Status: Active follow-on plan Scope: Extracts the projection-mode lane from 2026-02-24_physics_engine_extensibility_plan.md into a narrower execution plan for TwoD, TwoPointFive, and Isometric, without assuming full reorientable 3D. Related:

• 2026-02-24_physics_engine_extensibility_plan.md
• 2026-04-03_layout_backend_state_ownership_plan.md
• 2026-04-03_wasm_layout_runtime_plan.md
• 2026-04-03_layout_transition_and_history_plan.md
• 2026-04-10_vello_scene_canvas_rapier_scene_mode_architecture_plan.md
• view_dimension_spec.md
• 2026-02-27_viewdimension_acceptance_contract.md
• 2026-04-02_scene_mode_ux_plan.md
• ../aspect_render/render_backend_contract_spec.md

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The canonical ViewDimension contract currently covers TwoD and generalized ThreeD, but the most achievable next lane is narrower:

• preserve 2D layout truth
• derive ephemeral z placement from ZSource
• add projection/render behavior for TwoPointFive and Isometric
• keep full free-camera Standard 3D as a later, separate problem

This lane depends on the shared persisted carrier described in 2026-04-03_layout_backend_state_ownership_plan.md. It does not depend on the runtime-loaded guest layout work in 2026-04-03_wasm_layout_runtime_plan.md, though projection modes should be able to consume either built-in or guest-computed 2D layout truth later.

Architecture alignment (2026-04-10):

• the multi-layer scene/render substrate is now defined in 2026-04-10_vello_scene_canvas_rapier_scene_mode_architecture_plan.md,
• TwoPointFive and Isometric remain projection modes over canonical 2D layout truth,
• those projection modes now target the shared projected-scene + Vello world-render path,
• Standard remains architecture-only and is not part of this milestone.

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• a hard requirement to move to wgpu before TwoPointFive or Isometric can exist
• turning projection modes into a new source of graph truth
• using this plan to solve full free-camera 3D or arbitrary scene navigation
• making runtime-loaded WASM layouts a prerequisite for projection work

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TwoPointFive and Isometric should behave as projection modes over 2D layout truth, not as a different source of graph truth.

1. Define TwoPointFive and Isometric as modes that preserve (x, y) layout state.
2. Keep z derivation ephemeral and recomputable from ZSource plus node metadata.
3. Preserve the per-GraphViewId ownership model from view_dimension_spec.md.
4. Retain deterministic degradation to TwoD if the projection path is unavailable.
5. Route mode-switch animation/history through 2026-04-03_layout_transition_and_history_plan.md rather than inventing projection-specific one-offs.

• Unsupported projection path degrades to TwoD with diagnostics and no cross-pane impact.
• Switching projection modes preserves the underlying 2D layout truth.
• Projection-mode selection/restoration survives snapshot roundtrip through the shared carrier.

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TwoPointFive is the lowest-risk projection mode because it keeps a fixed camera and a mostly 2D interaction model while adding depth cues.

1. Implement a fixed-camera projection pass for node/edge rendering through the shared projected-scene + Vello world-render path.
2. Keep pan/zoom ownership aligned with existing 2D camera behavior.
3. Ensure selection, hover, and hit testing remain continuous under projection.
4. Expose diagnostic evidence for mode enter, mode exit, and degraded fallback.

• TwoPointFive preserves selection and hover continuity.
• Pan/zoom behavior remains aligned with 2D expectations.
• Entering and leaving TwoPointFive emits diagnosable mode-resolution evidence.

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Isometric is more semantically ambitious than TwoPointFive but still bounded because layer placement can remain a deterministic projection of 2D positions plus quantized depth.

1. Define layer quantization rules for ZSource inputs such as BFS depth, UDC level, or recency.
2. Render the same underlying graph with a fixed isometric projection on the shared projected-scene + Vello world-render path.
3. Preserve label, hover, and selection semantics across layer separation.
4. Keep the mode within the current graph-view interaction contract rather than requiring a separate 3D navigation model.

• Layer assignment is deterministic for the same graph state and ZSource.
• Isometric mode preserves active selection and focus-routing behavior.
• Returning to TwoD discards derived z without disturbing (x, y) positions.

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This plan should not quietly grow into a full 3D renderer effort.

1. Define the explicit boundary between projected 2.5D / isometric rendering and full 3D scene navigation.
2. Keep all mode transitions lossless with respect to underlying 2D layout truth.
3. Require a separate authority before adding orbit, tilt, true 3D camera persistence, or a non-projected Standard renderer path.

• Projection modes do not require full 3D navigation semantics.
• Degradation to TwoD is explicit and diagnosable.
• Full 3D work can remain deferred without blocking TwoPointFive and Isometric.

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This plan is complete when Graphshell can render TwoPointFive and Isometric as stable, per-view projection modes over the existing 2D layout truth, with deterministic degradation to TwoD, targeting the shared projected-scene + Vello world-render path, and without pretending that full reorientable 3D is already solved.