Decals

GoldSrc

Source Engine

…

A decal placed by a level designer.

Bullet impacts generated by the game.

Decals are materials projected onto existing surfaces. They can be placed by the level designer, and are also generated by the engine for bullet impacts, blood, and other effects.

Note:Each decal affects one entity only. (The world is one big entity.)

Decals are "sprayed" from a location and mark every surface in their path. For instance, a decal applied downward onto a staircase would cascade down onto the top (but not front) of each step.

Using

Note:VertexLitGeneric decals won't show up on brushes, and LightmappedGeneric ones won't appear on models. Decals using DecalModulate or $modelmaterial work everywhere (see below).

Hammer

Decal tool: Creates infodecal entities, which decal brushes and displacements.
info_projecteddecal: An entity that decals models or displacements (but not brushes). The angle of projection can be set, allowing for distortion.
Overlay tool: Overlays are more expensive decals that offer level designers more control.

All three of these methods create entities, albeit ones that are removed shortly after the map spawns.

Tip:If a decal entity has a name it will appear when used (i.e. blank input) or on receiving the activate input, instead of when the map starts. The entity then disappears.

C++

Standard

Created in response to impacts, blood, and so on. Limited in number by the value of r_decals, unreliable when transmitted, and not transmitted to players who connect after creation.

Note:Only materials registered in scripts/decals_subrect.txt can be used! The file isn't extracted from the GCFs by the mod wizard.

CBaseEntity::DecalTrace(trace_t* pTrace, const char* decalName): Shared code. The trace defines where the decal is projected from and its location.
Todo: How to avoid transmission being suppressed due to prediction?

C_BaseEntity::AddDecal( <lots of args> );: Client function where the decal is actually created. In most cases you are better off just calling DecalTrace().

Static

Normally created by the level designer. These decals last forever and are always transmitted, including to players who connect later on. Any material can be used.

engine->StaticDecal( Vector localOrigin, int decalIndex, int entityIndex, int modelIndex, bool lowpriority )

Server only. This system does not appear use entities.

Vector localOrigin: Where the decal will be created, relative to the target's origin.
int decalIndex: A value received from UTIL_PrecacheDecal(). Don't use GetDecalIndexForName().
int entityIndex
int modelIndex: Information about the target entity.
bool lowpriority: Low priority decals are not saved/restored, and can be "re-used on the client preferentially".

UTIL_PrecacheDecal(const char *filename, bool preload): Returns a decal index for use with StaticDecal(). If this is in precache(), remember that it may be called statically; store the result in a global var.

Creating

This section assumes that you understand the basics of material creation.

Any material can be used as a decal. They generally have an alpha channel however, and can use some special parameters and/or the DecalModulate shader.

Source RGB color channels
Source alpha channel
Decal in the world

Material parameters

$decal <bool>: Prevents decal texture clipping.
$decalscale <float>: Same as a brush face's texture scale value: the number of units that each texel covers. Normally 0.25 or lower.
Note:This defines the size of the decal's geometry, and cannot be changed after its creation. Use $basetexturetransform if you want motion.
$modelmaterial <material>: A separate VertexLitGeneric material to that will replace this one if the decal hits a model.
$decalfadeduration <float>: Amount of time to spend fading out. Requires $vertexcolor.
$decalfadetime <float>: Delay before fadeout begins.
$decalsecondpass <bool>: ALWAYS render this decal on top of decals without this parameter. If two decals with this parameter intersect, they will treat each other how two without the parameter would.
$fogscale <float>: (in all games since ) Scales the amount of fog affecting the decal. Useful for making important decals stand out in foggy levels.
$splatter <bool>: (in all games since )
Todo: What does this do?

Decals do not support $bumpmap in all engine branches prior to CS:GO.

Confirm:Is CS:GO the first game to support this feature?

DecalModulate

For decals intended to mimic the look of pock marks or dents in a surface, the DecalModulate (a.k.a. mod2x) shader is especially suitable: it lightens destination pixels for every source pixel that is over mid-range gray (128) and darkens any destination pixels for every source pixel that is below mid-range gray. This effect can be used to give the impression of depth when applied to a surface.

To begin, create a source image whose color channel will be used for the modulation's source values. Again, light values will lighten pixels they're drawn over, while dark values will darken the destination pixels. Mid-gray values will be treated as translucent.

Next, create an alpha channel that defines a mask for the decal. Because modulation cannot have an exact middle value currently, the mask is necessary to prevent "bordering" from occurring around the decal.

Tip:You can avoid "bordering" without creating an alpha channel simply by using BGR888 format instead of DXT1 when you create your VTF.

It has also been observed in Left 4 Dead 2 that DXT1 compressed textures using DecalModulate shader (such as blood splatters and "graffiti wall writing") with background colors RGB 124 126 124 are considered alpha, alleviating "bordering". L4D2's vtex will output that specific color from the source image pixels are gray RGB 127 127 127. This solution may also apply to other engines (and other versions of vtex) that use RGB 124 126 124 as the color indicating alpha in-game, but needs further testing.

Todo: Try converting source images with background colors RGB 127 127 127 with vtex found in other games other than L4D2 and see if it results in a background with RGB 124 126 124. Also, does RGB 124 126 124 apply to other versions of the Source engine as alpha for DXT1 compression? Does I8 work, or does the shader require RGB data?