Skyboxes and background vistas

A skybox is the background drawn behind all level geometry: the sky, and often the distant mountains, cities and planets under it. Valve's documentation separates the flat 2D skybox, a cube of images, from the 3D skybox, "an extra area constructed by the level designer" outside the playable space that the player can see but never enter (Valve Developer Community wiki: 3D Skybox).

Together with low-detail background meshes, these make up the backdrop or vista of a level. Done well, they make a small playable area feel like part of a large world. This page covers the types of sky, how studios build backdrops, and how to make them read as far away. For real-time cloud rendering see sky, clouds and volumetrics; for framing views see sightlines and vistas.

#Types of sky backgrounds

#Skies as a light source

In most engines the sky also lights the scene. In Godot, "by default, objects in the scene will reflect this sky material and absorb ambient light from it" (Godot Engine documentation: Environment and post-processing). Unreal's HDRI Backdrop includes its own Sky Light, and Epic warns that "if a regular Sky Light exists, it should be deleted" (Unreal Engine documentation: HDRI Backdrop Visualization Tool in Unreal…). Changing the sky therefore changes ambient light and reflections, not only the background. See light probes and reflection probes.

#3D skyboxes

The Source engine's 3D skybox is the best-documented example of a separate background scene. Its geometry is built at 1/16 scale, and "at run time, the engine scales them back up to match the scale of the rest of the map" (Valve Developer Community wiki: 3D Skybox). A sky_camera entity aligns the miniature with the playable map (Valve Developer Community wiki: 3D Skybox). Left 4 Dead changed the default to 1:32 (Valve Developer Community wiki: 3D Skybox).

The 3D skybox is drawn in front of the 2D skybox, not instead of it: "A 3D skybox is not a replacement for a 2D skybox" (Valve Developer Community wiki: 3D Skybox). Valve lists two performance notes that still apply to any backdrop. Lower texture and lightmap resolution makes skybox geometry "less expensive performance-wise than standard world geometry", but it is "not occluded or culled like the rest of the level geometry", so detailed or translucent models there "can severely affect performance" (Valve Developer Community wiki: 3D Skybox). Half-Life 2 keeps its skybox props in a separate models/props_skybox folder (Valve Developer Community wiki: 3D Skybox).

#Matte paintings and painted skies

Many AAA skies start as paintings. Mukul Soman, a skybox artist on Halo 4 and Halo 5, starts in Vue or Terragen, adds photographic elements and paints over them in Photoshop (80 Level: Creating Skyboxes for AAA Games). He works from the sky inwards: "I usually start with making the sky texture and then work my way closer and closer to the level" (80 Level: Creating Skyboxes for AAA Games). Mid-ground and foreground terrain is then modelled in World Machine, Geoglyph and ZBrush and placed in front of the sky (80 Level: Creating Skyboxes for AAA Games).

Clouds are often cards rather than volumes. Soman uses alpha-mapped cloud and fog cards with scrolling UVs to tie terrain and sky together (80 Level: Creating Skyboxes for AAA Games). Tony Arechiga, a sky and background artist on Destiny 2, describes alpha cloud cards cut from photos, 360° panoramas and matte paintings, and notes that "each card in Destiny had to be tuned for Time of Day, and this was extremely tedious" (80 Level: Creating Breathtaking Game Backgrounds).

#Low-detail background meshes and impostor skylines

In modern engines, much of a vista is real geometry outside the playable boundary. Arechiga builds background terrain with World Machine and Houdini heightfield generators, drawing on a library of several hundred terrain pieces (80 Level: Creating Breathtaking Game Backgrounds). His performance advice: "Delete out unseen polygons and crunch your textures down with MIPs", and use alpha cards sparingly, mainly to break up ridgelines and hide terrain seams (80 Level: Creating Breathtaking Game Backgrounds).

Objects that are only seen from far away can be flattened completely. On Destiny 2's Trials of the Nine, planets were modelled as spheres in 3ds Max and baked down to alpha cards on a spline path (80 Level: Creating Breathtaking Game Backgrounds). The same idea scales up to whole skylines: distant buildings or forests can be rendered as impostors and billboards or merged into HLOD meshes, since the player never gets close enough to see the difference. Background meshes need no collision and only one LOD, because the player's distance to them barely changes.

#Building a vista that sells scale

A backdrop looks large only if the eye has something to measure it against. Arechiga's advice (80 Level: Creating Breathtaking Game Backgrounds):

• Give a scale cue from the playspace. Show the same tree type near the player and in the distance, so distant terrain reads as larger. See scale and proportion.
• Use atmospheric fog. "Atmospheric Fog is extremely important for creating distance." Balance the values of fog, sky and terrain, and check them by converting reference images to black and white. See fog and atmosphere and value.
• Study real landscapes. Take reference photos on hikes and match how real distance desaturates and lightens.
• Reduce and reuse. His summary: "when in doubt – fog it out."

Movement matters too. Soman notes that skybox elements "are massive and far, so tend to move slowly" (80 Level: Creating Skyboxes for AAA Games). Fast-moving clouds or ships make a backdrop feel small.

Layering is how teams divide the work. On the Destiny 2: The Witch Queen Throneworld vistas, Mark Goldsworthy writes that Bungie's world artists built the close foreground while he handled the mid-ground and distance (ArtStation: Destiny 2 The Witch Queen / Throneworld…).

#How to set one up

Godot 4 (Godot Engine documentation: Environment and post-processing):

1. Add a WorldEnvironment node with a new Environment resource.
2. Set Background to Sky and create a new Sky resource.
3. Choose a sky material: PanoramaSkyMaterial for an HDRI (use .hdr or .exr, 2:1 aspect ratio), ProceduralSkyMaterial for a quick stylised sky, or PhysicalSkyMaterial for a realistic one.
4. Leave ambient light and reflections on their default source, so the scene is lit by the sky.

Unity (Unity 6 Manual: Using skyboxes):

1. Create a material and select a skybox shader from the Shader dropdown.
2. Open Window > Rendering > Lighting, go to the Environment tab and drag the material into Skybox Material.
3. For a single camera, add a Skybox component and set Custom Skybox; it overrides the global skybox for that camera.

Unreal Engine (HDRI Backdrop, for prop renders) (Unreal Engine documentation: HDRI Backdrop Visualization Tool in Unreal…):

1. Enable the HDRI Backdrop plugin under Rendering and restart.
2. Drag the HDRI Backdrop asset into the level and delete any existing Sky Light.
3. Assign an HDR image to Cubemap, then set Size (in metres), Projection Center and Intensity by hand.

Blocking out a vista (any engine), following Arechiga's and Soman's workflows (80 Level: Creating Breathtaking Game Backgrounds; 80 Level: Creating Skyboxes for AAA Games):

1. Set the sky and sun first, then work inwards to the level.
2. Block mid-ground and far terrain as simple meshes beyond the playable boundary; check silhouettes from the main viewpoints.
3. Add fog until each layer separates in value. Check in greyscale.
4. Place one scale cue (a known tree, building or character) between player and backdrop.
5. Delete back faces and unseen polygons, reduce textures, and add cards only where ridgelines need breaking up.

#Video tutorials

#Use in recent games

• Destiny 2: The Witch Queen (Bungie): Goldsworthy's Throneworld vistas combine a broken pyramid ship, swampy lowlands and hive towers, built in the mid-ground and distance behind the world artists' foreground (ArtStation: Destiny 2 The Witch Queen / Throneworld…).
• Destiny 2 (Bungie): the Titan sky is an animated 360° skybox with time of day, including non-playable arcology and hive structures (ArtStation: Destiny 2 : Titan skybox); Trials of the Nine uses layered terrain, fog cards, a colour dome and baked planet cards (80 Level: Creating Breathtaking Game Backgrounds).
• Halo 5: Guardians (343 Industries): painted skies with modelled terrain layers, and three stacked skyboxes for a space elevator sequence (80 Level: Creating Skyboxes for AAA Games).
• Left 4 Dead and Half-Life 2 (Valve): 3D skyboxes at 1:32 and 1:16 scale, with dedicated skybox props (Valve Developer Community wiki: 3D Skybox).

#In different tools

#Further reading and watching

Related pages: sightlines and vistas, sky, clouds and volumetrics, fog and atmosphere, impostors and billboards, hierarchical LOD, landmarks, forced perspective.