Heightfield erosion

Heightfield erosion is a simulation that runs over a terrain heightmap, moving material the way rain, rivers and gravity do: it cuts channels where water collects, wears down ridges and drops sediment at the bottom of slopes. SideFX describes its node as one that "Simulates hydraulic and thermal erosion at a specific scale to create more realistic terrain" (Houdini Documentation: HeightField Erode geometry node). QuadSpinner calls the erosion node in Gaea "the cornerstone of all terrains" (Gaea Documentation: Erosion).

Erosion makes a noise-based heightmap read as natural land. For the terrain system that receives the result, see terrain and landscapes.

#Two kinds of erosion

• Hydraulic erosion simulates water: it "uses rainfall and weathering to simulate the removal and transport of material" (Houdini Documentation: HeightField Erode geometry node). It carves gullies, valleys and branching drainage.
• Thermal erosion stands for weathering on slopes: loose material breaking off and moving downhill. Houdini exposes both as separate strengths, either of which can be set to 0 (Houdini Documentation: HeightField Erode geometry node).

#Erosion masks

Erosion tools also record where material went, and those masks drive texturing and scattering.

Houdini generates eroded height plus sediment, debris, flow and flowdir layers (Houdini Documentation: HeightField Erode geometry node). Gaea notes that its masks can be "used for texturing, or for driving other nodes", but warns that Flow "tends to create unrealistic textures and can make your terrain look fake and "CG"" if overused (Gaea Documentation: Erosion). Feed these masks into your terrain layers.

#Scale and resolution

Erosion has a physical scale, and it has to match your world.

• Gaea's Feature Scale sets "the lateral size of the largest erosion features in meters: width of largest valleys and ridges between them", and Real Scale can read the scale from the terrain definition (Gaea Documentation: Erosion). Its default Duration of 0.04 "is enough for most cases" (Gaea Documentation: Erosion).
• Houdini keeps features similar when you change resolution, but "The smallest possible feature size is 3× the input terrain's voxel size" (Houdini Documentation: HeightField Erode geometry node).
• Previews match the final build. Gaea says "a 512 x 512 preview build will maintain essential parity for all major erosion features" compared with a 4K or 8K build (Gaea Documentation: Erosion), so you can iterate fast and build big at the end.

#Building erosion at several scales

Real terrain is eroded at every scale at once. Houdini's documentation suggests: "You can chain Numerous HeightField Erode nodes together to create erosion features at multiple scales", and recommends freezing each result once it looks right, because unfrozen chained nodes "may not cook sequentially" (Houdini Documentation: HeightField Erode geometry node).

Gaea's output can vary between runs with parallel processing; turn on Deterministic when you need repeatable results, at some cost in speed (Gaea Documentation: Erosion).

#How to make one

The steps below erode a 4 × 4 km mountain range in Gaea and bring it into a game engine. A Houdini version follows.

Gaea

1. Start from a base shape. Use a mountain or noise node, or a real-world elevation map, and set the terrain's size so Real Scale can read it (Gaea Documentation: Erosion).
2. Add an Erosion node and set Feature Scale. Feature Scale is "the lateral size of the largest erosion features in meters: width of largest valleys and ridges between them" (Gaea Documentation: Erosion). For a 4 km range, try 1,000–2,000 m for the main valleys (example values).
3. Keep Duration short. The default of 0.04 "is enough for most cases" (Gaea Documentation: Erosion); longer runs tend to over-erode.
4. Iterate at low resolution. A "512 x 512 preview build will maintain essential parity for all major erosion features" with a 4K or 8K build (Gaea Documentation: Erosion), so tune at 512 and build big at the end.
5. Add a second, smaller pass. Chain another erosion at a smaller Feature Scale (for example 100–200 m) for gullies on the slopes. Houdini's documentation recommends the same multi-scale chaining (Houdini Documentation: HeightField Erode geometry node).
6. Turn on Deterministic for the final build so the result repeats between runs (Gaea Documentation: Erosion).
7. Export height and masks. Export the height at 16 bits, plus the Wear, Deposits and Flow masks (Gaea Documentation: Erosion). Use Flow sparingly; Gaea warns that it can make terrain look "fake and "CG"" (Gaea Documentation: Erosion).
8. Import and texture. Import the heightmap as the engine's landscape and use Wear for rock and Deposits for soil and scree as layer weights. See terrain layers and splat maps.

Houdini

Use a HeightField Erode node, set hydraulic and thermal strengths (either can be 0), and read the sediment, debris, flow and flowdir layers it outputs (Houdini Documentation: HeightField Erode geometry node). Remember that the smallest feature it can make is "3× the input terrain's voxel size" (Houdini Documentation: HeightField Erode geometry node), and freeze each pass in a chain so the nodes cook in order (Houdini Documentation: HeightField Erode geometry node).

#Video tutorials

#Use in recent games

• Star Wars Jedi: Survivor (Respawn, 2023). Senior skybox artist Eric Yi Lin calls Gaea his "go-to program" for erosion. On Koboh he used it to build the eroded, sloped bases of large rocks, "a connecting piece between the rocks and the ground", then sculpted the sharp peaks in ZBrush; Gaea also produced distant mountains and moon terrain for cinematics (QuadSpinner Blog: Artist Spotlight: Eric Yi Lin).
• The Last of Us Part I (Naughty Dog, 2022). Jared Sobotta's breakdown shows erosion built rather than simulated close to the player: prototype meshes, terrain geometry and instanced rocks, with a height map displacing the terrain for thicker erosion lines (see the views box above) (ArtStation: Erosion Breakdown).

#Common mistakes

• Wrong scale. Both tools erode at a set scale, measured in metres, so settings tuned for a 1 km terrain will not give the same features on an 8 km one; check Feature Scale or the node's scale whenever the terrain size changes (Gaea Documentation: Erosion; Houdini Documentation: HeightField Erode geometry node).
• Over-erosion. Longer simulation does not always look better; Gaea's short default Duration of 0.04 "is enough for most cases" (Gaea Documentation: Erosion).
• 8-bit export. Terraces appear; export at 16 bits (see terrain and landscapes and bit depth and banding).

Related: terrain layers and splat maps, sculpting rocks and cliffs, modeling rocks, trees and terrain pieces, vegetation and procedural tools.

#Further reading and watching