Modeling rocks, trees and terrain pieces

Natural environments in games are rarely modeled as one piece. Rocks, cliffs, embankments, cave walls and trees are built as reusable pieces, placed many times, rotated, scaled and partly buried in terrain, then blended together with shared materials. This page covers how those pieces are planned and built. For the sculpting itself, see sculpting rocks and cliffs; for ground surfaces, terrain and landscapes; for plants, foliage and vegetation.

#Organic pieces as a kit

Architecture uses modular kits on a grid. Nature uses kits as well, just without the grid. The Level Design Book notes that modules are "most effective for buildings and structures that make regular use of repetition along a grid" (The Level Design Book: Environment Art), and its list of kit types adds "platforms (landscape)", "flanges and arches (caves)" and "shells (caves)" for the natural cases (The Level Design Book: Modular kit design).

CD Projekt RED's Michał Janiszewski describes the same principle on The Witcher 3: "The basic approach to optimization is planning many modular meshes that can be reused and repurposed" (80 Level: World Building of Witcher 3). His example is one rock with three jobs: "Imagine taking a rock and using it as a cliff, rotating it to use as a climbable platform, or ceiling of a big cave" (80 Level: World Building of Witcher 3). The team also built sets: "city decoration, sets of rocks, and vegetation" (80 Level: World Building of Witcher 3).

#Rock production at Naughty Dog

Jared Sobotta's breakdowns of the river rocks and erosion he worked on for The Last of Us Part I at Naughty Dog show a complete production chain (ArtStation: River Rock Breakdown; ArtStation: Erosion Breakdown):

1. Sculpt the large rocks in ZBrush, starting from ZSpheres, then lay them out with deformation and in-house layout tools (ArtStation: River Rock Breakdown). See base meshes for sculpting and digital sculpting.
2. Mark the tiling. Polygroup edges on the finished sculpt mark where the texture will tile, and an ID mask is baked for Substance Designer (ArtStation: River Rock Breakdown). See tileable sculpts.
3. Texture procedurally. Height detail, colour and banding are built in Substance Designer, finished with a shared AO and crevice dust layer used across all his materials (ArtStation: River Rock Breakdown). See procedural materials.
4. Small rocks from atlases. Two sculpted atlases (smooth and sharp) feed seven Atlas Scatter nodes, largest to smallest (ArtStation: River Rock Breakdown). See texture atlases.
5. Geometry where texture is not enough. Because tessellation could not be used everywhere, small river-rock prototype meshes add real depth; Sobotta shows three sets of rocks plus adjustable clusters for placement (ArtStation: River Rock Breakdown).

The shoreline itself is built from two tileable textures plus geometry prototypes (ArtStation: River Rock Breakdown): a mix of tiling surfaces and placed meshes rather than unique sculpts everywhere.

#Terrain pieces and embankments

Between rocks and flat terrain sits a middle layer: embankments, eroded banks, landslides and ground shapes too steep or detailed for a heightfield. In Sobotta's erosion breakdown, the level is built from prototype meshes, terrain geometry and instanced rocks (ArtStation: Erosion Breakdown). The embankment prototype meshes were mostly modeled by Anthony Vaccaro, and terrain geometry is highlighted separately from the instanced assets (ArtStation: Erosion Breakdown).

The breakdown shows two techniques:

• One bake, many materials. Assets use a 1-to-1 bake "multiplied down the material layers" (ArtStation: Erosion Breakdown): the unique bake carries the shape, while tiling materials carry the surface.
• Displacement for big forms. On the landslide, a height map displaces the geometry to form thick erosion lines; in Maya he paints one erosion material over another and displaces it into new geometry before adding mud, debris and moss (ArtStation: Erosion Breakdown). See height and displacement maps and vertex painting and blending.

#Caves and cliffs off the grid

Organic spaces break the grid, so the seams need a plan:

• Shells and free pieces. Bethesda's Shell-Based Building system on Skyrim layered "freely placed walls, pillars and balconies" around grid-snapped shells to create organic shapes (Game Developer: Skyrim's Modular Approach to Level Design).
• Flanges and glue. Rotating tunnel pieces leaves gaps, which Skyrim's Ratway covered with archways (Game Developer: Skyrim's Modular Approach to Level Design). See glue pieces.
• Overlap and cover. For off-grid joins, The Level Design Book's advice is to let pieces intersect and "cover the messy intersection with another object", such as a rock (The Level Design Book: Environment Art). In organic scenes, rocks often serve as this glue.

#Trees

Trees are usually built with dedicated tools rather than by hand; Janiszewski says "Speedtree was a super helpful tool when working on environments for The Witcher 3: Wild Hunt" (80 Level: World Building of Witcher 3). The modeling choices (trunk, branch cards, leaf atlases, wind, LODs and billboards) are covered in foliage and vegetation, impostors and billboards and wind and world position offset.

#Practical checks for organic pieces

• Walkability. Organic slopes vary. Godot's character treats slopes up to 45 degrees as floor by default (Godot Engine: CharacterBody3D); a path with a steeper section stops the player. Test in a gym. See player collision capsule.
• Burial. Plan which part of a rock will be buried and skip detail there; it saves triangles and texture space.
• Instancing. Repeated rocks and cliff pieces should be instanced. Epic's Valley of the Ancient sample builds its world from kitbashed pieces using ISMs (Unreal Engine documentation: Instanced Static Mesh Component in Unreal…).
• Scale variety. Build a set in several sizes (for example large, medium and small) so repetition is hidden, as Sobotta's three sets and clusters show (ArtStation: River Rock Breakdown).
• Shared materials. A common dust or AO layer across all rock materials ties the set together (ArtStation: River Rock Breakdown).

#Further reading and watching