Decals

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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 manipulatable decals on brushes and displacements that offer level designers more control.

infodecal and info_projecteddecal create edicts that last until the decal is sprayed (for unnamed decals, this is a split second after the map spawns). info_overlay only creates edicts if named.

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.

Note:Decals can also be made out of brushes or displacements using $decal materials, by having the decal be coplanar to the face that it is decaling. Due to VBSP performing a CGG pass on map geometry, however, decal brushes which aren't part of a separate bmodel (i.e. using func_illusionary) may not reliably appear in-game. Additionally, brush-based decals will use additional brushsides.

Confirm:func_detail-based decals will reliably show up on structural world brushes and brush entities, but what about on other func_details?

Decals can also be made out of MDLs this way, but may not blend as cleanly due to being vertex-lit.

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!

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.; Note:This parameter is not exclusive to decal entities; any material can have $decal to modify how it is sorted when coplanar with another surface.
$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.

Bug: This is broken in

Source 2013, decals will not fade out [todo tested in ?]

$decalfadetime <float>: Delay before fadeout begins.

Bug: This is broken in

Source 2013, decals will not fade out [todo tested in ?]

$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 normal maps in all engine branches prior to CS:GO. The underlying brushes/displacements must have bumped lightmaps for the decal's normal map to have any effect.

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

DecalModulate

Split

It has been suggested that this article or section be split into multiple articles: DecalModulate. (Discuss)

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 with DXTn compression, the mask is necessary to prevent "bordering" from occurring around the decal. You can avoid "bordering" without creating an alpha channel simply by using BGR888 format instead of DXT1/DXT5 when you create your VTF, at the cost of being triple the file size.

Confirm:Does I8 (greyscale) work, like with $detailblendmode 0, or does the shader require RGB data? I8 is the same size as DXT5, and but accurately represent all 256 possible modulation values (albeit without color).

Note: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 unmodulated, 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 engine branches (and other versions of vtex) that use RGB 124 126 124 as the color indicating lack of modulation 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 unmodulated for DXT1 compression? It is possible that the upgraded shader has a range of a few values before modulation occurs.