Clouds

How to put clouds in a scene's sky and shape them: the fields, what each one draws, recipes to copy, what it costs and what it cannot do. Written for whoever composes a sky, a person or an agent. The engine side (how the clouds are built and drawn, and how to add a new form) is docs/status/hd-clouds-sculpted.md.

The short version

Clouds are one layer over an atmosphere sky, at meta.environment.sky.clouds. The HD renderer draws it.

"sky": { "kind": "atmosphere", "intensity": 1, "rotation": 0,
         "clouds": { "preset": "broken", "style": "sculpted" } }

• No clouds block: a clear sky. On any other sky kind the block is ignored with the warning render-clouds-without-atmosphere.
• Every field is optional. Start from a preset, then change one number at a time.
• An agent writes the block with scene.patch (docs/AGENT.md) like any other scene setting. A value outside its range is refused with sky-value-range, naming the field and the range.
• Everyone in a session gets the same clouds in the same places. They drift with the simulation clock, so a paused game holds them still and a recording plays them back as they were.

Two styles

The other fields mean the same in both. Choose noise for overcast and storm: the sculpted style is not tuned for closed decks and draws them as flat grey from below.

The layer

What the presets give:

Forms: composing your own sculpted sky

forms is read only with "style": "sculpted". Each field replaces what the preset would have built; leave one out and the preset's stays.

How they work together:

• coverage decides where. A pile or a tower stands where the cover reaches at its own middle, so a low coverage thins out everything, and the deck opens into breaks.
• deck, cumulus and towers decide what stands there. They are independent: a deck with nothing on it, towers with no deck, or all three.
• No two towers are built alike: each has its own width, taper, lean and side turrets. seed picks which set you get.
• A sky repeats every 40 km.
• A deck opens only where coverage runs out, in breaks. Inside covered sky it has a floor: no small holes.

Recipes

Each is the whole clouds block. All use the sculpted style.

A broken deck with towers, for flying over. The layer the engine's own flight footage uses.

{ "preset": "broken", "style": "sculpted", "coverage": 0.64, "altitudeM": 1500, "thicknessM": 3600, "density": 0.8 }

Fair-weather cumulus, seen from the ground.

{ "preset": "fair", "style": "sculpted", "coverage": 0.6 }

A rolling deck and nothing else. No towers, few piles: a sea of cloud to fly over.

{ "preset": "broken", "style": "sculpted", "coverage": 0.85, "altitudeM": 1500, "thicknessM": 3600, "density": 0.8,
  "forms": { "towers": 0, "cumulus": 0.05 } }

A field of towers with no deck. Tall cumulus standing on open air, half of them with anvils.

{ "preset": "broken", "style": "sculpted", "coverage": 0.7, "altitudeM": 1200, "thicknessM": 4500, "density": 0.8,
  "forms": { "deck": 0, "cumulus": 0.15, "towers": 0.8, "anvils": 0.5 } }

A crowded cumulus sky. A pile in every cell, no deck, no towers.

{ "preset": "fair", "style": "sculpted", "coverage": 0.8, "forms": { "deck": 0, "cumulus": 1, "towers": 0 } }

Low turrets on a deck. Many short towers and no anvils: a bubbling deck.

{ "preset": "broken", "style": "sculpted", "coverage": 0.7, "altitudeM": 1500, "thicknessM": 3600, "density": 0.8,
  "forms": { "towers": 0.7, "towerHeight": 0.6, "anvils": 0 } }

Smooth, stylised forms. The same sky with the lumps nearly off.

{ "preset": "broken", "style": "sculpted", "coverage": 0.64, "altitudeM": 1500, "thicknessM": 3600, "density": 0.8,
  "forms": { "lumps": 0.2 } }

Another sky with the same weather. Change only the seed.

{ "preset": "broken", "style": "sculpted", "coverage": 0.64, "altitudeM": 1500, "thicknessM": 3600, "density": 0.8,
  "forms": { "seed": 7 } }

To design a sky: pick the nearest recipe, set seed until the towers stand where the shot wants them, then move coverage for more or fewer breaks.

How finely it is drawn (render quality)

These are quality settings, per machine, not part of the scene (docs/SKY.md, "Sculpted clouds"). They trade speed for detail and change nothing about which clouds there are.

Measured on an RTX 3080 at 1920x1080, all cloud passes together, for the broken deck recipe from three views:

The sculpted forms are baked once into two small volumes (14 MB of GPU memory). They are baked again when the layer's coverage, preset, thicknessM, wind direction or any of forms but lumps changes, or when the sun or moon turns. So set a sky and leave it: do not change forms every frame.

How a sculpted cloud is drawn

Nothing here is a setting; it is what to expect in the picture.

• Soft, not solid. A cloud's edge is a shell about 35 m deep that the eye sees into, frayed by fine noise, so outlines are soft and thin parts glow against the light. clouds.sharpness sets how far in: at 2 a cloud is a crisp solid, at 0.5 softer still.
• Haze between the clouds. The air inside a cloud layer is hazy: a cloud 10 km off has lost about 40% of its contrast to the sky's colour, one 30 km off about 80%. Far towers stand as pale shapes, as they do from an aircraft.
• Gentle shade. The creases between lumps are shaded, not black; the big shadows come from the key light: the side of a bank away from a low sun or moon, the deck behind a tower. On a shaded face each lump is still modelled: its side toward the open sky is lighter than its lee.

What it cannot do

• Fly through a cloud. Inside one the view is a flat haze. Over, past and beside is what it draws.
• Storms and closed overcast in the sculpted style. Use noise for those.
• More than one layer. There are no high cirrus above a cumulus deck.
• Rain, lightning or fog banks. None come with the clouds.
• Close-up surfaces. Within a few hundred metres a cloud's own shape goes soft; the lumps carry the near detail.
• From the ground, the sculpted cumulus have smooth undersides. They read best low in the frame or from the air.