Skirts (terrain and blend skirts)

A skirt is a strip of extra geometry added along an edge to hide a seam. In terrain rendering, "each chunk has a skirt, which is a strip of triangles around the edge of the chunk extending down below the surface" (University of Chicago: Project 5: Terrain Rendering). Environment artists use the same word for a ring of faces around the base of a rock or building that fades it into the ground.

#Blend skirts on rocks, cliffs and buildings

When a rock or wall is pushed into the terrain, the two surfaces cross in a sharp, straight line where texture and lighting change at once, because the materials and normals differ.

A blend skirt is a band of faces built around the base of the mesh, lying on the ground surface. It usually carries:

• Vertex alpha (or one vertex colour channel) at 1 where it meets the rock and 0 at its outer edge, so the material can fade out.
• A terrain-matching material, so the faded edge has the same colour as the ground under it.

Artists discussing Star Wars Battlefront on Polycount describe the same ingredients without using the word skirt. One poster saw rocks fade at intersections "with terrain colour showing through", another guessed at "vertex alpha driven by real time AO", and doc rob said DICE "adjust the vertex normals and tangents to match between the terrain and the terrain meshes" (Polycount: Battlefront's Level Construction - how can…). Treat these as practitioners' readings of the game rather than an official breakdown.

The ground texture on a skirt should be projected in world space rather than through the skirt's own UVs. On Disintegration, Tony Arechiga's vista materials were "all using world coordinate alignment nodes", so meshes could be rotated, scaled and moved "without the texture looking stretched" (80 Level: Working on Game Vistas for Disintegration in…). The same property keeps a skirt's ground texture aligned with the real terrain around it. See triplanar and world-aligned mapping.

#Terrain and LOD skirts

Large terrains are split into tiles (chunks), and each tile switches between levels of detail on its own. Where a detailed tile meets a coarse one, their border vertices do not line up, which leaves T-junctions and thin cracks you can see through. Skirts are added "to avoid rendering artifacts that might be caused because of T-junctions" (University of Chicago: Project 5: Terrain Rendering).

The University of Chicago course notes describe the Chunked LOD algorithm, attributed to Thatcher Ulrich, in which skirts are "part of the precomputed triangle meshes"; each tile is stored as "five separate meshes, one for the terrain and four skirts" (University of Chicago: Project 5: Terrain Rendering).

Simplygon uses the same trick when remeshing terrain tiles for hierarchical LOD. Independently remeshed tiles produce "visible cracks for everything but the most trivial input geometry", so the tool adds a skirt that can "intersect and overlap the skirt of the next chunks" (Simplygon blog: Remeshing terrain chunks for hierarchical…). The code pushes skirt vertices down and outwards by an initial 1% of the tile size, and the author notes artefacts "on borders where the surface normal is facing down", such as overhangs (Simplygon blog: Remeshing terrain chunks for hierarchical…).

#How to make one

These steps build a blend skirt for a rock in Blender or Maya and set it up in Unreal Engine 5. The numbers are starting points, not rules.

1. Place the rock first. Put the rock in the level on the final terrain, then bring a small piece of that terrain back into your modeling tool as reference, so the skirt follows the real ground.
2. Select the ground contact. Select the edge loop where the rock meets the ground, or a loop just above it.
3. Extrude outwards along the ground. Extrude or inset that loop outwards by about 0.5–1 m for a 3–4 m rock, then move the new vertices down onto the reference terrain. Two rings of faces are enough for most rocks.
4. Paint the fade. Set vertex alpha to 1 on the inner ring and 0 on the outer ring. See vertex painting and blending; Hoj Dee's tutorial shows the same height-lerp and vertex-paint approach applied to material blends (YouTube: Detailed Texture Blending with Height Lerp &…).
5. Match the normals. Copy or transfer the terrain's normals onto the outer ring so the lighting matches the ground, the step doc rob credits to DICE (Polycount: Battlefront's Level Construction - how can…).
6. Build the material. In the rock's material, lerp from the rock surface to the terrain material using vertex alpha. Use world-aligned coordinates for the terrain part (80 Level: Working on Game Vistas for Disintegration in…), and add height blending so stones from the ground poke through before the fade ends.
7. Use a runtime virtual texture if you have one. In UE5, a mesh can sample the landscape's runtime virtual texture with the Runtime Virtual Texture Sample node, as long as it sits inside the RVT volume (Unreal Engine 5.8 documentation: Runtime Virtual Texturing in Unreal Engine). The skirt then shows exactly what the terrain is showing, including painted layers. See blending meshes into terrain.

#Video tutorials

#Use in recent games

Few studios document skirts by name; these sources show the problem and parts of the solution.

• The Last of Us Part I (Naughty Dog, 2022). In Jared Sobotta's breakdown, "the parts highlighted in blue represent the terrain geo" and the rest are "separate instanced assets placed in the level"; he lists "blend work" among his tasks (ArtStation: Erosion Breakdown).
• Disintegration (V1 Interactive, 2020). Terrain sections are "blended together via the material" and use world-aligned textures (80 Level: Working on Game Vistas for Disintegration in…).
• Star Wars Battlefront (DICE, 2015). The Polycount discussion above is a practitioner analysis of how its rocks and terrain blend (Polycount: Battlefront's Level Construction - how can…).

#Further reading and watching

Related pages: blending meshes into terrain, pixel depth offset, vertex painting and blending, height blending, terrain and landscapes, level of detail, glue pieces.