From 07ce64d6e7f8d984432927eb39c3dc597b7de25a Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Fri, 18 Aug 2023 18:44:52 +0200
Subject: [PATCH] refactor: `relative()` renamed to `quantity_from_origin()`

Resolves #479
---
 example/kalman_filter/kalman.h                |   2 +-
 .../kalman_filter/kalman_filter-example_6.cpp |   2 +-
 .../kalman_filter/kalman_filter-example_7.cpp |   2 +-
 .../kalman_filter/kalman_filter-example_8.cpp |   2 +-
 .../mp-units/bits/quantity_point_concepts.h   |  10 +-
 .../include/mp-units/customization_points.h   |   4 +-
 src/core/include/mp-units/quantity_point.h    |  39 ++--
 src/utility/include/mp-units/chrono.h         |   5 +-
 test/unit_test/static/quantity_point_test.cpp | 209 ++++++++++--------
 9 files changed, 149 insertions(+), 126 deletions(-)

diff --git a/example/kalman_filter/kalman.h b/example/kalman_filter/kalman.h
index 09867fa2..0d38ebaa 100644
--- a/example/kalman_filter/kalman.h
+++ b/example/kalman_filter/kalman.h
@@ -212,7 +212,7 @@ struct MP_UNITS_STD_FMT::formatter<kalman::estimation<Q>> {
       if constexpr (mp_units::Quantity<Q>)
         return t;
       else
-        return t.relative();
+        return t.quantity_from_origin();
     }(kalman::get<0>(e.state));
 
     std::string value_buffer;
diff --git a/example/kalman_filter/kalman_filter-example_6.cpp b/example/kalman_filter/kalman_filter-example_6.cpp
index d60b4b9a..1c31c199 100644
--- a/example/kalman_filter/kalman_filter-example_6.cpp
+++ b/example/kalman_filter/kalman_filter-example_6.cpp
@@ -45,7 +45,7 @@ template<QuantityPoint QP, QuantityOf<dimensionless> K>
 void print(auto iteration, K gain, QP measured, kalman::estimation<QP> current, kalman::estimation<QP> next)
 {
   std::cout << MP_UNITS_STD_FMT::format("{:2} | {:7%.4Q} | {:10%.3Q %q} | {:>18.3} | {:>18.3}\n", iteration, gain,
-                                        measured.relative(), current, next);
+                                        measured.quantity_from_origin(), current, next);
 }
 
 int main()
diff --git a/example/kalman_filter/kalman_filter-example_7.cpp b/example/kalman_filter/kalman_filter-example_7.cpp
index 9e1109d8..a8b67fd8 100644
--- a/example/kalman_filter/kalman_filter-example_7.cpp
+++ b/example/kalman_filter/kalman_filter-example_7.cpp
@@ -45,7 +45,7 @@ template<QuantityPoint QP, QuantityOf<dimensionless> K>
 void print(auto iteration, K gain, QP measured, kalman::estimation<QP> current, kalman::estimation<QP> next)
 {
   std::cout << MP_UNITS_STD_FMT::format("{:2} | {:7%.4Q} | {:10%.3Q %q} | {:>18.3} | {:>18.3}\n", iteration, gain,
-                                        measured.relative(), current, next);
+                                        measured.quantity_from_origin(), current, next);
 }
 
 int main()
diff --git a/example/kalman_filter/kalman_filter-example_8.cpp b/example/kalman_filter/kalman_filter-example_8.cpp
index cd7f8222..2765a1f3 100644
--- a/example/kalman_filter/kalman_filter-example_8.cpp
+++ b/example/kalman_filter/kalman_filter-example_8.cpp
@@ -45,7 +45,7 @@ template<QuantityPoint QP, QuantityOf<dimensionless> K>
 void print(auto iteration, K gain, QP measured, kalman::estimation<QP> current, kalman::estimation<QP> next)
 {
   std::cout << MP_UNITS_STD_FMT::format("{:2} | {:7%.3Q} | {:10%.3Q %q} | {:>16.2} | {:>16.2}\n", iteration, gain,
-                                        measured.relative(), current, next);
+                                        measured.quantity_from_origin(), current, next);
 }
 
 int main()
diff --git a/src/core/include/mp-units/bits/quantity_point_concepts.h b/src/core/include/mp-units/bits/quantity_point_concepts.h
index 0b8af05f..c3808fb9 100644
--- a/src/core/include/mp-units/bits/quantity_point_concepts.h
+++ b/src/core/include/mp-units/bits/quantity_point_concepts.h
@@ -22,6 +22,7 @@
 
 #pragma once
 
+#include <mp-units/bits/quantity_concepts.h>
 #include <mp-units/bits/quantity_spec_concepts.h>
 #include <mp-units/bits/reference_concepts.h>
 #include <mp-units/bits/representation_concepts.h>
@@ -163,7 +164,7 @@ concept QuantityPointOf =
  * all quantity_point-specific information.
  */
 template<typename T>
-concept QuantityPointLike = requires(T q) {
+concept QuantityPointLike = requires(T qp) {
   quantity_point_like_traits<T>::reference;
   quantity_point_like_traits<T>::point_origin;
   typename quantity_point_like_traits<T>::rep;
@@ -171,10 +172,11 @@ concept QuantityPointLike = requires(T q) {
   requires PointOrigin<std::remove_const_t<decltype(quantity_point_like_traits<T>::point_origin)>>;
   requires RepresentationOf<typename quantity_point_like_traits<T>::rep,
                             get_quantity_spec(quantity_point_like_traits<T>::reference).character>;
+  requires Quantity<std::remove_cvref_t<decltype(quantity_point_like_traits<T>::quantity_from_origin(qp))>>;
   requires std::constructible_from<
-    typename quantity_point<quantity_point_like_traits<T>::reference, quantity_point_like_traits<T>::point_origin,
-                            typename quantity_point_like_traits<T>::rep>::quantity_type,
-    decltype(quantity_point_like_traits<T>::relative(q))>;
+    quantity_point<quantity_point_like_traits<T>::reference, quantity_point_like_traits<T>::point_origin,
+                   typename quantity_point_like_traits<T>::rep>,
+    decltype(quantity_point_like_traits<T>::quantity_from_origin(qp))>;
 };
 
 }  // namespace mp_units
diff --git a/src/core/include/mp-units/customization_points.h b/src/core/include/mp-units/customization_points.h
index 316d1d18..a16d158a 100644
--- a/src/core/include/mp-units/customization_points.h
+++ b/src/core/include/mp-units/customization_points.h
@@ -149,8 +149,8 @@ struct quantity_like_traits;
  * @brief Provides support for external quantity point-like types
  *
  * The type trait should provide nested @c reference and @c origin objects,
- * a type alias @c rep, and a static member function @c relative(T) that will return
- * the quantity-like value of the quantity point.
+ * a type alias @c rep, and a static member function @c quantity_from_origin(T) that will
+ * return the quantity being the offset of the point from the origin.
  *
  * Usage example can be found in @c units/chrono.h header file.
  *
diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index 0e801d2d..41a6225b 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -121,7 +121,7 @@ public:
       q_([&] {
         if constexpr (is_same_v<std::remove_const_t<decltype(point_origin)>,
                                 std::remove_const_t<decltype(QP::point_origin)>>) {
-          return qp.relative();
+          return qp.quantity_from_origin();
         } else if constexpr (detail::is_derived_from_specialization_of_absolute_point_origin<
                                std::remove_const_t<decltype(point_origin)>>) {
           return qp.absolute();
@@ -137,7 +137,8 @@ public:
                           std::remove_const_t<decltype(point_origin)>> &&
              std::convertible_to<typename detail::quantity_point_like_type<QP>::quantity_type, quantity_type>
   constexpr explicit quantity_point(const QP& qp) :
-      q_(typename detail::quantity_point_like_type<QP>::quantity_type{quantity_point_like_traits<QP>::relative(qp)})
+      q_(typename detail::quantity_point_like_type<QP>::quantity_type{
+        quantity_point_like_traits<QP>::quantity_from_origin(qp)})
   {
   }
 
@@ -161,24 +162,24 @@ public:
   // data access
 #ifdef __cpp_explicit_this_parameter
   template<typename Self>
-  [[nodiscard]] constexpr auto&& relative(this Self&& self) noexcept
+  [[nodiscard]] constexpr auto&& quantity_from_origin(this Self&& self) noexcept
   {
     return std::forward<Self>(self).q_;
   }
 #else
-  [[nodiscard]] constexpr quantity_type& relative() & noexcept { return q_; }
-  [[nodiscard]] constexpr const quantity_type& relative() const& noexcept { return q_; }
-  [[nodiscard]] constexpr quantity_type&& relative() && noexcept { return std::move(q_); }
-  [[nodiscard]] constexpr const quantity_type&& relative() const&& noexcept { return std::move(q_); }
+  [[nodiscard]] constexpr quantity_type& quantity_from_origin() & noexcept { return q_; }
+  [[nodiscard]] constexpr const quantity_type& quantity_from_origin() const& noexcept { return q_; }
+  [[nodiscard]] constexpr quantity_type&& quantity_from_origin() && noexcept { return std::move(q_); }
+  [[nodiscard]] constexpr const quantity_type&& quantity_from_origin() const&& noexcept { return std::move(q_); }
 #endif
 
   [[nodiscard]] constexpr Quantity auto absolute() const noexcept
   {
     if constexpr (detail::is_derived_from_specialization_of_absolute_point_origin<
                     std::remove_const_t<decltype(point_origin)>>)
-      return relative();
+      return quantity_from_origin();
     else
-      return point_origin.quantity_point.absolute() + relative();
+      return point_origin.quantity_point.absolute() + quantity_from_origin();
   }
 
   template<Unit U>
@@ -247,9 +248,9 @@ template<auto R1, auto PO1, typename Rep1, auto R2, typename Rep2>
   requires ReferenceOf<std::remove_const_t<decltype(R2)>, get_quantity_spec(R1)>
 [[nodiscard]] constexpr QuantityPoint auto operator+(const quantity_point<R1, PO1, Rep1>& qp,
                                                      const quantity<R2, Rep2>& q)
-  requires requires { qp.relative() + q; }
+  requires requires { qp.quantity_from_origin() + q; }
 {
-  auto temp = qp.relative() + q;
+  auto temp = qp.quantity_from_origin() + q;
   using q_type = decltype(temp);
   return quantity_point<q_type::reference, PO1, typename q_type::rep>(std::move(temp));
 }
@@ -259,7 +260,7 @@ template<auto R1, typename Rep1, auto R2, auto PO2, typename Rep2>
   requires ReferenceOf<std::remove_const_t<decltype(R1)>, get_quantity_spec(R2)>
 [[nodiscard]] constexpr QuantityPoint auto operator+(const quantity<R1, Rep1>& q,
                                                      const quantity_point<R2, PO2, Rep2>& qp)
-  requires requires { q + qp.relative(); }
+  requires requires { q + qp.quantity_from_origin(); }
 {
   return qp + q;
 }
@@ -283,9 +284,9 @@ template<auto R1, auto PO1, typename Rep1, auto R2, typename Rep2>
   requires ReferenceOf<std::remove_const_t<decltype(R2)>, get_quantity_spec(R1)>
 [[nodiscard]] constexpr QuantityPoint auto operator-(const quantity_point<R1, PO1, Rep1>& qp,
                                                      const quantity<R2, Rep2>& q)
-  requires requires { qp.relative() - q; }
+  requires requires { qp.quantity_from_origin() - q; }
 {
-  const auto temp = qp.relative() - q;
+  const auto temp = qp.quantity_from_origin() - q;
   using q_type = decltype(temp);
   return quantity_point<q_type::reference, PO1, typename q_type::rep>(std::move(temp));
 }
@@ -306,7 +307,7 @@ template<QuantityPoint QP1, QuantityPointOf<QP1::absolute_point_origin> QP2>
                           std::remove_const_t<decltype(QP2::point_origin)>>) {
     constexpr auto common_qs = common_quantity_spec(QP1::quantity_spec, QP1::point_origin.quantity_spec,
                                                     QP2::quantity_spec, QP2::point_origin.quantity_spec);
-    return quantity_cast<common_qs>(lhs.relative() - rhs.relative());
+    return quantity_cast<common_qs>(lhs.quantity_from_origin() - rhs.quantity_from_origin());
   } else
     return lhs.absolute() - rhs.absolute();
 }
@@ -318,12 +319,12 @@ template<PointOrigin PO, QuantityPointOf<PO{}> QP>
   constexpr auto common_qs = common_quantity_spec(PO::quantity_spec, QP::quantity_spec, QP::point_origin.quantity_spec);
   if constexpr (detail::is_derived_from_specialization_of_absolute_point_origin<PO>) {
     if constexpr (is_same_v<std::remove_const_t<PO>, std::remove_const_t<decltype(QP::point_origin)>>)
-      return quantity_cast<common_qs>(qp.relative());
+      return quantity_cast<common_qs>(qp.quantity_from_origin());
     else
       return quantity_cast<common_qs>(qp.absolute());
   } else {
     if constexpr (is_same_v<std::remove_const_t<PO>, std::remove_const_t<decltype(QP::point_origin)>>)
-      return quantity_cast<common_qs>(qp.relative());
+      return quantity_cast<common_qs>(qp.quantity_from_origin());
     else
       return quantity_cast<common_qs>(qp.absolute() - PO::quantity_point.absolute());
   }
@@ -358,7 +359,7 @@ template<QuantityPoint QP1, QuantityPointOf<QP1::absolute_point_origin> QP2>
 {
   if constexpr (is_same_v<std::remove_const_t<decltype(QP1::point_origin)>,
                           std::remove_const_t<decltype(QP2::point_origin)>>)
-    return lhs.relative() <=> rhs.relative();
+    return lhs.quantity_from_origin() <=> rhs.quantity_from_origin();
   else
     return lhs.absolute() <=> rhs.absolute();
 }
@@ -369,7 +370,7 @@ template<QuantityPoint QP1, QuantityPointOf<QP1::absolute_point_origin> QP2>
 {
   if constexpr (is_same_v<std::remove_const_t<decltype(QP1::point_origin)>,
                           std::remove_const_t<decltype(QP2::point_origin)>>)
-    return lhs.relative() == rhs.relative();
+    return lhs.quantity_from_origin() == rhs.quantity_from_origin();
   else
     return lhs.absolute() == rhs.absolute();
 }
diff --git a/src/utility/include/mp-units/chrono.h b/src/utility/include/mp-units/chrono.h
index 42eef599..512b5df3 100644
--- a/src/utility/include/mp-units/chrono.h
+++ b/src/utility/include/mp-units/chrono.h
@@ -80,9 +80,10 @@ struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::durati
   static constexpr auto reference = detail::time_unit_from_chrono_period<Period>();
   static constexpr auto point_origin = chrono_point_origin<C>;
   using rep = Rep;
-  [[nodiscard]] static constexpr auto relative(const std::chrono::time_point<C, std::chrono::duration<Rep, Period>>& qp)
+  [[nodiscard]] static constexpr auto quantity_from_origin(
+    const std::chrono::time_point<C, std::chrono::duration<Rep, Period>>& qp)
   {
-    return qp.time_since_epoch();
+    return quantity{qp.time_since_epoch()};
   }
 };
 
diff --git a/test/unit_test/static/quantity_point_test.cpp b/test/unit_test/static/quantity_point_test.cpp
index 87f0b388..c171fc7d 100644
--- a/test/unit_test/static/quantity_point_test.cpp
+++ b/test/unit_test/static/quantity_point_test.cpp
@@ -198,15 +198,20 @@ static_assert(is_same_v<quantity_point<isq::length[m], absolute_point_origin<isq
 // static member functions
 ////////////////////////////
 
-static_assert(quantity_point<isq::length[m]>::zero().relative().number() == 0);
-static_assert(quantity_point<isq::length[m]>::min().relative().number() == std::numeric_limits<double>::lowest());
-static_assert(quantity_point<isq::length[m]>::max().relative().number() == std::numeric_limits<double>::max());
-static_assert(quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::zero().relative().number() ==
-              0);
-static_assert(quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::min().relative().number() ==
-              std::numeric_limits<int>::lowest());
-static_assert(quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::max().relative().number() ==
-              std::numeric_limits<int>::max());
+static_assert(quantity_point<isq::length[m]>::zero().quantity_from_origin().number() == 0);
+static_assert(quantity_point<isq::length[m]>::min().quantity_from_origin().number() ==
+              std::numeric_limits<double>::lowest());
+static_assert(quantity_point<isq::length[m]>::max().quantity_from_origin().number() ==
+              std::numeric_limits<double>::max());
+static_assert(
+  quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::zero().quantity_from_origin().number() ==
+  0);
+static_assert(
+  quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::min().quantity_from_origin().number() ==
+  std::numeric_limits<int>::lowest());
+static_assert(
+  quantity_point<isq::length[m], absolute_point_origin<isq::length>{}, int>::max().quantity_from_origin().number() ==
+  std::numeric_limits<int>::max());
 
 
 //////////////////////////////
@@ -383,42 +388,51 @@ static_assert(!std::convertible_to<sys_seconds, quantity_point<isq::time[s]>>);
 // obtaining a relative quantity
 //////////////////////////////////
 
-static_assert(quantity_point(42 * m).relative() == 42 * m);
-static_assert(quantity_point(isq::height(42 * m)).relative() == 42 * m);
+static_assert(quantity_point(42 * m).quantity_from_origin() == 42 * m);
+static_assert(quantity_point(isq::height(42 * m)).quantity_from_origin() == 42 * m);
 
-static_assert(quantity_point(1 * one).relative() == 1 * one);
-static_assert(quantity_point(dimensionless(1 * one)).relative() == 1 * one);
+static_assert(quantity_point(1 * one).quantity_from_origin() == 1 * one);
+static_assert(quantity_point(dimensionless(1 * one)).quantity_from_origin() == 1 * one);
 
-static_assert(quantity_point<isq::height[m], mean_sea_level>(42 * m).relative() == 42 * m);
-static_assert(quantity_point<isq::height[m], ground_level>(42 * m).relative() == 42 * m);
-static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).relative() == 42 * m);
+static_assert(quantity_point<isq::height[m], mean_sea_level>(42 * m).quantity_from_origin() == 42 * m);
+static_assert(quantity_point<isq::height[m], ground_level>(42 * m).quantity_from_origin() == 42 * m);
+static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).quantity_from_origin() == 42 * m);
 
 static_assert(quantity_point<isq::height[m], mean_sea_level>(quantity_point<isq::height[m], ground_level>(42 * m))
-                .relative() == 84 * m);
+                .quantity_from_origin() == 84 * m);
 static_assert(quantity_point<isq::height[m], mean_sea_level>(quantity_point<isq::height[m], tower_peak>(42 * m))
-                .relative() == 126 * m);
+                .quantity_from_origin() == 126 * m);
 
 static_assert(quantity_point<isq::height[m], ground_level>(quantity_point<isq::height[m], mean_sea_level>(84 * m))
-                .relative() == 42 * m);
+                .quantity_from_origin() == 42 * m);
 static_assert(quantity_point<isq::height[m], ground_level>(quantity_point<isq::height[m], tower_peak>(42 * m))
-                .relative() == 84 * m);
+                .quantity_from_origin() == 84 * m);
 
 static_assert(quantity_point<isq::height[m], tower_peak>(quantity_point<isq::height[m], mean_sea_level>(42 * m))
-                .relative() == -42 * m);
+                .quantity_from_origin() == -42 * m);
 static_assert(quantity_point<isq::height[m], tower_peak>(quantity_point<isq::height[m], ground_level>(84 * m))
-                .relative() == 42 * m);
+                .quantity_from_origin() == 42 * m);
 
-static_assert(quantity_point<isq::height[m], mean_sea_level>(42 * m).point_from(mean_sea_level).relative() == 42 * m);
-static_assert(quantity_point<isq::height[m], ground_level>(42 * m).point_from(mean_sea_level).relative() == 84 * m);
-static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(mean_sea_level).relative() == 126 * m);
+static_assert(
+  quantity_point<isq::height[m], mean_sea_level>(42 * m).point_from(mean_sea_level).quantity_from_origin() == 42 * m);
+static_assert(quantity_point<isq::height[m], ground_level>(42 * m).point_from(mean_sea_level).quantity_from_origin() ==
+              84 * m);
+static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(mean_sea_level).quantity_from_origin() ==
+              126 * m);
 
-static_assert(quantity_point<isq::height[m], ground_level>(84 * m).point_from(ground_level).relative() == 84 * m);
-static_assert(quantity_point<isq::height[m], mean_sea_level>(84 * m).point_from(ground_level).relative() == 42 * m);
-static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(ground_level).relative() == 84 * m);
+static_assert(quantity_point<isq::height[m], ground_level>(84 * m).point_from(ground_level).quantity_from_origin() ==
+              84 * m);
+static_assert(quantity_point<isq::height[m], mean_sea_level>(84 * m).point_from(ground_level).quantity_from_origin() ==
+              42 * m);
+static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(ground_level).quantity_from_origin() ==
+              84 * m);
 
-static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(tower_peak).relative() == 42 * m);
-static_assert(quantity_point<isq::height[m], mean_sea_level>(42 * m).point_from(tower_peak).relative() == -42 * m);
-static_assert(quantity_point<isq::height[m], ground_level>(84 * m).point_from(tower_peak).relative() == 42 * m);
+static_assert(quantity_point<isq::height[m], tower_peak>(42 * m).point_from(tower_peak).quantity_from_origin() ==
+              42 * m);
+static_assert(quantity_point<isq::height[m], mean_sea_level>(42 * m).point_from(tower_peak).quantity_from_origin() ==
+              -42 * m);
+static_assert(quantity_point<isq::height[m], ground_level>(84 * m).point_from(tower_peak).quantity_from_origin() ==
+              42 * m);
 
 static_assert(is_of_type<quantity_point<isq::height[m], ground_level, short>(42 * m).point_from(mean_sea_level),
                          quantity_point<isq::height[m], mean_sea_level, int>>);
@@ -468,13 +482,15 @@ static_assert(quantity_point<isq::height[m], ground_level>(84 * m).point_from(to
 // converting to a different unit
 ///////////////////////////////////
 
-static_assert(quantity_point<isq::length[km]>(2. * km)[km].relative().number() == 2.);
-static_assert(quantity_point<isq::length[km]>(2. * km)[m].relative().number() == 2000.);
-static_assert(quantity_point<isq::length[m]>(2000. * m)[km].relative().number() == 2.);
-static_assert(
-  quantity_point<isq::length[km], absolute_point_origin<isq::length>{}, int>(2 * km)[km].relative().number() == 2);
-static_assert(
-  quantity_point<isq::length[km], absolute_point_origin<isq::length>{}, int>(2 * km)[m].relative().number() == 2000);
+static_assert(quantity_point<isq::length[km]>(2. * km)[km].quantity_from_origin().number() == 2.);
+static_assert(quantity_point<isq::length[km]>(2. * km)[m].quantity_from_origin().number() == 2000.);
+static_assert(quantity_point<isq::length[m]>(2000. * m)[km].quantity_from_origin().number() == 2.);
+static_assert(quantity_point<isq::length[km], absolute_point_origin<isq::length>{}, int>(2 * km)[km]
+                .quantity_from_origin()
+                .number() == 2);
+static_assert(quantity_point<isq::length[km], absolute_point_origin<isq::length>{}, int>(2 * km)[m]
+                .quantity_from_origin()
+                .number() == 2000);
 
 #if MP_UNITS_COMP_GCC != 10 || MP_UNITS_COMP_GCC_MINOR > 2
 template<template<auto, auto, typename> typename QP>
@@ -521,18 +537,18 @@ static_assert(([]() {
                 quantity_point l1{1 * m}, l2{2 * m};
                 return l2 = l1;
               }())
-                .relative() == 1 * m);
+                .quantity_from_origin() == 1 * m);
 static_assert(([]() {
                 const quantity_point l1{1 * m};
                 quantity_point l2{2 * m};
                 return l2 = l1;
               }())
-                .relative() == 1 * m);
+                .quantity_from_origin() == 1 * m);
 static_assert(([]() {
                 quantity_point l1{1 * m}, l2{2 * m};
                 return l2 = std::move(l1);
               }())
-                .relative() == 1 * m);
+                .quantity_from_origin() == 1 * m);
 
 
 ////////////////////
@@ -562,14 +578,14 @@ static_assert([](auto v) {
 ////////////////////////
 
 // same type
-static_assert((quantity_point(1 * m) += 1 * m).relative().number() == 2);
-static_assert((quantity_point(2 * m) -= 1 * m).relative().number() == 1);
+static_assert((quantity_point(1 * m) += 1 * m).quantity_from_origin().number() == 2);
+static_assert((quantity_point(2 * m) -= 1 * m).quantity_from_origin().number() == 1);
 
 // different types
-static_assert((quantity_point(2.5 * m) += 3 * m).relative().number() == 5.5);
-static_assert((quantity_point(123 * m) += 1 * km).relative().number() == 1123);
-static_assert((quantity_point(5.5 * m) -= 3 * m).relative().number() == 2.5);
-static_assert((quantity_point(1123 * m) -= 1 * km).relative().number() == 123);
+static_assert((quantity_point(2.5 * m) += 3 * m).quantity_from_origin().number() == 5.5);
+static_assert((quantity_point(123 * m) += 1 * km).quantity_from_origin().number() == 1123);
+static_assert((quantity_point(5.5 * m) -= 3 * m).quantity_from_origin().number() == 2.5);
+static_assert((quantity_point(1123 * m) -= 1 * km).quantity_from_origin().number() == 123);
 
 
 template<template<auto, auto, typename> typename QP>
@@ -950,18 +966,21 @@ static_assert(is_of_type<(1 * m + tower_peak) - (1 * m + other_ground_level), qu
 
 // check for integral types promotion
 static_assert(
-  is_same_v<decltype((quantity_point{std::uint8_t(0) * m} + std::uint8_t(0) * m).relative().number()), int&&>);
+  is_same_v<decltype((quantity_point{std::uint8_t(0) * m} + std::uint8_t(0) * m).quantity_from_origin().number()),
+            int&&>);
 static_assert(
-  is_same_v<decltype((std::uint8_t(0) * m + quantity_point{std::uint8_t(0) * m}).relative().number()), int&&>);
+  is_same_v<decltype((std::uint8_t(0) * m + quantity_point{std::uint8_t(0) * m}).quantity_from_origin().number()),
+            int&&>);
 static_assert(
-  is_same_v<decltype((quantity_point{std::uint8_t(0) * m} - std::uint8_t(0) * m).relative().number()), int&&>);
+  is_same_v<decltype((quantity_point{std::uint8_t(0) * m} - std::uint8_t(0) * m).quantity_from_origin().number()),
+            int&&>);
 static_assert(
   is_same_v<decltype((quantity_point{std::uint8_t(0) * m} - quantity_point{std::uint8_t(0) * m}).number()), int&&>);
-static_assert((quantity_point{std::uint8_t(128) * m} + std::uint8_t(128) * m).relative().number() ==
+static_assert((quantity_point{std::uint8_t(128) * m} + std::uint8_t(128) * m).quantity_from_origin().number() ==
               std::uint8_t(128) + std::uint8_t(128));
-static_assert((std::uint8_t(128) * m + quantity_point{std::uint8_t(128) * m}).relative().number() ==
+static_assert((std::uint8_t(128) * m + quantity_point{std::uint8_t(128) * m}).quantity_from_origin().number() ==
               std::uint8_t(128) + std::uint8_t(128));
-static_assert((quantity_point{std::uint8_t(0) * m} - std::uint8_t(1) * m).relative().number() ==
+static_assert((quantity_point{std::uint8_t(0) * m} - std::uint8_t(1) * m).quantity_from_origin().number() ==
               std::uint8_t(0) - std::uint8_t(1));
 static_assert((quantity_point{std::uint8_t(0) * m} - quantity_point{std::uint8_t(1) * m}).number() ==
               std::uint8_t(0) - std::uint8_t(1));
@@ -1051,32 +1070,32 @@ static_assert(is_of_type<quantity_point{1 * km} - quantity_point{1. * m}, quanti
 static_assert(is_of_type<quantity_point{1. * km} - quantity_point{1. * m}, quantity<si::metre, double>>);
 
 
-static_assert((quantity_point{1 * m} + 1 * m).relative().number() == 2);
-static_assert((1 * m + quantity_point{1 * m}).relative().number() == 2);
-static_assert((quantity_point{1 * m} + 1 * km).relative().number() == 1001);
-static_assert((1 * m + quantity_point{1 * km}).relative().number() == 1001);
-static_assert((quantity_point{1 * km} + 1 * m).relative().number() == 1001);
-static_assert((1 * km + quantity_point{1 * m}).relative().number() == 1001);
-static_assert((quantity_point{2 * m} - 1 * m).relative().number() == 1);
-static_assert((quantity_point{1 * km} - 1 * m).relative().number() == 999);
+static_assert((quantity_point{1 * m} + 1 * m).quantity_from_origin().number() == 2);
+static_assert((1 * m + quantity_point{1 * m}).quantity_from_origin().number() == 2);
+static_assert((quantity_point{1 * m} + 1 * km).quantity_from_origin().number() == 1001);
+static_assert((1 * m + quantity_point{1 * km}).quantity_from_origin().number() == 1001);
+static_assert((quantity_point{1 * km} + 1 * m).quantity_from_origin().number() == 1001);
+static_assert((1 * km + quantity_point{1 * m}).quantity_from_origin().number() == 1001);
+static_assert((quantity_point{2 * m} - 1 * m).quantity_from_origin().number() == 1);
+static_assert((quantity_point{1 * km} - 1 * m).quantity_from_origin().number() == 999);
 
-static_assert((quantity_point{1.5 * m} + 1 * m).relative().number() == 2.5);
-static_assert((1.5 * m + quantity_point{1 * m}).relative().number() == 2.5);
-static_assert((quantity_point{1.5 * m} + 1 * km).relative().number() == 1001.5);
-static_assert((1.5 * m + quantity_point{1 * km}).relative().number() == 1001.5);
-static_assert((quantity_point{1.5 * km} + 1 * m).relative().number() == 1501);
-static_assert((1.5 * km + quantity_point{1 * m}).relative().number() == 1501);
-static_assert((quantity_point{2.5 * m} - 1 * m).relative().number() == 1.5);
-static_assert((quantity_point{1.5 * km} - 1 * m).relative().number() == 1499);
+static_assert((quantity_point{1.5 * m} + 1 * m).quantity_from_origin().number() == 2.5);
+static_assert((1.5 * m + quantity_point{1 * m}).quantity_from_origin().number() == 2.5);
+static_assert((quantity_point{1.5 * m} + 1 * km).quantity_from_origin().number() == 1001.5);
+static_assert((1.5 * m + quantity_point{1 * km}).quantity_from_origin().number() == 1001.5);
+static_assert((quantity_point{1.5 * km} + 1 * m).quantity_from_origin().number() == 1501);
+static_assert((1.5 * km + quantity_point{1 * m}).quantity_from_origin().number() == 1501);
+static_assert((quantity_point{2.5 * m} - 1 * m).quantity_from_origin().number() == 1.5);
+static_assert((quantity_point{1.5 * km} - 1 * m).quantity_from_origin().number() == 1499);
 
-static_assert((quantity_point{1 * m} + 1.5 * m).relative().number() == 2.5);
-static_assert((1 * m + quantity_point{1.5 * m}).relative().number() == 2.5);
-static_assert((quantity_point{1 * m} + 1.5 * km).relative().number() == 1501);
-static_assert((1 * m + quantity_point{1.5 * km}).relative().number() == 1501);
-static_assert((quantity_point{1 * km} + 1.5 * m).relative().number() == 1001.5);
-static_assert((1 * km + quantity_point{1.5 * m}).relative().number() == 1001.5);
-static_assert((quantity_point{2 * m} - 1.5 * m).relative().number() == 0.5);
-static_assert((quantity_point{1 * km} - 1.5 * m).relative().number() == 998.5);
+static_assert((quantity_point{1 * m} + 1.5 * m).quantity_from_origin().number() == 2.5);
+static_assert((1 * m + quantity_point{1.5 * m}).quantity_from_origin().number() == 2.5);
+static_assert((quantity_point{1 * m} + 1.5 * km).quantity_from_origin().number() == 1501);
+static_assert((1 * m + quantity_point{1.5 * km}).quantity_from_origin().number() == 1501);
+static_assert((quantity_point{1 * km} + 1.5 * m).quantity_from_origin().number() == 1001.5);
+static_assert((1 * km + quantity_point{1.5 * m}).quantity_from_origin().number() == 1001.5);
+static_assert((quantity_point{2 * m} - 1.5 * m).quantity_from_origin().number() == 0.5);
+static_assert((quantity_point{1 * km} - 1.5 * m).quantity_from_origin().number() == 998.5);
 
 static_assert((quantity_point{2 * m} - quantity_point{1 * m}).number() == 1);
 static_assert((quantity_point{1 * km} - quantity_point{1 * m}).number() == 999);
@@ -1098,18 +1117,18 @@ static_assert(quantity_point(42 * m, tower_peak) - quantity_point(42 * m, other_
 
 
 // commutativity and associativity
-static_assert((quantity_point{10 * isq::length[si::metre] / (2 * isq::time[s])} + 5 * isq::speed[m / s]).relative() ==
-              10 * isq::speed[m / s]);
-static_assert((10 * isq::length[si::metre] / (2 * isq::time[s]) + quantity_point{5 * isq::speed[m / s]}).relative() ==
-              10 * isq::speed[m / s]);
-static_assert((quantity_point{5 * isq::speed[m / s]} + 10 * isq::length[m] / (2 * isq::time[s])).relative() ==
-              10 * isq::speed[m / s]);
-static_assert((5 * isq::speed[m / s] + quantity_point{10 * isq::length[m] / (2 * isq::time[s])}).relative() ==
-              10 * isq::speed[m / s]);
-static_assert((quantity_point{10 * isq::length[m] / (2 * isq::time[s])} - 5 * isq::speed[m / s]).relative() ==
-              0 * isq::speed[m / s]);
-static_assert((quantity_point{5 * isq::speed[m / s]} - 10 * isq::length[m] / (2 * isq::time[s])).relative() ==
-              0 * isq::speed[m / s]);
+static_assert((quantity_point{10 * isq::length[si::metre] / (2 * isq::time[s])} + 5 * isq::speed[m / s])
+                .quantity_from_origin() == 10 * isq::speed[m / s]);
+static_assert((10 * isq::length[si::metre] / (2 * isq::time[s]) + quantity_point{5 * isq::speed[m / s]})
+                .quantity_from_origin() == 10 * isq::speed[m / s]);
+static_assert((quantity_point{5 * isq::speed[m / s]} + 10 * isq::length[m] / (2 * isq::time[s]))
+                .quantity_from_origin() == 10 * isq::speed[m / s]);
+static_assert((5 * isq::speed[m / s] + quantity_point{10 * isq::length[m] / (2 * isq::time[s])})
+                .quantity_from_origin() == 10 * isq::speed[m / s]);
+static_assert((quantity_point{10 * isq::length[m] / (2 * isq::time[s])} - 5 * isq::speed[m / s])
+                .quantity_from_origin() == 0 * isq::speed[m / s]);
+static_assert((quantity_point{5 * isq::speed[m / s]} - 10 * isq::length[m] / (2 * isq::time[s]))
+                .quantity_from_origin() == 0 * isq::speed[m / s]);
 static_assert(quantity_point{10 * isq::length[m] / (2 * isq::time[s])} - quantity_point{5 * isq::speed[m / s]} ==
               0 * isq::speed[m / s]);
 static_assert(quantity_point{5 * isq::speed[m / s]} - quantity_point{10 * isq::length[m] / (2 * isq::time[s])} ==
@@ -1136,17 +1155,17 @@ static_assert(
              quantity<isq::speed[m / s], int>>);
 
 // NOTE: 1 / isq::time[s] works for quantities but not for quantity_point (origin can't be weakened)
-static_assert((quantity_point{10 / (2 * isq::period_duration[s])} + 5 * isq::frequency[Hz]).relative() ==
+static_assert((quantity_point{10 / (2 * isq::period_duration[s])} + 5 * isq::frequency[Hz]).quantity_from_origin() ==
               10 * isq::frequency[Hz]);
-static_assert((10 / (2 * isq::period_duration[s]) + quantity_point{5 * isq::frequency[Hz]}).relative() ==
+static_assert((10 / (2 * isq::period_duration[s]) + quantity_point{5 * isq::frequency[Hz]}).quantity_from_origin() ==
               10 * isq::frequency[Hz]);
-static_assert((quantity_point{5 * isq::frequency[Hz]} + 10 / (2 * isq::period_duration[s])).relative() ==
+static_assert((quantity_point{5 * isq::frequency[Hz]} + 10 / (2 * isq::period_duration[s])).quantity_from_origin() ==
               10 * isq::frequency[Hz]);
-static_assert((5 * isq::frequency[Hz] + quantity_point{10 / (2 * isq::period_duration[s])}).relative() ==
+static_assert((5 * isq::frequency[Hz] + quantity_point{10 / (2 * isq::period_duration[s])}).quantity_from_origin() ==
               10 * isq::frequency[Hz]);
-static_assert((quantity_point{10 / (2 * isq::period_duration[s])} - 5 * isq::frequency[Hz]).relative() ==
+static_assert((quantity_point{10 / (2 * isq::period_duration[s])} - 5 * isq::frequency[Hz]).quantity_from_origin() ==
               0 * isq::frequency[Hz]);
-static_assert((quantity_point{5 * isq::frequency[Hz]} - 10 / (2 * isq::period_duration[s])).relative() ==
+static_assert((quantity_point{5 * isq::frequency[Hz]} - 10 / (2 * isq::period_duration[s])).quantity_from_origin() ==
               0 * isq::frequency[Hz]);
 static_assert(quantity_point{10 / (2 * isq::period_duration[s])} - quantity_point{5 * isq::frequency[Hz]} ==
               0 * isq::frequency[Hz]);