Killing Floor SDK: CullDistance and Distance Fog, and How to Pair Them
Two engine settings decide how far a Killing Floor map is drawn: CullDistance on actors and distance fog on zones. Used separately they cause either low FPS or ugly pop-in. Used together they are the cheapest performance lever a mapper has. This glossary entry explains both and gives a rule for pairing them. It applies to Killing Floor 1 and other Unreal Engine 2.5 (UT2004 branch) maps.
CullDistance
Actor.CullDistance is a property of an actor, a float in Unreal units, not a separate object. You set it on individual actors, most often on StaticMeshActors (in the editor: Display → CullDistance).
- What it does: the engine stops drawing the actor when the camera is farther away than
CullDistance. 0means never cull: the actor is drawn at any distance. This is the default.- Put it on props, not on world geometry. Trees, crates, grass, barrels and other placed static meshes are the candidates. Never cull the map's BSP shell (walls, floors): it would open holes when players look into the distance.
- Why it matters in KF: inside one large zone there is no automatic occlusion culling: props hidden behind other objects are still drawn. On a dense map with 1000-2600 actors and 300-400 thousand prop triangles, everything is rendered every frame and FPS drops whenever a player looks down a long sightline. Distance culling fixes that without touching close-up quality.
- It culls by distance, not visibility. Too small a value makes props visibly pop in; too large does nothing. Pick it for the map's size and sightlines.
As a reference point, maps converted from Source or GoldSrc use a scale of about 1.9165 KF units per Source unit, so 8000 KF units ≈ 4175 Source units ≈ 80 metres.
Distance fog
Distance fog is a set of ZoneInfo properties. It is radial: it thickens with distance from the camera. For an outdoor map you usually set it on the zone that covers the whole map (ZoneInfo0).
| Property | Type | Meaning |
|---|---|---|
bDistanceFog | bool | Turns fog on in the zone |
bClearToFogColor | bool | Paints the whole background beyond the fog in the fog colour. This also hides the skybox seam and the white far plane |
DistanceFogColor | color | Fog colour. In exported T3D it is written as an FColor in B, G, R, A order |
DistanceFogStart | float | Where fog begins (0% opacity), in KF units |
DistanceFogEnd | float | Where fog becomes fully opaque (100%). Beyond it only the fog colour is visible |
Class defaults: DistanceFogStart=3000, DistanceFogEnd=8000, colour (128,128,128).
Because the fog is radial, it hides distant props near the ground and the sky overhead equally: both are the same distance away. Match the fog colour to the sky colour and the result looks like natural haze or an overcast day rather than a wall. On a night map the fog colour has to be dark.
Pairing them
Fog and culling always go together. Tune them from one value, the cull distance:
DistanceFogEnd ≈ 0.92 × CullDistance
DistanceFogStart ≈ 0.45 × DistanceFogEnd- With fog end slightly before the cull distance, a prop is already fully hidden in fog when it stops being drawn, so it disappears instead of popping out.
- Fog start at about 45% of fog end gives a gradual build-up rather than a hard band.
Example for CullDistance=8000: DistanceFogEnd≈7360, DistanceFogStart≈3310, with bDistanceFog=True and bClearToFogColor=True.
Things to keep in mind
- Fog boundaries between zones are switched on the player's location, not on line of sight, so a different fog in the next room changes the moment you step through the portal.
- Partial distance fog counts as a line-of-sight blocker for zed spawning, even when it is not opaque.
- Players on weak PCs may lower draw distance in their own settings; still tune for the majority.
- Press F in the editor viewport to toggle distance fog while you work.
