Optimising

A generated city is already cheap by construction — one mesh and one material per building, sharing four materials across the whole city. This page is about the further levers, when each one is worth pulling, and which ones are traps.

What is already free

Before you optimise anything, know what you are starting from:

• One draw call per building before any batching. Because every building shares one of four materials, Unity's static batching and the SRP Batcher then collapse them much further.
• No runtime cost from the tool. Generation happens in the Editor. Your shipped city is ordinary meshes and prefabs.
• No lights for night windows. The night glow is material emission, not light sources.
• No lightmap unwrap by default, which is the single most expensive part of generating a building.

That means the honest answer for most projects is: generate at Standard detail, turn on LODs, and stop. The rest of this page is for when you need more.

LODs

Generate LODs (in Generation Settings ▸ Geometry, off by default) builds simplified meshes alongside each building and wires them into a LODGroup.

LOD1 drops the recessed window relief (facades go flush), the roof clutter, and the House eave detail and chimney. Rooftop props are kept, so nothing visibly pops at the swap — a disappearing antenna is far more noticeable than flattened window reveals.

The transition points differ by detail tier:

For Standard buildings, LOD0 is held all the way down to 10% of screen height on purpose. An earlier build swapped at 60% and the flush LOD1 was visibly popping in while the building still filled most of the screen. Culling at 1% keeps a 30 m tower visible to roughly 2.6 km, so distant skylines never blink out in a driving game.

Pair LODs with Detailed. A Detailed building carries three to fourteen times the vertices of a Standard one (see what Detailed costs). Without LODs it carries all of them at every distance. With them, the cost is confined to buildings that are actually close.

Notes:

• Works for both prefab output (…_LOD1 / …_LOD2 mesh assets, GUID-stable) and scene-only output.
• Preview In Scene stays full detail — it is an audition view.
• Regenerate All preserves each prefab's LOD-ness as authored, whatever the toggle currently says.
• Static batching bakes both LOD meshes into its combined buffers, so distant draw cost falls at a small memory cost.
• With Bake Lightmap UVs on, every LOD gets its own UV set and contributes GI. With it off, the LOD children keep the cheaper UV-less layout.

Choosing a detail tier

Detail is a per-district field, so mixing tiers across one city is normal — run Detailed only in the block the player actually walks through.

Fewer unique meshes

Two settings covered in Creating Many Buildings matter as much for runtime as for disk:

• Reuse Existing Assets (on by default) skips rebuilding assets that already match.
• Mesh Variety caps how many distinct designs a fill may use per archetype. Buildings sharing a footprint then share a mesh, which means fewer assets, faster fills, and identical scene-only meshes that static-batch together.

For background filler, a pool of 8–20 per archetype is usually indistinguishable from unlimited once palettes and row facing are mixed in.

Optimize City (combining)

Optimize City (Combine Meshes) — row 4 of the checklist on 4 Ship ▸ Finalize — merges every generated building into a handful of district-scale meshes, bucketed by shared material and split below 65,000 vertices per chunk.

Be honest about what this buys you. It collapses GameObject and renderer counts — the per-object CPU overhead that hurts mobile most. Draw calls were already being handled by static batching and the SRP Batcher, so this is a finalise step, not a frame-rate multiplier. On a demo-scale city it typically takes renderer counts from four figures to double digits.

What you need to know before running it:

• It is fully reversible. Source buildings are disabled, never destroyed. De-Combine City restores them exactly, and the button only appears while a combined city exists.
• Run it last. While a city is combined, the placement guard cannot see the disabled buildings, so anything you generate afterwards will happily overlap them. Generate everything, then combine.
• Foundation skirts ride along, and LOD chains contribute their LOD0.
• Baked lightmap textures do not carry over. GI-enabled chunks receive a fresh unwrap over the final combined geometry and are ready for a new bake; you must re-bake lighting afterwards. Sources with GI off stay in separate, unwrapped-free chunks.
• Combined renderers use Simple reflection-probe blending, since one probe now covers a district-sized mesh.

Download size

Four import-and-geometry decisions keep the shipped size down. They are automatic, but two of them only apply to newly generated assets, so a library made with an older version needs a Regenerate All to benefit:

One important exception. A mesh that carries lightmap UVs is never packed. Unity's lightmapper rejects any mesh outside the standard float layout and silently drops it from the bake. So if you bake GI on your buildings, those meshes keep the wider stride — correct lighting beats a smaller download. Meshes without lightmap UVs (the default) get the full saving.

A mobile checklist

1. Detail = Simple or Standard. Never Detailed for filler.
2. Generate LODs on.
3. Mesh Variety set to something finite — 8 to 16 per archetype.
4. Fake Interiors off (the default). It is a per-fragment cost across every facade.
5. Lit Signage is free (it rides the existing atlas), so leave it on if you like the look.
6. Street furniture in combined mode (the default), not separate props.
7. Regenerate All once, so the packed vertex format and 16-bit indices apply.
8. Optimize City as the final step, after everything is placed.
9. Light probes on a modest budget — 4096 is usually plenty; use Near roads + buildings coverage rather than raising the budget.

Common questions

Combining did not improve my frame rate. It reduces renderer and GameObject overhead, not draw calls — those were already batched. If you are GPU-bound on overdraw or shader cost, look at Fake Interiors and your detail tier instead.

Should I combine before or after baking lighting? Combine first, then bake. Combining discards existing lightmaps and re-unwraps the combined geometry, so baking beforehand wastes the bake.

My meshes are still the old size after updating. Run Regenerate All — the size wins for indices and vertex format apply when a mesh is written, and existing assets keep their old format until they are rebuilt.

Can I use LODs and combining together? Yes. Combining takes each building's LOD0. You lose the LOD chain on combined output, which is the trade — combine district-scale background, keep LODs on anything the camera approaches.

Where to go next

• Finishing and Shipping — the checklist that puts these steps in the right order.
• Customising Buildings — the detail tiers and what each costs.
• Materials and Lighting — baked lighting and probes.