[#mp_units-frame_projection]
= xref:mp_units.adoc[mp_units]::frame_projection
:relfileprefix: ../
:mrdocs:

User‐specializable variable template for runtime coordinate frame transformations.

== Synopsis

Declared in `<mp‐units/framework/quantity_point_concepts.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    /* implementation-defined */ auto From,
    /* implementation-defined */ auto To>
requires (From != To)
inline constexpr /* implementation-defined */ frame_projection = frame_projection;
----

== Description

Connects pairs of absolute point origin values with user‐supplied callables. Specialize per (From, To) pair with a callable invocable as: frame_projection<From, To>(quantity_point_at_From, extra_args...) ‐> quantity_point_at_To

Both directions must be explicitly specialized — the library never derives an inverse. The primary template defaults to `detail::undefined_t{}` so that `detail::HasFrameProjection` can detect whether a specialization exists.


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