Tutorial — Streets to a PCG city

One Streets Network in, a city out: this tutorial wires lots to buildings and road edges to props, and by the end the graph rebuilds all of it whenever you move a street - that is what the clip shows. Work in a staging level or a duplicate of the example map.

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1. Author the street graph

1. Place a Spline Architect Streets Network.
2. Add at least one Street Config with a width and material.
3. Press Edit, choose Draw, and create connected roads that enclose one or more blocks.
4. Press Enter to finish each stroke. In Select mode, straighten intended spans and adjust fillet rings.
5. Set Output Mode = Generate and press Rebuild Network.

Confirm that intersection patches are closed and visible and that block interiors resolve as lots. If not, fix the authored graph before adding PCG.

Choose Default Lot Config → Elevation Mode before building the PCG graph:

• Follow Elevation preserves the road-derived boundary elevations;
• Flatten to Lowest Boundary creates a flat lot at its lowest edge;
• Flatten to Highest Boundary creates a flat lot at its highest edge.

Use Lot Overrides when one block needs a different mode. The visible surface and the boundary read by PCG will stay aligned. Flattening does not create retaining walls or cut the terrain by itself; feed the lots to SA Landscape Patch when the ground should follow them.

2. Add semantic zones

Draw closed Lot Zone actors over parts of the blocks and set names such as Residential, Commercial, and Park. Rebuild the Streets Network after moving a zone. The smallest overlapping zone wins.

3. Read lots into PCG

Create a PCG Graph on a PCG Volume or PCG Component whose bounds include the network. Add SA Get Lots:

• configure Actor Selector to find the intended Streets Network;
• keep Include Empty Lots enabled while debugging so SA_LotEmpty stays visible;
• enable Emit Lot Surfaces only if the graph will sample or spawn on them;
• use Ground Surface spacing/smoothing appropriate to the terrain.

Inspect the Lot Boundaries output. Each closed spline carries SA_LotIndex, SA_LotEmpty, SA_LotArea, SA_LotSeed, SA_LotZone, SA_LotSurfaceMaterial, and SA_SourceActor.

4. Filter and subdivide

Add Filter Data By Attribute after Lot Boundaries and keep SA_LotEmpty == false. Branch by SA_LotZone if different districts need different presets.

Subdivision shapes the city's fabric - block sizes, alleys, parcel rhythm - and both building routes in step 6 sit on top of it. Feed a branch to SA Subdivide Lots. Start with:

• Lot Target Area: 6,000,000 cm²;
• Max Subdivisions: 2;
• Cut Mode: Angled;
• Irregularity: 0.25;
• Gap: a small firebreak/alley width;
• Knockout: 0 while validating;
• Boundary Filter: All;
• Elevation Mode: Follow Elevation;
• fixed Seed.

Follow Elevation preserves each reconstructed child boundary. Lowest/Highest first reconstruct the terrain Z, then flatten every child independently to its own extreme boundary elevation. In all modes, each child's boundary spline, Dynamic Mesh, and Ground Surface agree. Use SA_IsExterior downstream if perimeter and interior lots need different rules.

5. A setback, for the lots you will generate on

Connect Sub-Lot Boundaries to SA Polygon Offset and use a negative Offset for a building setback. A large inset can split or remove a narrow lot, so preview this output before spawning.

Only lots headed for SA Spawn Building need this - the instancing route in the next step lines buildings along road edges and never looks at the sub-lot shape.

6. Fill the lots with buildings

There are two ways to do this, and picking the wrong one is the most common way to make a city that will not run.

	Instance finished buildings	Generate buildings in the graph
What it places	One static mesh per building, from a pool you built earlier	Every wall piece, floor, and roof, generated per footprint
Cost per building	One instance	Dozens of pieces, generated on every regeneration
Use it for	The city - blocks, districts, everything in the background	Hero buildings, the street the player walks down, anything the camera gets close to
Footprint	Whatever the mesh is	Fits the lot exactly, whatever its shape

Build the city out of instanced meshes, and spend SA Spawn Building only where it shows. A district of procedurally generated buildings looks the same from a distance as a district of instanced ones and costs enormously more, in generation time and in draw calls.

Instance finished buildings

Author a handful of buildings first, the normal way: draw them, stack the walls, get them looking right. Then convert each one to a static mesh - one mesh per building, with a proxy LOD if they will be seen from far away. That library is what the city is made of.

To line them along the streets:

SA Get Road Edges → SA Edge Placer → Static Mesh Spawner (By Attribute: SA_Mesh)

Put your building meshes in the Mesh Pool with weights, set Fill = Packed so each one advances by its own true width, and Facing = Away From Edge so they front the street. Packed fitting means a pool of differently sized buildings lines up without gaps or overlaps - the same behavior that packs mixed-size barriers, applied to whole buildings.

Vary it by district: branch on SA_LotZone before the Edge Placer and give each branch its own pool.

Generate buildings in the graph

Where a building needs to fit its lot exactly, or the camera gets close enough that repetition shows, generate it.

Tag your source rows first - Building Presets get values like Residential, Small, or Tower; Wall Presets get layer-precise ones like BaseFloor, Facade, or Trim.

Connect Polygon Offset's Offset Polygons to SA Spawn Building → Footprints, choose Preset Source = Random From DataTable, assign the Building Preset DataTable, and keep Output = Data. For a small residential pool, add Residential and Small to Filter Terms with Term Match Mode = All. Empty Filter Terms include the whole table; Any widens it to rows matching at least one term. Include Row Names is on by default - turn it off when terms should match only authored Preset Tags.

Selection is equal-probability with replacement, and repeats exactly for fixed graph and footprint seeds. Only Building Preset tags classify a Building row; tags on the Wall Presets it references do not widen the pool.

Then:

• Generated → stock Static Mesh Spawner, with Mesh Selector Type = By Attribute and Attribute Name = SA_Mesh.
• Dynamic Meshes → SA Spawn Dynamic Mesh for floors, roofs, and other non-static pieces.
• To reach individual pieces - every BaseFloor, say - use Array Contains on SA_PresetTags, then Attribute Filter.
• To keep a whole generated building tagged Small, use Filter Data By Tag.

Branch on SA_LotZone and give each district its own query on its own SA Spawn Building node.

7. Place road-edge props

Add SA Get Road Edges for the same network and connect Road Edges to SA Edge Placer.

1. Add meshes or actor classes to Mesh Pool; set exactly one asset kind per entry.
2. Choose Left, Right, or Center and a Facing mode.
3. Use Packed for bounds-fitted curbs/barriers, Fixed Spacing for lamps, or Random Spacing for loose detail.
4. Add SA Prune Footprints when rotated props or terrain slopes make stock 3D Self Pruning unsuitable.
5. Send mesh picks to a Static Mesh Spawner by SA_Mesh; actor-class picks to Spawn Actor by SA_Actor.

8. Validate ownership and determinism

• Regenerate the graph twice and confirm fixed seeds produce identical lots and picks.
• Confirm the same footprints choose the same random Building rows after a second regeneration.
• Move one street node, rebuild Streets, then regenerate PCG; confirm lots and downstream output update.
• Move a Lot Zone, rebuild, regenerate, and confirm the SA_LotZone branch changes.
• Delete/regenerate the PCG component and confirm it cleans its own output.
• Keep SA Spawn output in Data mode unless live managed actors are a deliberate requirement.

Continue with recipes, node reference, and metadata.