# Design

## Architecture

`fountain.py` will join the existing bpy-layer feature modules under
`blender/osmassets/`. It may import `bpy`, `math`, and the collection helper
needed to preserve current object linking behavior.

The module will expose one function:

```python
def assemble(name, x, y, collection, materials):
    ...
```

The signature mirrors the existing inline `add_fountain(name, x, y, collection,
materials)` call site to keep the migration mechanical and parity-friendly.

## Boundaries

- `generate_scene.py` keeps OSM feature filtering, projection, material creation,
  and count ownership.
- `fountain.py` owns only Blender object creation for one fountain at already
  projected `(x, y)` coordinates.
- No pure-Python module may import `bpy`; this module is explicitly in the
  bpy layer, like `water.py`, `grass.py`, `scrub.py`, and `tree.py`.

## Data Flow

1. `generate_scene.py` creates `fountain_mats` from `catalog.MATERIALS`.
2. `generate_scene.py` finds point features where `tags.amenity == "fountain"`.
3. `Projector.xy()` converts the OSM coordinate to local meters.
4. `fountain.assemble("Fountain_" + id, fx, fy, props_c, fountain_mats)` creates
   the existing basin, water disk, pedestal, crown, and droplets.
5. `generate_scene.py` increments `counts["fountain_count"]`.

## Compatibility

The refactor must preserve all generated object names, primitive dimensions,
materials, smoothing flags, and custom properties. Cesium export reads the
resulting `.blend`; it should see an equivalent scene graph.

## Trade-Offs

A full feature registry is intentionally deferred. Starting with a one-module
extraction keeps the diff small and lets parity isolate any regression to the
fountain move before higher-risk building or road work.

## Rollback

Rollback is straightforward: remove `fountain.py`, restore the inline
`add_fountain()` function, remove the module import, and restore the original
call site.