Modular kits

A modular kit is a set of environment pieces built to a shared grid so they snap together in many combinations: walls, floors, corners, doorways, stairs, pillars and the small "glue" pieces that join them. The Level Design Book calls it "a 3D tileset of modules, meshes designed to snap together", borrowed from "the real-life building practice of prefabricated modular construction" (The Level Design Book: Modular kit design).

Kits are how a small art team supports a large world. Build a few dozen good pieces once, and level designers can assemble hundreds of rooms from them.

#Why studios use kits

The clearest numbers come from Bethesda. In their GDC 2013 talk, Joel Burgess and Nate Purkeypile described how two full-time kit artists supported eight level designers on The Elder Scrolls V: Skyrim. Together they produced seven kits, used to build "well over 400 cells" of dungeon content in about two and a half years (Game Developer: Skyrim's Modular Approach to Level Design). Skyrim's Cave kit alone was used over 200 times (Game Developer: Skyrim's Modular Approach to Level Design).

Burgess lists the benefits (Game Developer: Skyrim's Modular Approach to Level Design):

• Scope. Reuse makes a huge world possible.
• Ratio. A few artists can supply many designers.
• Iteration. Designers work in the editor with final art, not grey boxes waiting for an art pass.
• Global fixes. Improve a kit piece and every instance in the game updates.

Bethesda's GDC 2016 follow-up on Fallout 4 described the same approach as one that "empowers a relatively small content team to make an enormous world" (Game Developer: Come to GDC and learn how Bethesda built the…).

The Level Design Book adds that kits work best for "buildings and structures that make regular use of repetition along a grid" (The Level Design Book: Environment Art). Caves, cliffs and ruins are possible, but need more tricks (see below).

#Anatomy of a kit

A kit is a planned system of pieces rather than a loose collection of meshes. Burgess's smallest example is a Fallout 3 pipe kit of "four simple pieces of art" that can build endless pipe runs (Game Developer: Skyrim's Modular Approach to Level Design).

#Footprint and tiling

The footprint is the full bounds of a piece, not just its walkable floor; walls and alcoves sit inside it (Game Developer: Skyrim's Modular Approach to Level Design). All footprints should be multiples of each other, or pieces drift apart (see grids and metrics).

Decide which axes a kit tiles on before you model anything. Burgess advises against kits that tile on every axis: hallways usually tile on one axis and rooms on two (Game Developer: Skyrim's Modular Approach to Level Design). In a later interview he notes that "Equilateral kits are very versatile but will tend to look the most repetitive" (80 Level: Building Huge Open Worlds: Modularity, Kits &…).

#Pivots, snaps and doorways

• Pivots go on a grid corner or edge, the same on every piece. Burgess lists "pivot point location, base footprint size, and transition metrics" among the decisions that are very costly to change late (80 Level: Building Huge Open Worlds: Modularity, Kits &…). See pivot point.
• Snap size. Skyrim's designers usually snapped to half the footprint size; larger snaps make misalignment obvious (Game Developer: Skyrim's Modular Approach to Level Design).
• One doorway. Use "a standard doorway system" so different kits connect (80 Level: Building Huge Open Worlds: Modularity, Kits &…). Burgess recommends one uniform door frame dimension across all kits (Game Developer: Skyrim's Modular Approach to Level Design).
• Floor thickness. Decide it up front to avoid co-planar faces and Z-fighting (Game Developer: Skyrim's Modular Approach to Level Design).

#Piece types

A typical architectural kit contains:

Burgess also describes sub-kits: the Skyrim Cave kit had seven, and its small-hallway sub-kit alone had about 50 pieces, while the Ratway's hall sub-kit had seven (Game Developer: Skyrim's Modular Approach to Level Design).

Kit-piece sheets from shipped games show this anatomy directly. Adrien Thierry's post on Dishonored's Golden Cat bath house lays out "All exterior modules" next to the one "Texture sheet" that covers them (ArtStation: The Golden Cat); Paweł Kostecki's tomb kit for Indiana Jones and the Great Circle, "used to assemble all of the tombs" on the Gizeh map, is shown piece by piece with a matching wireframe (ArtStation: Indiana Jones and the Great Circle - Tomb Kit). More in game art breakdowns: a curated library.

To see a kit built in practice, Class Creatives' modular guide has Jon Arellano of Santa Monica Studio work through the Maya side and Jacob Norris, formerly of Naughty Dog, the Unreal Engine side (YouTube: Ultimate Guide to Modular Assets | Sony Santa…). EMC3D's look at Hogwarts Legacy picks apart a modular stone fence and modular structures from the shipped game (YouTube: Why Does Hogwarts Legacy Environment Art Look…).

The artists' own posts show what such kits look like when shipped:

• A shared kit across studios. Arellano's God of War Ragnarök kit-assets breakdown for Asgard notes that many props were kitbashed, and that many pieces were concepted and built by artists across several studios (ArtStation: God Of War Ragnarok - Kit Assets).
• Instances inside buildings. Edgar A. Martinez's wireframes of Bill's Town in The Last of Us Part I come with the note that "All building share many instances" (ArtStation: "The Last of Us: Part I"…): each building is itself assembled from repeated parts.
• One team's kit, another team's level. On Hogwarts Legacy, Molly Warner of Certain Affinity built an open-world castle ruin by kitbashing castle pieces, crediting the Avalanche environment art team for the assets (ArtStation: Hogwarts Legacy - Castle Ruins).

All three are annotated, with images, in game art breakdowns: a curated library.

#Testing a kit

The Level Design Book warns: "Your first kit will probably be very hard to use" (The Level Design Book: Modular kit design). Its advice is to stress-test the kit, and "do NOT just stop at an ideal case" (The Level Design Book: Modular kit design):

• Stack test. Put floors on top of each other. Floors "should NOT be paper thin, they have thickness and mass just like walls" (The Level Design Book: Modular kit design).
• Loopback and gap tests. The book names these too, with little detail (The Level Design Book: Modular kit design). A practical reading: build a loop of corridors that must return exactly to its start, and hunt for any seam where pieces fail to meet.
• Collision. The book calls collision "one of the biggest issues"; make sure players "can't get stuck" and pieces "stay in footprint / bounding box" (The Level Design Book: Modular kit design). See collision meshes.
• Showcase map. The environment-art chapter suggests making "a scene or level with every prop / art asset, and walk around in it" (The Level Design Book: Environment Art).

Bethesda's own schedule allowed for this: a proof phase of typically 1–3 weeks, a greybox phase of 1–4 weeks, then a build-out that could take a few months (Game Developer: Skyrim's Modular Approach to Level Design).

#Art fatigue and variation

The cost of reuse is repetition. Burgess calls it art fatigue, and it matters more the longer people play: he cites a Steam median play time for Skyrim of "well over 100 hours" (Game Developer: Skyrim's Modular Approach to Level Design). Oblivion's copied-and-pasted "warehouse" cells drew "cookie-cutter" criticism, and players reacted more negatively to repeated detail than to repeated architecture (Game Developer: Skyrim's Modular Approach to Level Design).

Ways to reduce visible repetition:

1. Mix kits. Burgess's Dwemer example combines a dwarven hallway with ice caves to change the tone; he calls this "kit-bashing" (Game Developer: Skyrim's Modular Approach to Level Design). See kitbashing.
2. Don't marry kits to content. "Seeing a posh castle kit covered up with grimy kobold assets and moody lighting will shake up player expectations" (80 Level: Building Huge Open Worlds: Modularity, Kits &…).
3. Break the grid. Shell-based building adds free-placed walls and pillars around grid-snapped shells to make organic caves (Game Developer: Skyrim's Modular Approach to Level Design).
4. Vary surfaces, not meshes. "Texture variations go a long way" (The Level Design Book: Modular kit design). Use trim sheets, decals, vertex painting and blending and detail maps and macro variation to change the look of identical geometry.
5. Cover the joins. For off-grid joins, The Level Design Book suggests letting modules intersect and covering "the messy intersection with another object" (The Level Design Book: Environment Art). See set dressing and art passes.

#Performance

Kits are usually good for performance because the same mesh is drawn many times; engines can instance repeated meshes, and shared materials keep draw calls down. The trade-off is many small objects to cull and many seams to light. See draw calls, level of detail, hierarchical LOD and lightmap UVs (each kit piece needs clean lightmap UVs if you bake lighting).

#Further reading and watching

• Kit-piece posts from shipped games — Thierry's Dishonored Golden Cat, Kostecki's tomb kit, Wilmsen's Baldur's Gate 3 glasshouse and more, annotated in game art breakdowns: a curated library. Free.