const auto m = q.to_rotation_matrix3();
+
+    // manual mat-vec multiply (row-major)
+    const float rx = m.at(0, 0) * v.x + m.at(0, 1) * v.y + m.at(0, 2) * v.z;
+    const float ry = m.at(1, 0) * v.x + m.at(1, 1) * v.y + m.at(1, 2) * v.z;
+    const float rz = m.at(2, 0) * v.x + m.at(2, 1) * v.y + m.at(2, 2) * v.z;
+
+    EXPECT_NEAR(rotated.x, rx, kEps);
+    EXPECT_NEAR(rotated.y, ry, kEps);
+    EXPECT_NEAR(rotated.z, rz, kEps);
+}
+
+// ── to_rotation_matrix4 ───────────────────────────────────────────────────────
+
+TEST(Quaternion, RotationMatrix4FromIdentityIsIdentityMatrix)
+{
+    constexpr Quaternion<float> id;
+    constexpr auto m = id.to_rotation_matrix4();
+    for (size_t i = 0; i < 4; ++i)
+        for (size_t j = 0; j < 4; ++j)
+            EXPECT_NEAR(m.at(i, j), i == j ? 1.f : 0.f, kEps);
+}
+
+TEST(Quaternion, RotationMatrix4HomogeneousRowAndColumn)
+{
+    const auto q = Quaternion<float>::from_axis_angle({1.f, 0.f, 0.f}, std::numbers::pi_v<float> / 5.f);
+    const auto m = q.to_rotation_matrix4();
+
+    // Last row and last column must be (0,0,0,1)
+    for (size_t i = 0; i < 3; ++i)
+    {
+        EXPECT_NEAR(m.at(3, i), 0.f, kEps);
+        EXPECT_NEAR(m.at(i, 3), 0.f, kEps);
+    }
+    EXPECT_NEAR(m.at(3, 3), 1.f, kEps);
+}
+
+TEST(Quaternion, RotationMatrix4Upper3x3MatchesMatrix3)
+{
+    const auto q = Quaternion<float>::from_axis_angle({0.f, 1.f, 0.f}, std::numbers::pi_v<float> / 7.f);
+    const auto m3 = q.to_rotation_matrix3();
+    const auto m4 = q.to_rotation_matrix4();
+
+    for (size_t i = 0; i < 3; ++i)
+        for (size_t j = 0; j < 3; ++j)
+            EXPECT_NEAR(m4.at(i, j), m3.at(i, j), kEps);
+}
+
+// ── as_array ──────────────────────────────────────────────────────────────────
+
+TEST(Quaternion, AsArray)
+{
+    constexpr Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    constexpr auto arr = q.as_array();
+    EXPECT_FLOAT_EQ(arr[0], 1.f);
+    EXPECT_FLOAT_EQ(arr[1], 2.f);
+    EXPECT_FLOAT_EQ(arr[2], 3.f);
+    EXPECT_FLOAT_EQ(arr[3], 4.f);
+}
+
+// ── std::formatter ────────────────────────────────────────────────────────────
+
+TEST(Quaternion, Formatter)
+{
+    const Quaternion<float> q{1.f, 2.f, 3.f, 4.f};
+    const auto s = std::format("{}", q);
+    EXPECT_EQ(s, "[1, 2, 3, 4]");
+}