# Spatial Contraints

It is common to ask for the spatial relationship or extrinsics between two components. A Plex incorporates this information in the form of spatial constraints. A spatial constraint can be broken down as:

Field
Type
Description
Extrinsics Extrinsics The extrinsics describing the "To" from "From" transformation.
Covariance ExtrinsicCovariance The covariance of the extrinsics described by this constraint.
From Uuid The UUID of the component that describes the "From" or base coordinate frame.
To Uuid The UUID of the component that describes the "To" coordinate frame, which we are transforming into. This can be considered the "origin" of the extrinsics matrix

## Extrinsics, "From", and "To"

tl;dr: the reference frame for the extrinsics transform is in the coordinate frame of the "to" component. It would move points from the coordinate frame of the "from" component.

Our extrinsics type is essentially a way to describe how to transform points in one coordinate system into another. Anyone who has ever worked with transforms has experienced confusion in convention. In order to cut through the ambiguity of extrinsics, every spatial constraint has a from and to field. Let's dive into how this works.

We can think of an extrinsics transform between components $$A$$ and $$B$$ using the following notation:

$\Gamma_{B}^{A} :: \Gamma_{from\ B}^{to\ A}$

If we wanted to move a point $$p$$ from the frame of reference of component $$B$$ to that of $$A$$, we would use the following math:

$\vec{p}_{W}^{\ A} = \Gamma_{B}^{A} \cdot \vec{p}_{W}^{\ B}$

...also read as "$$p_A$$ equals $$p_B$$ to $$A$$ from $$B$$".

Thus, when constructing a spatial constraint, the reference frame for the extrinsics transform is in the coordinate frame of component $$A$$, and would move points from the coordinate frame of component $$B$$. In code, this would look like:

let to: component_A_uuid
let from: component_B_uuid


Both from and to must reference components within the Plex.

## Spatial Covariance

Spatial covariance is generally presented as a 6×6 matrix relating the variance-covariance of an se3 lie group:

$\begin{bmatrix} v_1 & v_2 & v_3 & \omega_1 & \omega_2 &\omega_3 \end{bmatrix}$

When traversing for spatial constraints within the Plex, the constraint returned will always contain the extrinsic with the minimum overall covariance. This ensures that users will always get the extrinsic that has the smallest covariance (thus, the highest confidence / precision), even if multiple spatial constraints exist between any two components.