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).

#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