Add ROADMAP.md with revised Phase 1 architecture

ROADMAP.md

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+# rtxpy Roadmap
+
+Evolution path from GPU ray-tracing geospatial engine to a real-time digital twin platform.
+
+Tracked in [#57](https://github.com/makepath/rtxpy/issues/57).
+
+---
+
+## Current State
+
+rtxpy is a geospatial visualization engine built on NVIDIA OptiX with:
+- Hardware-accelerated ray tracing (triangles, B-spline curves, heightfields) with OptiX 9.1 cluster acceleration
+- Interactive GLFW + Jupyter viewers with 30+ keyboard controls
+- Multi-GAS scene architecture with per-geometry transforms and visibility
+- GPU-accelerated GIS analysis (viewshed, hillshade, slope, aspect)
+- Data integration: Overture Maps, OSM, GTFS, NASA FIRMS, NOAA wind, DEMs (Copernicus, USGS 3DEP)
+- Keyframe animation, flyover tours, GIF/MP4 export
+
+---
+
+## Phase 1 — Architecture
+
+Decompose the monolithic `InteractiveViewer` (6400+ lines, ~80 instance variables) into composed, testable subsystems. Approach: plain Python composition — no ECS, no scene graph trees, no event buses. Complexity is added only where profiling justifies it.
+
+### Viewer Decomposition via Composition ([#61](https://github.com/makepath/rtxpy/issues/61))
+
+Extract subsystem classes from the monolithic `InteractiveViewer`:
+
+- **`CameraState`** — position, yaw, pitch, fov, speeds, `apply_input()`, `view_matrix()`
+- **`TerrainState`** — raster data, mesh cache, resolution management, basemap cycling
+- **`WindState`** — particle arrays, GPU buffers, `step()` simulation
+- **`OverlayManager`** — ordered overlay layers, compositing, alpha control
+- **`ObserverManager`** — observer slots, drone modes, viewshed state
+- **`GeometryLayerManager`** — geometry grouping, visibility cycling, chunk loading (see #62)
+- **`RenderSettings`** — shadows, AO, DOF, denoise, colormap, VE, sun position
+- **`InputState`** — held keys, mouse state, modifiers
+- **`HUDState`** — help overlay, minimap, FPS counter, title text
+
+`InteractiveViewer` holds these as composed objects. `_tick()` delegates to subsystem methods. Selective `@njit` acceleration applied only to proven CPU hotspots (wind particle updates, bilinear Z-sampling).
+
+### Geometry Layer Manager ([#62](https://github.com/makepath/rtxpy/issues/62))
+
+Replace scattered geometry management variables with a `GeometryLayerManager`:
+
+- Flat layer grouping with ordered visibility cycling — matches OptiX's flat IAS model
+- `GeometryLayer` groups related GAS entries with shared visibility toggle
+- Chunk managers owned by their respective layers
+- Generic `cycle_index()` helper replaces 4 duplicated cycling implementations
+- No scene graph tree — OptiX IAS is inherently flat, hardware BVH already handles frustum culling
+
+### Declarative Key-Binding Map ([#63](https://github.com/makepath/rtxpy/issues/63))
+
+Replace the 280-line if/elif chain in `_handle_key_press()`:
+
+- Dict-based `(key, shift) → action` dispatch table
+- Direct method calls to subsystem objects — no pub/sub event bus
+- REPL command queue stays as-is (already correct)
+- Help text auto-generation from binding table (stretch goal)
+
+### Main Loop Cleanup ([#64](https://github.com/makepath/rtxpy/issues/64))
+
+Fix phase ordering and separate concerns in the main loop:
+
+- Move `glfw.poll_events()` before `_tick()` — fixes one-frame input lag
+- Split `_tick()` into clear input / simulation / render sections
+- Extract `_drain_command_queue()` and `_present_if_dirty()` methods
+- Separate render (ray trace + post-process) from present (readback + composite) in `_update_frame()`
+- No fixed-timestep accumulator (no physics), no Platform ABC (two backends don't justify it)
+
+---
+
+## Phase 2 — Rendering
+
+Modern material and effects pipeline.
+
+### PBR Materials
+- Metallic-roughness workflow (glTF PBR model)
+- Per-geometry material assignment
+- Material library: concrete, glass, vegetation, water, asphalt, terrain variants
+
+### Texture & UV Support
+- GLB/glTF texture loading (baseColor, normal, metallic-roughness)
+- UV coordinates via barycentric interpolation in closest-hit
+- Terrain texture splatting, satellite imagery draping
+
+### Render Graph
+- Configurable DAG of render passes (GBuffer → Shadow → AO → GI → Denoise → Tonemap → Composite)
+- Easy to add/remove passes per capability
+
+### Level of Detail
+- Distance-based terrain LOD (subsample far tiles)
+- Instanced geometry LOD, billboard imposters
+- Hybrid: cluster GAS close, simplified GAS far
+
+### Particle Systems
+- GPU particle simulation (CUDA compute kernels)
+- Emitters: point, line, area, volume
+- Applications: smoke/fire, rain/snow, traffic flow, pollution dispersion
+
+---
+
+## Phase 3 — Simulation
+
+Time-evolving behavior.
+
+### Physics
+- Rigid body dynamics, fluid simulation on terrain
+- Structural load visualization
+- Integration: cuPhysics or bullet3 via CUDA interop
+- Fixed-timestep accumulator pattern added here when physics demands it
+
+### Animation System
+- Property animation: any component value over time
+- Skeletal animation (glTF), procedural animation (flag waving, tree sway)
+- Timeline editor
+
+### AI & Pathfinding
+- Navigation mesh generation from terrain + buildings
+- A*/Dijkstra pathfinding, agent simulation (pedestrians, vehicles, drones)
+- Crowd simulation, GTFS integration for transit agents
+
+### Environmental Simulation
+- Solar exposure accumulation, shadow analysis (hours of sun per location)
+- Noise propagation via ray tracing
+- Line-of-sight network analysis, flood inundation modeling
+
+---
+
+## Phase 4 — Platform
+
+Accessibility beyond a single Python process.
+
+### Web Viewer
+- Server-side rendering + frame streaming (WebRTC / WebSocket + H.264)
+- Progressive resolution, thin browser client
+- Long-term: WebGPU client with scene serialization
+
+### Multi-User
+- Shared scene state, per-user camera/selection
+- Collaborative annotation, role-based access
+
+### API Layer
+- REST/GraphQL for scene management
+- Python client library (same interface as local accessor)
+- Webhook/event streams, batch rendering API
+
+### IoT & Sensor Integration
+- MQTT/AMQP subscriber → entity binding → visual update
+- Time-series storage and playback, alert rules
+
+---
+
+## Phase 5 — Digital Twin
+
+Domain-specific capabilities.
+
+### BIM & CityGML
+- IFC, CityGML LOD1–3, 3D Tiles / Cesium ion import
+- Indoor/outdoor seamless navigation
+
+### Temporal Dimension
+- Time slider with historical scene states
+- Version-controlled snapshots, change detection
+
+### What-If Scenarios
+- Fork scenes, modify parameters, compare outcomes
+- Side-by-side comparison views, scenario libraries
+
+### Domain Verticals
+- **Urban Planning**: zoning, density, transit accessibility
+- **Disaster Response**: flood modeling, fire spread, evacuation routing
+- **Telecommunications**: RF propagation via ray tracing, tower placement
+- **Renewable Energy**: solar panel placement, wind farm siting
+- **Environmental Monitoring**: air quality, noise mapping, vegetation health
+- **Infrastructure Management**: utility networks, maintenance scheduling
+
+---
+
+## Key Advantages
+
+- **Ray tracing as rendering primitive** — viewshed, shadows, AO, GI all share the OptiX pipeline; new analyses = new launch kernel
+- **GIS-native coordinates** — real DEMs with real CRS metadata, GeoJSON/Overture/OSM data flows directly in
+- **xarray accessor** — `dem.rtx.explore()` zero-boilerplate interface
+- **GPU-native data path** — xarray → CuPy → OptiX device buffers, no CPU round-trips
+- **Real-time data patterns** — GTFS-RT, FIRMS, NOAA wind already demonstrate fetch → transform → place → update
+- **Zarr persistence** — spatially chunked mesh store, foundation for LOD streaming and temporal versioning