# `Labs.mirror`

> Contract 2.0.0 · package sha256 `a857eca95b35921109faf997094bb4192f12527d9d0fcd4be7e97f06425bad6e` · generated from `configurator-runtime.d.ts` by `services/onsite-scripter/scripts/build-contract-docs.mjs`; edit the source, not this file.

Method of [`Labs`](Labs.md) · labs · since 2.0.0 · called as `session.labs.mirror`

```ts
mirror(axis: 'x' | null, options: C & SignalOption & {
  owner: Collection;
}): Promise<Report>;
```

A geometry mirror of models and collections about the world plane x = 0. Lights that are
collection items mirror with their collection; the stage's own rig, the ground and the
environment don't. Reads return mirrored world coordinates. null removes the mirror. Look-only;
owned by `owner`. Writes stay in placement space, before the mirror: the scene model's
transforms, pose rules on a model root ([`node(id)`](Writer.node.md)) or a collection root, and an item's `at`
(local to its `parent`), which the runtime then mirrors. So un-mirror a position you read
(`bounds()`, a world `pose`, a connector, a pick) before you write it: negate x, swap a box's
`min.x` and `max.x` as you negate them, and reflect a world pose's matrix before copying it
back; then take a local `at` through the inverse of its parent's pose. The camera is not
mirrored: [`camera.project`](Camera.project.md) takes a read's mirrored coordinates as they are, as do an overlay
anchor's world `at` (no `on`), [`Frame.camera`](Frame.camera.md) and every `camera.*` input. [`Frame.pose`](Frame.pose.md) and a
drag's poses ([`DragMove.pose`](DragMove.pose.md), what `constrain` returns, the 'drag:*' events' `pose`) are in
placement space, as the writes above, so a pose computed from [`DragMove.ray`](DragMove.ray.md) or a read is
un-mirrored before `constrain` returns it.
Faces keep their sides: front faces stay front (the runtime corrects the winding), so no
material's `side` changes with the mirror.

## Options

Through [`Session`](Session.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `actor` | `Actor` | the `expectGroup`'s actor when one is given, else 'app' | Who made the write. |
| `dryRun` | `boolean` | false | Runs and reports, then rolls back by design ('dry-run'). |
| `expectGroup` | `HistoryGroup` | undefined | A guard, checked at the call: this write belongs to the undo group you are running. |
| `expectRevision` | `number` | undefined (no check) | If-match on [`session.revision`](Session.revision.md). |
| `origin` | `string` | null | Your tag, echoed unchanged in [`Report.origin`](ReportFields.origin.md) and `cause.origin`. |
| `owner` | `Collection` | — |  |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |
| `supersede` | `boolean` | false (the write is queued behind the transaction and applies after it closes) | While a transaction is open: roll it back (it resolves 'superseded'); the writes queued behind it apply in call order, then this one, which supersedes an… |
| `undo` | `'rebase' \| (string & {})` | a recorded-state change gets one step labelled 'Change'; a look-only write gets none; the session's first model (cause 'load') gets none, whatever the label: it is the floor of history | An undo label makes this write one undo step. |

Through [`Tx`](Tx.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `owner` | `Collection` | — |  |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |

Refused here (typed `never`, so passing one fails to compile): `actor`, `checkpoint`, `dryRun`, `expectGroup`, `expectRevision`, `origin`, `supersede`, `undo`.

## Abort (`signal`)

This call is a write (`SignalOption.signal`, [the kinds of call](../read-this-first.md#conventions)).

> A write: before the commit point it rolls back and rejects AbortError; after it, no effect, except that a running camera move stops where the last frame drew it (`CameraMoveOptions`).
>
> Through a `tx` the commit point is the transaction's: aborting a write still loading rejects AbortError and stages nothing (catch it: unobserved, it rolls the transaction back, spec section 6.1, O6); aborting one already staged unstages it, its paths keeping the latest other write to them as after a failed savepoint (O7), and its Promise stays resolved 'staged'.

## Example

Imports: values (`has`, `validate`, the `Holo*Error` classes, the pure helpers) from `@holostep-configurators/runtime-client`, types from `@holostep-configurators/runtime-client/contract`, `/math` and `/testing` names from those entries; `boot` from the runtime module the root's `loadRuntimeAtFloor` resolves to. A type your page defines is your app's: declare it there, never import it from the package.

Compiled under the contract's strict settings (`examples/declaration/20-Labs.mirror.ts`):

```ts
import type { Collection, Session } from '@holostep-configurators/runtime-client/contract';
declare const session: Session;
declare const signal: AbortSignal;
declare const dresserId: string;
declare const room: Collection;
await session.labs.mirror('x', { owner: room });
// the dresser's bounds read mirrored; the wall's `at` is written before the mirror
const box = session.find({ node: dresserId, kind: 'mesh' }).bounds();
const x = box ? -box.center.x : 0;
await room.set({ items: [{ key: 'wall', kind: 'group', at: { position: { x, y: 0, z: 0 } } }] });
```

## Generic-API test (spec 3.1)

| Rule | Result |
| --- | --- |
| 1. Neutral name | pass: `mirror` names no product |
| 2. A precedent | pass: [glTF 2.0 transformations (a negative determinant reverses the winding)](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#transformations) (row `E3`) |
| 3. Two apps, or labs | labs: it graduates when a second app uses it; apps today: app-0c773a4ee3, app-b362a81c85, app-c1384707fc |
| 4. The cheaper fix first | decided per need in Appendix B: rows `E3`, `K31` at tier labs (a member only where no rule, data, asset or app code serves the need) |