Procedural modeling

Procedural modeling means building geometry from rules and parameters rather than placing every vertex by hand. You describe how a thing is made (a fence is posts every 2 m with two rails between them) and the software builds it. Change the input, such as the fence's length or path, and the result rebuilds itself.

Blender describes its procedural system, Geometry Nodes, as "a system for modifying the geometry of an object with node-based operations" (Blender Manual: Introduction). SideFX's John Moncrief presents Houdini as a "Tool Building Tool" and frames proceduralism as a way of making artistic decisions while generating models (SideFX Houdini on Vimeo: Intro to Procedural Modeling: Not Just…). Procedural modeling is therefore partly a modeling technique and partly a way of making tools for other people.

#What makes a model procedural

A hand-built model is a result. A procedural model is a recipe that produces a result. Three properties follow:

• Parameters. Numbers and curves you can change: height, number of floors, plank width, random seed.
• A live history. Every step stays editable. In Blender, the node tree connected to a Geometry Nodes modifier is a node group; geometry enters through a Group Input node, is processed and leaves through a Group Output node to the next modifier (Blender Manual: Introduction).
• Inputs from elsewhere. A curve drawn by a level designer, a terrain, or a blockout mesh can drive the result, so the tool adapts to the level instead of the other way round.

#Three levels of proceduralism

#1. Procedural steps inside normal modeling

Most modelers already work partly procedurally: modifiers that stay live until you apply them (arrays, bevels, mirrors, subdivision) are small rules on top of hand-made geometry. Geometry Nodes extends Blender's modifier stack with full node graphs (Blender Manual: Introduction). This is the easiest starting point: model by hand, add rules where repetition starts.

#2. Fully procedural assets

Here the whole asset is a node network. Scott Keating's GDC 2016 talk shows procedural techniques for game art such as columns, bridges and stone stairs, and how thinking procedurally produces efficient game assets (SideFX Houdini on Vimeo: Thinking Procedurally with Houdini). Moncrief's GDC 2019 introduction is subtitled "Not Just Another Rock Generator": rocks are a common first procedural asset, but the same thinking applies to architecture and props (SideFX Houdini on Vimeo: Intro to Procedural Modeling: Not Just…).

Procedural assets do not have to be big systems. Davin Saputra's weeds breakdown for a personal project includes a free Houdini Digital Asset (HDA) download for the weeds (ArtStation: Project: Villa | Part 01b - Weeds Breakdown): one small, shareable tool.

#3. Tools inside the level editor

Procedural work saves the most time when a procedural asset becomes a tool that level artists use in the engine. With Houdini Engine, the art is delivered into the game editor (SideFX Houdini on Vimeo: Thinking Procedurally with Houdini). On Far Cry 5, Ubisoft's procedural tech art team built Houdini tools for fresh water, power lines, sands, cliffs, biomes, a fog density map and the world map, integrated into Ubisoft's editor so that "game artists could craft the world using the Houdini tools" (SideFX community: Far Cry 5). SideFX's write-up credits the approach with removing "tedious and time-consuming tasks" so artists can focus on the world and gameplay (SideFX community: Far Cry 5).

For placing many objects over a landscape, see procedural placement and scattering; for node-based textures, see procedural materials.

#When to use procedural modeling

Procedural work costs more up front: you build a system instead of an object. It pays off when one or more of these is true:

• Many variations. Dozens of buildings, fences, cliffs or rocks that share a logic.
• Layouts that change. Paths, rivers and power lines that follow a designer's curve and keep changing during production (SideFX community: Far Cry 5).
• Repetitive pipeline steps. Unwrapping, naming, collision or LOD generation applied to every asset (ArtStation: The Last of Us: Part I...Trim Pipeline).
• Large worlds with small teams. The Far Cry 5 tools existed so artists could build at scale inside their normal editor (SideFX community: Far Cry 5).

It rarely pays off for a single hero prop, a one-off set piece or anything whose charm comes from specific, deliberate decisions. See hero assets, props and filler.

#In different tools

• Houdini: node-based procedural modeling, with Houdini Engine for using procedural assets inside game editors (SideFX Houdini on Vimeo: Thinking Procedurally with Houdini; SideFX community: Far Cry 5). See vegetation and procedural tools.
• Blender: Geometry Nodes, added as a modifier and built from node groups (Blender Manual: Introduction).
• Unreal Engine: the PCG framework for rule-based generation and placement in the level; see procedural placement and scattering.
• Substance 3D Designer: procedural textures rather than meshes; see procedural materials.

#Further reading and watching