Performance and optimization

8.1 Purpose

Define performance targets, typical bottlenecks, and optimization techniques for levels.

Optimization should happen throughout production, not only at the end. It is much easier to keep a level fast while building it than to fix an already fully dressed map.

8.2 Core concepts

Performance is driven by a lot of different elements. The list below present some of the major areas to keep under control.

• Draw calls

  • A draw call is the CPU telling the GPU to render a batch of geometry with a specific mesh/submesh, material/shader, and render state. Changing material, submesh, render pass (base, shadow, depth), or camera view creates a new draw call. Each call has CPU cost, so many small or diverse objects can bottleneck.
  • Quick examples
    • One mesh, one material: 1 draw call
    • One mesh with 3 materials/submeshes: 3 draw calls.
    • 100 identical props:
      • Without batching/instancing: ~100 draw calls.
      • With GPU instancing (same material): ~1 draw call.
    • UI using one atlas/material: often 1 draw call; separate materials: ~1 per element.
    • Decals: typically ~1 draw call per decal.

• Overdraw

  • Overdraw is how many times the same screen pixel gets shaded because multiple triangles or transparent layers overlap there. Each extra layer re-runs the pixel shader (and often blending), increasing GPU fill-rate cost. Opaque geometry with depth testing can minimize overdraw; transparency usually increases it.

  Quick examples

  • Dense alpha-blended foliage/grass cards: very high overdraw (many overlapping leaves).
  • Particle smoke/fog: lots of overlapping quads = heavy overdraw.
  • Multiple decals on the same surface: each decal adds another layer to shade.
  • Looking through several glass/window panes: each pane adds a layer.

• Triangle count

• Texture memory

• Decalroads amount

• Dynamic lights

• Vegetation density

• Collisions complexity

Early, recurring profiling avoids costly late fixes.

8.3 Guidelines and budgets

• Geometry: keep triangle counts proportional to distance and importance; use LODs for large assets.
• Materials: limit unique materials per view; reuse and atlas where possible.
• Textures: Texture Cooker PNG only; use power-of-two sizes and mipmaps; choose the smallest size that holds up at driving distance.
• Vegetation: control species count and density; prefer impostors/far LODs where available.
• Lights: keep local dynamic light counts conservative.

Practical rules:

• Reuse materials across related props instead of creating one-off materials for every object.
• Prefer atlases or trim sheets for small repeated details.
• Use Forest or GroundCover for repeated natural scatter instead of many separate static objects.
• Avoid high collision detail on objects that vehicles cannot reach.
• Add LODs for objects that remain visible at distance.
• Keep decal and DecalRoad overlap intentional; every extra layer has a cost.

8.3.1 What to optimize first

When a scene is slow, check the biggest cost first:

1. Too many unique materials or draw calls in view.
2. Dense vegetation or ground cover.
3. Too many dynamic lights or shadow-casting lights.
4. High overdraw from transparent foliage, glass, particles, or decals.
5. Very large textures or too many high-resolution textures visible together.
6. Heavy collision meshes on frequently touched objects.

Do not reduce visual quality randomly. Find the actual bottleneck, then adjust the content that causes it.

8.4 Visibility and overdraw

• Avoid coplanar overlaps (roads/decals/intersections).
• Reduce large, opaque surfaces stacked in the same view.
• Use occlusion/visibility tools where applicable (e.g., tunnels, enclosed spaces).

Overdraw is especially common in:

• Grass and leaf cards
• Dense bushes
• Layered decals
• Glass, fences, and alpha-tested materials
• Smoke, dust, fog, or rain effects

For vegetation, fewer larger clumps are often cheaper than many tiny overlapping cards. For roads and markings, remove hidden decal layers below newer ones.

8.5 Terrain considerations

• Match heightmap resolution to level size.
• Tune terrain LOD and draw distance for target hardware.
• Keep terrain material layer count manageable.

8.5.1 Collision considerations

Collision is part of performance and gameplay quality.

• Use simple collision meshes for buildings, walls, props, and barriers.
• Avoid visible mesh collision unless the object really needs it.
• Split large collision objects logically when it improves stability.
• Disable collision for small decorative objects that vehicles should not hit.
• Test collision at speed, not only by slowly driving over it.

Bad collision can feel like bad road design: invisible bumps, sticky edges, tire snags, or sudden impacts.

8.6 Profiling

• Use the in‑engine profiler and statistics overlays in representative areas (dense forest, urban cluster, long vistas).
• Verify frame time stability during sustained driving.

Profile these cases before release:

• Default spawn view
• Fast drive through the main route
• Dense vegetation area
• Urban/detail-heavy area
• Night scene with active lights
• Water/reflection view
• Worst long-distance vista

Keep notes while profiling. If a later change makes the level slower, you need to know which area changed.

8.7 Validation criteria

• Stable frame times across dressed areas and long road sections.
• No hotspots caused by excessive materials, decals, or lights.
• Texture memory within acceptable limits for target hardware.
• Collision is simple enough to be stable and cheap.
• LODs, impostors, and draw distances do not produce obvious popping while driving.
• Performance remains acceptable after a full reload, not only during an editor session.

See also: Props, buildings, and prefabs , Vegetation , Materials , Testing and validation .