✨ feat: tangent representations

docs/spec.md

@@ -946,7 +946,7 @@ A non-exhaustive table of exported objects are:
 | `coordinax.angles` | `AbstractAngle`, `Angle`, `wrap_to` |
 | `coordinax.distances` | `AbstractDistance`, `Distance` |
 | `coordinax.charts` | `CartesianProductChart`, </br> `cartesian_chart`, `guess_chart`, `cdict`, `pt_map`, `jac_pt_map`, </br> `cart0d`, </br> `cart1d`, `radial1d`, `time1d`, </br> `cart2d`, `polar2d`, </br> `cart3d`, `cyl3d`, `sph3d`, `lonlat_sph3d`, `loncoslat_sph3d`, `math_sph3d`, </br> `cartnd`, </br> `spacetimect` |
-| `coordinax.representations` | `cconvert`, `change_basis`, </br> `Representation`, `point`, `coord_disp`, `coord_vel`, `coord_acc`, `phys_disp`, `phys_vel`, `phys_acc`, </br> `PointGeometry`, `point_geom`, `TangentGeometry`, `tangent_geom`, </br> `NoBasis`, `no_basis`, `CoordinateBasis`, `coord_basis`, `PhysicalBasis`, `phys_basis`, </br> `Location`, `loc`, `Displacement`, `dpl`, `Velocity`, `vel`, `Acceleration`, `acc`, </br> `guess_geometry_kind`, `guess_semantic_kind`, `guess_rep` |
+| `coordinax.representations` | `cconvert`, `change_basis`, `tangent_map`, </br> `Representation`, `point`, `coord_disp`, `coord_vel`, `coord_acc`, `phys_disp`, `phys_vel`, `phys_acc`, </br> `PointGeometry`, `point_geom`, `TangentGeometry`, `tangent_geom`, </br> `NoBasis`, `no_basis`, `CoordinateBasis`, `coord_basis`, `PhysicalBasis`, `phys_basis`, </br> `Location`, `loc`, `Displacement`, `dpl`, `Velocity`, `vel`, `Acceleration`, `acc`, </br> `guess_geometry_kind`, `guess_semantic_kind`, `guess_rep` |
 | `coordinax.vectors` | `Point`, `ToUnitsOptions` |
 | `coordinax.manifolds` | `guess_manifold`, `scale_factors`, `angle_between`, </br> `EuclideanManifold`, `EuclideanMetric`, `euclidean3d`, </br> `EmbeddedManifold`, `EmbeddedChart` </br> `twosphere`, `embedded_twosphere`, </br> `CustomManifold`,`CustomAtlas`, |
 | `coordinax.transforms` | `act`, `simplify`, `compose`, `materialize_transform`, </br> `AbstractTransform`, `Identity`, `Composed`, `Translate`, `Rotate`, `Reflect`, `Scale`, `Shear`, `identity`, </br> `AbstractTransformGroup`, `IdentityGroup`, `DiffeomorphismGroup`, `AffineGroup`, `EuclideanGroup`, `OrthogonalGroup`, `SpecialOrthogonalGroup`, `PoincareGroup`, `LorentzGroup`, `ProperOrthochronousLorentzGroup` |
@@ -1728,6 +1728,44 @@ A representation is therefore **not** the same thing as a chart: the chart deter
 
     - Inherits all failure semantics from `guess_geometry_kind`.
 
+!!! info `tangent_map`
+
+    Transform a tangent vector from one chart to another.
+
+    **Signature:**
+
+    ```text
+    tangent_map(v, from_chart, from_geom, from_rep, to_chart, to_geom, to_rep, /, *, at) -> CDict
+    ```
+
+    A 4-argument shorthand form is also supported:
+
+    ```text
+    tangent_map(v, from_chart, from_rep, to_chart, /, *, at) -> CDict
+    ```
+
+    **Arguments:**
+
+    - `v`: `CDict` — tangent vector components in `from_chart` with basis `from_rep.basis`.
+    - `from_chart`: source chart.
+    - `from_geom`: source geometry (e.g. `TangentGeometry`).
+    - `from_rep`: source `Representation` (must have `TangentGeometry`).
+    - `to_chart`: target chart.
+    - `to_geom`: target geometry.
+    - `to_rep`: target `Representation`.
+    - `at`: `CDict` — base point in `from_chart` coordinates at which the tangent space is attached. Required for non-Cartesian charts (since the Jacobian depends on the base point).
+
+    **Semantics by basis:**
+
+    - **`CoordinateBasis`**: delegates to `jac_pt_map(at, from_chart, to_chart)` to obtain the Jacobian $J^j{}_i = \partial\tilde{q}^j/\partial q^i$, then applies $\tilde{v}^j = J^j{}_i v^i$.
+    - **`PhysicalBasis`**: fetch the orthonormal frame matrices $B_{\rm from}$ (columns = physical basis vectors in Cartesian) and $B_{\rm to}$ via `frame_cart`, compute $R = B_{\rm to}^T B_{\rm from}$, apply $\hat{v}' = R \hat{v}$.
+
+    **Same-chart optimisation:** when `from_chart is to_chart`, returns `v` unchanged.
+
+    **`cconvert` integration:** `cconvert` dispatches to `tangent_map` when the source representation has `TangentGeometry`, passing `at` through the `at` keyword argument.
+
+    **Same-chart basis conversion:** `change_basis(v, chart, from_basis, to_basis, /, *, at)` changes tangent component conventions without changing charts. In v1 it is defined only for Cartesian charts and `CoordinateBasis` $
+
 </br>
 
 ### Geometric Kind

packages/coordinax.api/src/coordinax/api/representations.py

@@ -32,6 +32,23 @@ def change_basis(*args: Any, **kwargs: Any) -> Any:
     raise NotImplementedError  # pragma: no cover
 
 
+@plum.dispatch.abstract
+def tangent_map(*args: Any, **kwargs: Any) -> Any:
+    """Compute the tangent map (Jacobian) of a chart transition.
+
+    This is an abstract API definition. See the main coordinax package for
+    concrete implementations.
+
+    Examples
+    --------
+    >>> import jax.numpy as jnp
+    >>> import coordinax.charts as cxc
+    >>> import coordinax.representations as cxr
+
+    """
+    raise NotImplementedError  # pragma: no cover
+
+
 @plum.dispatch.abstract
 def cconvert(*args: Any, **kwargs: Any) -> Any:
     """Transform the current vector to the target chart.

src/coordinax/main/__init__.py

@@ -78,6 +78,7 @@
     "phys_disp",
     "phys_vel",
     "phys_acc",
+    "tangent_map",
     "change_basis",
     # vectors
     "Point",
@@ -151,6 +152,7 @@
     point_geom,
     subtract,
     tangent_geom,
+    tangent_map,
     vel,
 )
 from coordinax.transforms import (

src/coordinax/representations/__init__.py

@@ -96,6 +96,7 @@
     "guess_rep",
     "guess_semantic_kind",
     "subtract",
+    "tangent_map",
     "change_basis",
     # Representations
     "Representation",
@@ -171,6 +172,7 @@
         point,
         point_geom,
         tangent_geom,
+        tangent_map,
         vel,
     )
     from coordinax.api.representations import (

src/coordinax/representations/_src/__init__.py

@@ -8,3 +8,4 @@
 from .register_cx import *
 from .rep import *
 from .semantics import *
+from .tangent_map import *

src/coordinax/representations/_src/basis_change.py

@@ -3,7 +3,7 @@
 __all__ = ("change_basis",)
 
 from jaxtyping import ArrayLike
-from typing import Any
+from typing import Any, TypeVar
 
 import jax
 import jax.scipy.linalg
@@ -28,6 +28,8 @@
 from coordinax.internal import QuantityMatrix, UnitsMatrix
 from coordinax.internal.custom_types import CDict, OptUSys
 
+T = TypeVar("T", bound=u.Quantity)
+
 _RAD = u.unit("rad")
 
 

src/coordinax/representations/_src/core.py

@@ -330,19 +330,14 @@ def cconvert(
     at: CDict | None = None,
     usys: OptUSys = None,
 ) -> Any:
-    r"""Convert tangent data between basis conventions in the same chart.
-
-    Tangent conversions are basis changes when source and target charts are
-    identical. In this case, `cconvert` redispatches to `change_basis`.
+    r"""Convert tangent data between charts via Jacobian pushforward.
 
     Examples
     --------
-    Convert tangent data between coordinate and physical basis in the same
-    chart:
-
     >>> import jax.numpy as jnp
     >>> import coordinax.charts as cxc
     >>> import coordinax.representations as cxr
+
     >>> v = {"r": jnp.array(5.0), "theta": jnp.array(1.0), "phi": jnp.array(2.0)}
     >>> at = {"r": jnp.array(3.0), "theta": jnp.array(0.5), "phi": jnp.array(0.0)}
     >>> cxr.cconvert(v, cxc.sph3d, cxr.tangent_geom, cxr.coord_disp,
@@ -351,28 +346,15 @@ def cconvert(
      'theta': Array(3., dtype=float64, ...),
      'phi': Array(..., dtype=float64, ...)}
 
-    Tangent conversion across different charts is not implemented by this
-    dispatch:
-
-    >>> cxr.cconvert(v, cxc.sph3d, cxr.tangent_geom, cxr.coord_disp,
-    ...              cxc.cart3d, cxr.tangent_geom, cxr.coord_disp, at=at)
-    Traceback (most recent call last):
-    ...
-    NotImplementedError: Tangent cconvert between different charts is not implemented;
-    use the same chart for basis changes.
+    >>> v = {"x": jnp.array(1.0), "y": jnp.array(0.0)}
+    >>> at = {"x": jnp.array(1.0), "y": jnp.array(0.0)}
+    >>> cxr.cconvert(v, cxc.cart2d, cxr.coord_disp, cxc.polar2d, cxr.coord_disp, at=at)
+    {'r': Array(1., ...), 'theta': Array(0., ...)}
 
     """
-    del from_geom, to_geom  # represented by dispatch signature
-
-    if from_chart != to_chart:
-        msg = (
-            "Tangent cconvert between different charts is not implemented; "
-            "use the same chart for basis changes."
-        )
-        raise NotImplementedError(msg)
-
-    return cxrapi.change_basis(
-        x, from_chart, from_rep.basis, to_rep.basis, at=at, usys=usys
+    del from_geom, to_geom
+    return cxrapi.tangent_map(
+        x, from_chart, from_rep, to_chart, to_rep, at=at, usys=usys
     )