Volumetric Clouds
Renzora's clouds are raymarched through a real volume, not painted onto a sphere. Every pixel of sky walks a ray through a cloud deck wrapped around a virtual planet, and every sample along that ray takes a second, shorter walk toward the sun to find out how much light reaches it. That second march is why clouds have bright sunward faces and dark undersides, why one cloud shadows the one behind it, and why the whole deck curves down and compresses into the horizon on its own.
The model is the one from "The Real-Time Volumetric Cloudscapes of Horizon Zero Dawn" (Andrew Schneider and Nathan Vos, Guerrilla Games), with the scattering integration and dual-lobe phase function from "Physically Based Sky, Atmosphere and Cloud Rendering in Frostbite" (Sébastien Hillaire). The implementation follows bevy-volumetric-clouds (MIT) by Erik Vroon.
Quick start
- Select an entity — usually the same one carrying the scene's environment settings — and add the Clouds component from the inspector (Rendering category).
- Drag Coverage to decide how much sky is cloud, from clear to overcast.
- Drag Scale to decide how big the formations are: lower means larger, broader weather systems.
Everything else updates live. The defaults are a fair-weather cumulus deck from 1690 m to 2960 m, drifting at 40 m/s — cloud features are kilometres across, so wind has to be weather-system fast before the sky reads as moving at all.
The clouds pick up the scene's sun automatically — direction, colour,
elevation and illuminance all come off the brightest DirectionalLight, so this
works in any scene that has a sun at all, with or without a Sun component.
The deck relights as the day advances and fades out as the sun sets. It also
reads the scene's atmosphere, so sunlight reaching it reddens and dims
exactly as the sky does; see Atmosphere coupling.
How it works
Two GPU noise fields are baked once, at startup, and then never touched again:
- a 1024² base map, sampled by world XZ. Its red channel is Perlin FBM multiplied by Worley noise — the Perlin gives connected, wispy structure and the Worley carves the puffy cauliflower edges neither produces alone. The other two channels widen or narrow the coverage threshold per region, and modulate the height profile.
- a 32³ detail volume of high-frequency Worley, which erodes the base silhouette into wisps.
Both tile seamlessly, which is what lets a ray travel any distance and the wind offset grow without bound while still landing inside them.
Per pixel, the dome shader then:
- Solves where the view ray enters and leaves the cloud shell. All three cases are handled — under the deck, inside it, above it — so a camera can fly through the deck and come out on top of it. See Flying through.
- Marches that span, sampling the base map, applying a height profile that flattens bases and billows tops, and eroding with the detail volume.
- At every sample with density, marches again toward the sun with geometrically growing steps to get that sample's optical depth.
- Accumulates radiance with Hillaire's energy-conserving integration, a dual-lobe Henyey-Greenstein phase — a tight forward lobe for the silver lining, a wide backward one for clouds lit from behind — and three multiple-scattering octaves, each carrying less light, extincting less, and scattering more isotropically than the last.
- Fades the result into the horizon haze by distance, and outputs premultiplied colour that composites over whatever sky and stars are already there.
Two of those exist to fix things a naive port gets wrong, and are worth knowing about because their symptoms are distinctive:
- Multiple scattering. A single dual-lobe phase spans a 400:1 range across the sky, so everything more than about 60° off the sun ends up lit by the ambient term alone — flat, grey, and unresponsive to where the sun is. The octaves reach deeper into the cloud with a progressively more isotropic phase, which is what keeps a shaded cloud body lit.
- A weather map. The base atlas repeats every ~13 km of world at the default scale, which along the horizon is close enough together to read as wallpaper. The same atlas sampled an order of magnitude wider, and rotated so the two lattices never line up, modulates coverage per region: every repeat of the silhouette then carries a different amount of cloud, and the pair only comes back into phase after a thousand km.
The erosion detail also fades out with distance. Its volume is 32 cells repeating every few hundred metres — close up that reads as wisps, but a few km out it is below a pixel and all that survives of it is the repeat, tiled across the horizon.
Morphing
Wind only translates the deck. A cloud whose silhouette never changes reads as a cutout sliding across the sky however fast it moves, so Morph Speed runs shape evolution on its own clock, crossing the wind rather than following it:
- A warp field — a smooth vector field at a third of the silhouette's frequency — displaces where the base map is sampled, and scrolls along its own axis. Because the field moves, the displacement at any fixed point in the world keeps changing, so clouds stretch and fold in place rather than passing through unaltered. One cloud spans several times its own width of the field, which is what makes it deform rather than merely shift.
- The detail volume walks its third axis over time, so the erosion evolves instead of being carried along rigidly. This is free — the volume is 3D whether or not anything moves through it.
Set it to 0 for a frozen sky that only drifts.
The mesh is a dome centred on the camera, but only as a way to get one fragment per sky pixel — nothing about the shading uses its surface. It is sized from the camera's far plane so scene geometry occludes it correctly.
Atmosphere coupling
Leave Atmosphere Lighting on and every colour in the Lighting and Atmosphere sections becomes a noon value that the scene's atmosphere then modulates. Drop the sun toward the horizon and the light reaching the deck reddens and dims on its own, the skylight filling the shadows cools and darkens, and the haze splits into a warm half toward the sun and a cool half away from it. Nothing is keyframed — it falls out of the same scattering medium the sky itself is rendered from, so changing the atmosphere (or swapping Earth's medium for Mars') changes the clouds with it.
The sun's height changes their shape as well as their colour, because the sun march lengthens as it drops: light from overhead crosses the deck by the shortest path there is, while light from 20° up crosses three times as much cloud to reach the same point. That is the difference between a bright, flat midday sky and a deep, modelled evening one, and it happens on its own.
Be aware the coupling is a relative model, so between 90° and 45° of elevation it multiplies by 0.97–0.86 — deliberately almost nothing. The sun has to get below about 20° before it is obvious, and below 10° it is dramatic.
Two situations look like "no atmosphere" and neither switches this off: a scene that never spawned one because it is lit by a skybox or an HDRI, and a scene that turned its procedural sky off. Both still have a sun in them, so both measure Earth's medium instead — the alternative is a deck that stays noon-white while everything underneath it goes to dusk, which is the single most conspicuous way for a sky to look wrong.
The only thing that hands the authored colours straight through untouched is turning Atmosphere Lighting off, which is what you want for a stylised sky that should not care how high the sun is.
Mechanically this is evaluated once per frame on the CPU rather than sampled per pixel, because Bevy's atmosphere LUTs live in the render world where a material cannot reach them. That costs almost nothing and loses almost nothing: the two terms that matter most to a cloud — the sunlight surviving to the deck, and the colour of the sky filling its shadows — depend on the sun's elevation and the deck's altitude, not on which pixel is asking. Only the haze genuinely varies across the sky, and that is handled by evaluating the horizon twice, once toward the sun and once away, and blending between them by bearing.
Two things it does not model: multiple scattering, so twilight reads slightly darker here than in the sky behind it; and per-pixel aerial perspective, so the haze follows a smooth falloff toward the horizon rather than the exact depth of each cloud.
Settings
Shape
| Field | What it does |
|---|---|
| Coverage | 0 = clear, 1 = solid overcast. |
| Density | How opaque the cloud material is. |
| Scale | Size of the formations. Lower = larger systems. |
| Detail Scale | Frequency of the erosion detail relative to the base shape. |
| Detail Strength | How much that detail eats into the silhouette. |
| Edge Softness | Width of the coverage threshold. Low = crisp cauliflower, high = soft haze. |
| Base Softness | Fraction of the deck's depth over which density fades in from the base. |
Deck geometry
| Field | What it does |
|---|---|
| Bottom Height | Altitude of the base of the deck, in metres. |
| Top Height | Altitude of the top of the deck, in metres. |
| Planet Radius | Radius of the virtual planet the deck wraps, in metres. This is what sets how sharply clouds compress toward the horizon — shrink it for a small, storybook world; leave it at Earth's 6 371 000 for a realistic sky. |
Wind
| Field | What it does |
|---|---|
| Follow World Wind | Take heading and drift from the scene's wind instead of the two fields below. On by default. |
| Wind Speed | Metres per second. Weather-system speeds, not breezes — the default 40 m/s moves the deck about one cloud every thirty seconds. While Follow World Wind is on this is the drift the deck reaches at reference wind, and the world wind scales it. |
| Wind Direction | Degrees, 0–360. Ignored while Follow World Wind is on. |
| Morph Speed | How fast shapes evolve, independent of the wind carrying them. 0 freezes them into pure drift. |
Wind moves the sample position through the noise, not the geometry, so the deck drifts without the horizon moving with it. See Morphing for why drift alone is not enough.
The deck ignores gusts, and never fully stops. Cloud features are kilometres across, so a two-second gust does not move them — the deck scales off sustained wind only. And even in a dead-calm scene it keeps a slow drift, because air aloft is always moving and a frozen sky looks more wrong than a drifting one. Turn Follow World Wind off for a deliberately decoupled sky.
Lighting
With Atmosphere Lighting on, every colour here is the value at noon; the atmosphere shifts it from there.
| Field | What it does |
|---|---|
| Color | Tint of the sunlight scattering out of the cloud. |
| Brightness | Overall level of the deck. A trim on the scene sun, not a substitute for it — both the sunlight and the skylight scale with the Sun component's illuminance first, so dimming the scene's sun dims the clouds with everything else, and this only says how bright the deck sits within that. |
| Ambient Color | Skylight filling the top of the deck. |
| Shadow Color | Skylight filling the base — scattered blue sky, which is what actually lights a real cloud's underside. Keep it saturated, not merely dark: grey cloud is almost never a brightness problem, it is a warm direct term summed with a near-neutral fill, and the only way out of neutral is for the two to disagree about hue. |
| Ambient | Multiplier on both ambient colours. |
| Absorption | How fast light is absorbed inside the cloud, and therefore how hard the lit/shadowed contrast is. |
| Forward Scattering | Eccentricity of the forward lobe — the silver lining. |
| Backward Scattering | Eccentricity of the backward lobe. Negative by convention. |
| Scattering Blend | Mix between the two lobes. 0 = all forward, 1 = all backward. |
| Powder Effect | Darkens thin sunlit edges, which have scattered little light back toward the eye yet. Without it, rims look like cut paper. |
March
| Field | What it does |
|---|---|
| Raymarch Steps | Samples along each view ray. The dominant cost knob. |
| Shadow Steps | Samples along each sun-shadow ray. Set to 0 to drop self-shadowing entirely — much cheaper, and much flatter. |
Atmosphere
| Field | What it does |
|---|---|
| Atmosphere Lighting | Drive the lighting from the scene's atmosphere. See above. |
| Horizon Color | Colour distant cloud fades into, at noon. |
| Atmosphere | How much haze the deck picks up at distance. |
Haze accumulates with the distance the ray travelled before it met cloud, not with how low in the sky the pixel is — depth of air is what the eye reads as depth in a sky, and keying it that way also does the work of hiding the base map's repeat, which only becomes visible at the distances haze is thickest at.
Flying through
The march starts at the camera and solves its span against the real shell, so all three positions work: under the deck, inside it, and above looking down. Fly up into it and you are enveloped; keep going and you come out on a cloud tops view. Nothing needs switching on for this.
Two details make it hold up rather than merely not crash. The view march steps geometrically rather than uniformly — a uniform step has to choose between resolving the cloud you are inside and reaching the one on the horizon, and sized for the horizon it would put two samples between you and the far side of the cloud you are in, turning the envelope into a flat wall. And the dome is centred using the camera's global transform, so a camera parented to a flying rig still gets the deck put where its eye actually is.
Day and night
The deck thins out as the sun sets and is gone by the time it reaches the horizon. Above 8° of sun elevation the clouds are fully solid; through the last few degrees of the descent they fade steadily; at 0° and below there is no deck at all, so night skies show stars rather than a bank of lit cloud hanging over them.
Ending the fade at the horizon rather than below it is deliberate. The sky and the stars are driven by the atmosphere, which reaches night at 0° — so a deck still lit at −1° reads as broken, however good it looks on its own. Matching the horizon keeps the deck and the sky telling the same story.
Performance
Both marches run per pixel, so clouds are a fullscreen, resolution-bound cost that is the same for an empty scene as for a full level. The graphics quality tier is the main control:
- High — your configured step counts.
- Medium — capped at 16 view steps and 4 shadow steps.
- Low — the dome is despawned entirely.
Below that, in rough order of what buys the most frame time back:
- Raymarch Steps is close to linear in cost. 32 is comfortable; 16 still reads well with TAA on.
- Shadow Steps multiplies the cost of every sample that has density in it.
- 3D render scale in the viewport settings cuts pixel count directly.
Ray starts are dithered with a hash of the view direction rather than of time or screen position, so the dither is pinned to the sky: it does not crawl as the camera turns or flicker frame to frame, which leaves it in a form TAA resolves cleanly. If you run with TAA off, raise Raymarch Steps.