BSP surface flags

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This will explain all surface flags you can use. Surface flags can be accessed either by right-clicking a surface and choosing Surface Properties, or by selecting a face and then cliking on the Surface Properties button in the upper toolbar.


The surface is invisible. Light goes through it. If you set a surface on a solid or a subtracted brush as invisible, players will see a Hall of Mirror as there is nothing to render behind the surface, except the void, which renders as a HOM.


Surface is rendered as masked. This makes All black areas on the texture go invisible. Some textures are already masked (have invisible parts) and do not require this flag to be set manually. Just like invisible surfaces, masked textures and surfaces will create HOMs if they are applied to solids or subtracted brush.


Surface is translucent. It is only partly visible and you can see through it. Light goes through it. Fog and translucent surfaces do not work together.

It is not possible to control how translucent a surface is, but how much light falls on the surface somewhate influences the visibility of the translucent surface.. Brighter lights make it surface more visible, while complete darkness make that surface invisible.

Translucent surface may show HOMs like masked and invisible surfaces, for the same reasons.

Force view zone

Render the poly regardless of its zone mapping.

When building a map, polys will sometimes be flagged as part of Zone 0, causing them to be invisible when the player is within the level. This happens usually on semi-solids, which may have one surface suddenly go missing for no apparent reason. This class of hole can be detected by moving "into geometry" (i.e., outside the level or inside a solid brush) in UnrealEd, and setting the 3D Viewport to "Zone/Portal View." All polygons in a given zone have different nuances of a same colour (i.e, various shades of blue or red, etc.) If the hole is replaced by a polygon with a colour strikingly different from the rest of the zone (or bright red since 227j), the "hole" can usually be patched by setting the Force View Zone flag.

The fix is instantaneous and does not need rebuilding can immediately be verified by moving inside the level.


Render surface as modulated (uses the grey colour as the transparency colour). Thus acts like Masked but using a different colour for its transparency base. Does not support fog.

Fake backdrop

Render the surface as sky (using a skybox outside the level and placing a SkyZoneInfo actor in center of it).

Surfaces flagged with fake backdrop are skipped during the lightmap stage, i.e they are not lit. If there is no skybox to render, surfaces with this flag will be displayed unlit in game and in editor.

Two Sided

Surface is on both sides. Normal polygons are only visible on one side, as is evident with sheets which are just a single flat polygon, invisible on their "back".

This is usually used for flat decorations to be seen from both sides, or for zone portals to be visible.

Two sided sheets can't be two sided mirrors. It flickers and doesn't work properly.

U/V - Pan

The texture moves in a given direction at a constant speed (its direction depends on the surface's orientation and its speed is defined in ZoneInfo > ZoneLight > TexUPanSpeed/TexVPanSpeed).

High/Low shadow detail

Controls sharpness of light shadows. High detail makes them sharper, low detail makes them fuzzier. Both flags together makes it even fuzzier, basically Super Low Detail.

More specifically, controls the relative resolution of a surface's lightmap. For example: a lightmap can be a 256 x 256 texture map for the whole level. Surfaces may take up 16 x 16 or 64 x 16 pixels of that lightmap's area, but can never go beyond 256 x 256 pixels. Higher scale values = lower surface's lightmap resolution. Lower scale values = higher surface's lightmap resolution.

No smooth

Draw the surface with no texture smoothing.

Special poly

No effect.

Special lit

The surface does not receive light, except from lights with bSpecialLit=True.

Small wavy

Texture is slowly panning in random, changing directions.

Dirty shadows

Makes the surface be covered with random shadow spots. The lightmap on that surface looks "dirty", like splotches of mud, etc.

Bright corners

Prevents the creation of darker areas on the corners of a surface. These dark areas are usuallyc reated because of how Unreal manages light.

Often, lamp textures or small lamps made out of brushes are not lit properly because of the extremly low resolution of lightmaps. This flag may help.

No bounds reject

Some polygons may seem to flicker in and out of view when looking around. This is caused by a failure of the bound rejection code, a performance optimization. Flagging the polygon (or in some cases, several adjacent polygons as well) with No Bound Rejection will eliminate this problem.

To verify that the polygon in question is suffering from this problem, select all surfaces, then toggle the flag. The missing polygon should appear when "No Bound Rejection" is flagged.

Entirely disabling bound rejection may result in a performance reduction ranging from 10% to 30% depending on level complexity.


Surface is unlit and appears fullbright.


Special flag for sheets for defining where zone changes cut.


Surface is rendered as mirror (can be combined with Translucent/Environment flag).

Caution! Mirrors can cause huge slowdowns on slower machines, because the engine has to render things twice for the mirror's reflections.

If one mirror is in the line of sight of another, the engine has a hard limit of up to 3 inter-reflections to prevent it from wasting power calculating infinite reflections. This limit is also used by Zone Portals on a warp zone: you cannot see more than 3 times the same scene.

Mirrors do not reflect light.

Mirrors can be combined with translucent to make a shiny surface. However, it's not possible to control how much translucent the surface's texture is and the diffuse / specular reflection levels of that surface.

Environment (227g only)

Draw the surface with an environment mapped texture overlay on it. Does not support volumetric fog.