The sky tag, often called a skybox, models the environment outside the BSP. It contains radiosity parameters that affect lightmaps like ambient light and directional lights, defines general atmospheric fog (not to be confused with planar fog), and can have a 3D model that draws behind level geometry.

The sky is not responsible for weather effects, which are instead assigned to clusters in Sapien.

Indoor skies

Even fully indoor levels can use a sky. Sky tags contain both outdoor/exterior and indoor/interior options for ambient light, fog, and directional light. The first sky in the scenario's skies block (sky 0) has special meaning and its indoor options are used for any indoor clusters the BSP has. A fully indoor level with no visible sky faces will only have indoor clusters.

However, it is not mandatory that indoor levels use a sky. Without any skies in the scenario the space outside the BSP will simply be black and there will be no interior fog or ambient light, as is the case with Chiron TL34. Indoor levels can still be lit using light-emitting shaders in the BSP and objects that affect lightmaps.

Multiple skies

Scenarios can reference up to 8 skies. This is mostly useful for long singleplayer levels where the artist wants to portray a change in weather, time of day, or lighting in a different BSP. This is accomplished by using the +sky0 and +sky1 special material names, for example. Tool will not permit any cluster to have a mix of these special materials.

The scenario should reference a sky in the skies block for each sky index used in special materials. For example, if you used +sky0, +sky1, and +sky2 then the skies block should contain 3 sky references. The game and lightmapping process is tolerant if this is not the case, however. Any missing skies block entries or empty/null sky references will simply result in black being seen through those sky faces and those clusters receiving no sky lighting and fog.

When the game changes to a different sky the fog colour will smoothly transition over cumulative camera movement of approximately 20 world units. In other words, the fog doesn't change if the camera isn't moving and it needs to move about 20 units (even back and forth) to fully transition. The model itself does not transition in any way and changes instantly. Transitions can happen as the camera/player moves between clusters with different skies, between indoor and outdoor clusters, or during a BSP switch. An example of the latter can be seen in Assault on the Control Room when the player reaches the first chasm and Cortana says "the weather patterns here seem natural"; look up and you should see the fog transition as you move.

Animation

The model of a skybox can be rigged and animated with an overlay (JMO) model_animations. Don't forget to set an animation period in the sky's animations block. By animating the skybox you can have moving moons, ships, clouds, and even moving sun markers. This cannot be used to achieve dynamic time of day lighting for your map because lightmaps and dynamic object shadow directions are precomputed based on the sky's rest position and cannot be animated.

You can also animate the sky's shaders for things like sliding cloud textures or a twinkling star effect.

Ray of Buddha

Sunlight shining through an alpha-tested texture

The "Ray of Buddha" effect is a glowing disk of light that simulates light beams when obstructed by the BSP and objects within it (even by alpha-tested transparent textures like foliage). This effect will appear automatically for any sky light which has a lens flare referenced. Halo supports multiple Rays of Buddha drawing simultaneously if there are multiple sky lights with lens flares. It will always render over any other part of the skybox.

The referenced lens_flare tag itself is not responsible for this effect but rather creates the faint circular rings of light across the screen. The appearance of Ray of Buddha is hard-coded into the engine and you cannot change its size or colour.

Related debug globals

The following are related debug globals that you can enter into the developer console for troubleshooting.

Global

(rasterizer_environment_fog)

Toggles environmental sky fog and fog plane colors. Does not affect fog screen.

(rasterizer_ray_of_buddha [boolean])

Toggles the sky's sun effect.

Structure and fields

Field Type Comments
model
TagDependency

An optional reference to a model which is rendered for this sky. Fully indoor maps where the sky isn't visible don't need to have a model since there would be no way to see it (and it doesn't render from indoor clusters anyway).

animation graph TagDependency: model_animations

An animation which can move parts of the sky model, including markers. This must be an overlay (JMO) animation to work.

indoor ambient radiosity color ColorRGB

Sets the colour used for ambient light in indoor clusters. This can be used to brighten indoor spaces to a minimum level.

indoor ambient radiosity power float
  • Min: 0

Sets the strength of indoor ambient light. This value must not be negative (causes a crash in pre-H1A versions).

outdoor ambient radiosity color ColorRGB

Sets the colour used for ambient light in outdoor clusters. It is like a fill light that approximates light coming from all directions and will be most visible in shadowed areas that would not be lit otherwise. This is typically a similar colour to the sky's backdrop, e.g. blue for a blue sky and black in space.

outdoor ambient radiosity power float
  • Min: 0

Sets the strength of outdoor ambient light. Values around 0.2 are common. If this value is too high the level will appear flat and shapeless from a lack of strong shadows. This value must not be negative (causes a crash in pre-H1A versions).

outdoor fog color ColorRGB

The colour of the atmospheric fog which draws over the BSP by distance while in outdoor clusters.

outdoor fog maximum density float
  • Min: 0
  • Max: 1
  • Default: 1

Limits the maximum density of outdoor fog. For example, a value of 0.75 would mean that no matter how far away parts of the level and objects are, the fog will only ever reach 75% opacity and you will still be able to see their textures and details faintly. If this value is 1 then distant areas beyond outdoor_fog_opaque_distance will always be outdoor_fog_color.

outdoor fog start distance float
  • Unit: world units

Within this distance there will be no outdoor fog drawn. This is the start distance where fog begins to fade to its maximum density. A small value like 0 or 1 is typical.

outdoor fog opaque distance float
  • Unit: world units

The distance where fog reaches its maximum density. Note that the outdoor_fog_maximum_density may limit how opaque it actually is at this distance. For comparison, Blood Gulch's sky uses 100 while The Maw's uses 20.

indoor fog color ColorRGB

The colour of the atmospheric fog which draws over the BSP by distance while in indoor clusters.

indoor fog maximum density float
  • Min: 0
  • Max: 1
  • Default: 1

Limits the maximum density of indoor fog. If this value is 1.0 then distant clusters beyond indoor_fog_opaque_distance will be culled from the PVS.

indoor fog start distance float
  • Unit: world units

Within this distance there will be no indoor fog drawn. This is the start distance where fog begins to fade to its maximum density. A small value like 0 or 1 is typical.

indoor fog opaque distance float
  • Unit: world units

The distance where fog reaches its maximum opacity for indoor clusters. When indoor fog maximum density is 1.0, this distance helps cull visible clusters when Tool is generating the PVS.

indoor fog screen TagDependency: fog
  • Unused

This field is seemingly unused but was probably intended to allow indoor clusters to have a fog screen effect like planar fog. Setting a reference here does not cause anything to render even in H1A where fog screen effects were fixed for planar fog.

shader functions Block
  • HEK max count: 8
  • Unused

Not used by the engine. Ignore this field.

animations Block
  • HEK max count: 8

Defines the repeating period (duration) of sky model animations.

Field Type Comments
animation index int16

An index into the model_animations animations block.

period float
  • Unit: seconds

How long it takes the animation to complete 1 repeat.

lights Block
  • HEK max count: 8

A list of directional light sources that cast light on the level. These are mainly used to represent incoming sunlight, but you can include multiple lights to model different light sources. For example, if you were making a night sky with two moons you could have two lights, or if your sky is cloudy you may wish to have a direct overhead light with a high diameter and low power to model soft lighting bounced down from the clouds. A sky doesn't have to have any lights. Indoor skies like the one for The Library have no need unless you have a special reason to force directional lights indoors with the affects interiors flag.

Field Type Comments
lens flare TagDependency: lens_flare

If provided, this light will appear in the sky with a Ray of Buddha and a lens_flare. Otherwise the light source itself will be invisible (but still casts light for lightmaps).

lens flare marker name TagString

If provided, the lens flare will be rendered in the sky at the location of this marker in the model. If a marker name is provided but no such marker exists in the model then no lens flare will be rendered at all. Otherwise, when a marker name is not provided the game will render the lens flare according to the light's direction.

flags bitfield

Flags that control which cluster types this light affects.

Flag Mask Comments
affects exteriors 0x1

If enabled, this light will affect outdoor clusters.

affects interiors 0x2

If enabled, this light will affect indoor clusters. Remember to set this light's test distance low enough or else this flag will seem to have no effect. You will not need to set this flag except in special circumstances.

color ColorRGB

The colour of the directional light. Overhead sunlight will be a yellowish white, while sunsets will be more orange.

power float
  • Min: 0

The irradiance power of the sky light. This value must not be negative (causes a crash in pre-H1A versions).

test distance float

The maximum distance to search for a shadow-casting surface from a surface being lit towards the sky light. The radiosity process shoots rays toward sky lights to determine if they are directly lit, but they can be limited with this field. If you are finding that distant parts of the BSP are not casting shadows where you expect, try increasing this. Increasing the distance likely has a time cost for radiosity since more surfaces need to be considered.

direction Euler2D

The direction toward the sky light from the origin. The yaw is in a counterclockwise direction from the +X axis while the pitch ranges from -90 (straight below) to 90 (straight above), with 0 being at the horizon.

Field Type Comments
yaw float

Rotation to the left or right around the Z (vertical) axis.

pitch float

Rotation up or down.

diameter float

Angular diameter of the light source. It is entered in degrees in Guerilla but internally stored in radians. A wider angular diameter makes shadows softer. Increase this diameter if your light source is diffuse like as an overcast sky.

Acknowledgements

Thanks to the following individuals for their research or contributions to this topic:

  • Galap (Researching the purpose of affects interiors)
  • Kavawuvi (Invader tag definitions)
  • MosesOfEgypt (Tag structure research)