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Projection Layout Strategies Specification

Purpose

This specification defines layout as a distinct concern of projection products. VGM describes visual graph data. The projection pipeline creates products. Layout chooses coordinates, dimensions, spacing, routing, and text measurement for products that need visual geometry.

Scope

This document covers:

layout strategy boundaries;
meet-in-the-middle layout;
text metrology;
deterministic and external layout engines;
product-specific layout constraints.

Non-Goals

This document does not define semantic graph construction; see LogosSvoSemanticsSpec.md.
This document does not define VGM schema fields; see ProjectionVgmSpecification.md.
This document does not define BPMN notation behavior; see ProjectionBpmn.md.

Terminology

Term	Meaning
Layout strategy	Algorithm or engine that assigns positions, sizes, and routes.
Text metrology	Measurement of rendered text dimensions before final geometry.
Meet-in-the-middle layout	Separation between semantic model, measured text boxes, and final renderer coordinates.
Visual product	Pipeline product that requires stable geometry, such as SVG, BPMN DI, Mermaid, D2, or UI canvas.

Model or Contract

Layout consumes target-aware projection data and produces geometry:

render plan
  -> text measurement
  -> node sizing
  -> node placement
  -> edge routing
  -> target geometry

The layout stage may use:

built-in deterministic algorithms;
Graphviz;
ELK or other graph layout engines;
target-native layout where the target owns layout;
manual coordinates supplied by a .view profile.

Rules

Layout must not infer domain semantics.
Text measurement must happen before final node sizing when labels affect geometry.
The same semantic graph may have several valid layouts.
Layout diagnostics must distinguish impossible geometry from unsupported renderer capability.
Product-specific layout constraints belong near the target specification.

Examples

Meet-in-the-Middle Layout

semantic graph
  -> render plan with labels
  -> measured text boxes
  -> coordinate solver
  -> target renderer

This keeps the semantic model independent from concrete SVG, BPMN DI, or canvas coordinates while still allowing accurate text-aware layout.

BPMN Layout

BPMN layout must respect pool/lane containment, sequence-flow legality, message flow across pool boundaries, and data object association placement. These rules are target constraints layered on top of general layout strategy.

Validation

Layout validation should verify:

no node has missing dimensions;
label text fits within the computed node box;
edge routing does not require impossible target constructs;
target-specific containment rules are respected;
fallback layout produces deterministic output when an external engine is unavailable.

Compatibility

This document replaces the earlier "meet-in-the-middle" document as the home for layout strategy. Meet-in-the-middle remains one strategy, not the name of the whole layout concern.

Open Questions

Which visual products require SOTER-owned deterministic layout?
Which products should delegate layout to native target tooling?