---
id: PLN-0077
ticket: render-frame-packet-boundary
title: Composer Buffer and Game2D Packet
status: done
created: 2026-05-25
ref_decisions: [DEC-0030]
tags: [gfx, renderer, runtime, frame-composer, architecture, ui, pipeline]
---

## Briefing

Source decision: `DEC-0030`.

Convert current `composer.*` frame state into a mutable `ComposerBuffer` that closes into `Game2DFramePacket` at frame boundary.

## Target

Game 2D scene, camera, sprites, HUD, and frame orchestration are represented as domain-owned state until `RenderManager` closes them into a typed packet.

## Scope

- Add `ComposerBuffer`.
- Move scene, camera, sprite, HUD, layer ordering, and Game 2D orchestration state into the buffer.
- Close the buffer into `Game2DFramePacket`.
- Preserve the v1 four-layer scene/sprite composition model and HUD-above-game rule.
- Ensure fade is absent from Game 2D packet state.

## Out of Scope

- Primitive `gfx2d.*` command buffering.
- Shell UI packet work.
- Renderer consumption refactor beyond packet shape.
- New Game 2D features.

## Execution Sequence

1. Locate current `FrameComposer`, composer syscalls, scene/camera/sprite/HUD state, and tests.
2. Introduce `ComposerBuffer` and migrate composer mutation paths into it.
3. Implement closure into `Game2DFramePacket` with immutable packet contents.
4. Preserve integer layer ordering across the four Game 2D layers and HUD final composition.
5. Route frame closure through `RenderManager`.
6. Update tests that currently inspect `FrameComposer` internals.

## Acceptance Criteria

- `composer.*` owns Game 2D high-level frame state.
- Closed `Game2DFramePacket` contains all data required by the Game 2D renderer without live VM or `Gfx` access.
- Scene/sprite layer behavior and HUD ordering are preserved.
- Fade is not represented.

## Tests / Validation

- Add unit tests for buffer mutation and packet closure.
- Add composition ordering tests for scene, sprites, and HUD.
- Run current frame composer/render tests and compare existing expected output.

## Risks

- Existing composer code may mix orchestration with drawing; split state capture from rendering before deeper migration.
- Layer behavior is user-visible and must not drift.

## Affected Artifacts

- Composer/frame-composer modules under `crates/`
- Game 2D tests