affn

Affine geometry primitives for strongly-typed coordinate systems.

affn is a small, domain-agnostic geometry kernel for scientific and engineering software. It provides:

• Reference centers (C): the origin of a coordinate system (optionally parameterized via C::Params)
• Reference frames (F): the orientation of axes
• Typed coordinates: Cartesian, spherical, and ellipsoidal positions plus directions/vectors
• Affine operators: Rotation3, Translation3, and Isometry3
• Units: distances/lengths are carried via qtty units at the type level

The goal is to make invalid operations (like adding two positions) fail at compile time.

Quick Start

Add the dependency:

[dependencies]
affn = "0.4"
qtty = "0.4"

Define a center + frame and do basic affine algebra:

use affn::cartesian::{Displacement, Position};
use affn::centers::ReferenceCenter;
use affn::frames::ReferenceFrame;
use qtty::*;

#[derive(Debug, Copy, Clone)]
struct World;
impl ReferenceFrame for World {
    fn frame_name() -> &'static str { "World" }
}

#[derive(Debug, Copy, Clone)]
struct Origin;
impl ReferenceCenter for Origin {
    type Params = ();
    fn center_name() -> &'static str { "Origin" }
}

let a = Position::<Origin, World, Meter>::new(0.0, 0.0, 0.0);
let b = Position::<Origin, World, Meter>::new(3.0, 4.0, 0.0);

// Position - Position -> Displacement
let d: Displacement<World, Meter> = b - a;
assert!((d.magnitude().value() - 5.0).abs() < 1e-12);

// Position + Displacement -> Position
let c = a + d;
assert!((c.y().value() - 4.0).abs() < 1e-12);

Core Concepts

• Position<C, F, U>: an affine point (depends on both center and frame)
• Direction<F>: a unit vector (frame-only, translation-invariant)
• Vector<F, U> / Displacement<F, U> / Velocity<F, U>: free vectors (frame-only)
• EllipsoidalPosition<C, F, U>: geodetic longitude/latitude/height on an ellipsoid-aware frame

The type system enforces the usual affine rules:

• ✅ Position - Position -> Displacement
• ✅ Position + Displacement -> Position
• ❌ Position + Position (does not compile)

Affine Operators

affn includes typed affine operators for pure geometric transforms:

• Rotation3
• Translation3
• Isometry3

These operators preserve the existing frame tag. When you intentionally rotate from one frame into another, retag the result explicitly with reinterpret_frame:

use affn::cartesian::Position;
use affn::ops::Rotation3;
use affn::prelude::*;
use qtty::*;

#[derive(Debug, Copy, Clone, ReferenceFrame)]
struct FrameA;

#[derive(Debug, Copy, Clone, ReferenceFrame)]
struct FrameB;

#[derive(Debug, Copy, Clone, ReferenceCenter)]
struct Center;

let pos_a = Position::<Center, FrameA, Meter>::new(1.0 * M, 0.0 * M, 0.0 * M);
let rot = Rotation3::rz(90.0 * DEG);
let pos_b = (rot * pos_a).reinterpret_frame::<FrameB>();

assert!(pos_b.y().value() > 0.999);

Defining Custom Systems (Derive Macros)

For zero-sized marker types, use the derive macros re-exported by the crate:

use affn::prelude::*;

#[derive(Debug, Copy, Clone, ReferenceFrame)]
struct MyFrame;

#[derive(Debug, Copy, Clone, ReferenceCenter)]
struct MyCenter;

Some centers need runtime parameters (e.g. “topocentric” depends on an observer):

use affn::prelude::*;

#[derive(Clone, Debug, Default, PartialEq)]
struct Observer {
    lat_deg: f64,
    lon_deg: f64,
}

#[derive(Debug, Copy, Clone, ReferenceCenter)]
#[center(params = Observer)]
struct Topocentric;

Spherical ↔ Cartesian

Cartesian and spherical positions can be converted losslessly (up to floating point error):

use affn::cartesian::Position as CPos;
use affn::spherical::Position as SPos;
use affn::centers::ReferenceCenter;
use affn::frames::ReferenceFrame;
use qtty::*;

#[derive(Debug, Copy, Clone)]
struct Frame;
impl ReferenceFrame for Frame { fn frame_name() -> &'static str { "Frame" } }

#[derive(Debug, Copy, Clone)]
struct Center;
impl ReferenceCenter for Center { type Params = (); fn center_name() -> &'static str { "Center" } }

let cart = CPos::<Center, Frame, Meter>::new(1.0, 1.0, 1.0);
let sph: SPos<Center, Frame, Meter> = cart.to_spherical();
let back: CPos<Center, Frame, Meter> = CPos::from_spherical(&sph);
assert!((back.z().value() - 1.0).abs() < 1e-10);

Ellipsoidal Coordinates

affn::ellipsoidal::Position<C, F, U> models geodetic longitude, latitude, and height above an ellipsoid. Conversions to and from Cartesian coordinates are available when the frame implements HasEllipsoid.

Built-in terrestrial frames under the astro feature already carry ellipsoids:

• ECEF uses Wgs84
• ITRF uses Grs80

Example:

use affn::centers::ReferenceCenter;
use affn::ellipsoidal::Position;
use qtty::*;

#[derive(Debug, Copy, Clone)]
struct Geocentric;
impl ReferenceCenter for Geocentric {
    type Params = ();
    fn center_name() -> &'static str { "Geocentric" }
}

# #[cfg(feature = "astro")]
# {
let geodetic = Position::<Geocentric, affn::frames::ECEF, Meter>::new(
    -17.89 * DEG,
    28.76 * DEG,
    2200.0 * M,
);
let cart = geodetic.to_cartesian::<Meter>();
let roundtrip = Position::<Geocentric, affn::frames::ECEF, Meter>::from_cartesian(&cart);
assert!((roundtrip.height.value() - geodetic.height.value()).abs() < 1e-6);
# }

Built-In Astro Frames (optional)

Enable the astro feature to use the built-in astronomy and geodesy marker frames:

[dependencies]
affn = { version = "0.4", features = ["astro"] }
qtty = "0.4"

Available built-ins include:

• Equatorial: ICRS, ICRF, EquatorialMeanJ2000, EME2000, EquatorialMeanOfDate, EquatorialTrueOfDate, GCRS, CIRS, TIRS, FK4B1950, TEME
• Ecliptic and galactic: EclipticMeanJ2000, EclipticOfDate, EclipticTrueOfDate, Galactic
• Terrestrial and horizontal: Horizontal, ECEF, ITRF
• Planetary body-fixed: MercuryFixed, VenusFixed, MarsFixed, MoonPrincipalAxes, JupiterSystemIII, SaturnFixed, UranusFixed, NeptuneFixed, PlutoFixed

Examples

Run the included examples:

• cargo run --example basic_cartesian
• cargo run --example parameterized_center
• cargo run --example spherical_roundtrip
• cargo run --example serde_roundtrip --features serde

Serde (optional)

affn supports serde serialization for its core coordinate types behind an opt-in feature flag.

Enable it in your Cargo.toml:

[dependencies]
affn = { version = "0.4", features = ["serde"] }
qtty = "0.4"

This feature also forwards serialization support to dependencies where needed (e.g. qtty/serde, and nalgebra's serde-serialize).

To run the serde example:

• cargo run --example serde_roundtrip --features serde

Or to run tests including serde round-trips:

• cargo test --features serde

License

Licensed under AGPL-3.0-only. See LICENSE.