sky

Everything in GREEN SKY that is above the ground.

The slice registers itself with greensky/film as sky (order 20), builds once from M.build(), and is ticked once per rendered frame with the film context. It owns five things:

It also owns the background gradient the clouds composite over: the scene ships no sky, so the slice spawns sky_backdrop with a ProceduralSky and drives its horizon/zenith/ground colours across the three regimes. Its own sun disc is off — the deck covers the sky for most of the film and a disc in the wrong place is worse than no disc.

Entities

All parented under the SKY anchor, all rebuilt from scratch by build():

SKY/
  sky_backdrop                     the ProceduralSky gradient
  sky_rain_shaft_1 .. 6            the shafts
  sky_rainbow_primary_1 .. 46      the 42-degree arc
  sky_rainbow_secondary_1 .. 46    the 51-degree arc
  sky_rainbow_reflected_1 .. 46    the arc lying in the flood

The deck itself is the scene's own volumetric_sky entity, and the two lights are the scene's own sun and ambient. The slice drives them; it does not create them.

Greybox materials live in /zero/source/greensky/sky/: gs_shaft, gs_bow, gs_bow_dim. They are flat PBR colours and nothing more — the refine stage replaces the arcs with a spectral surface shader and the shafts with precipitation.

The two suns

sun (the scene's directional Light) lights geometry. The cloud preset's sunDir / sunColor / sunIntensity light the cloud volume. They are entirely separate systems and nothing couples them, so every lightning event and every step of the sun's arc writes both. The cloud volume also keeps a legible sun through act II — a supercell's mass is a bright bruised grey while the ground under it is nearly black, and crushing both to the ground's exposure loses the middle of the film to a black frame.

Quality knobs

build() sets stepSize = 42, maxSteps = 288 on the component. The raymarch advances stepSize per iteration and stops at maxSteps, so the two together bound how far a ray travels inside the volume. The shipped 4 × 224 reaches 896 m, which crosses the package demo's 700-metre layer; this deck is 4,500 m thick and 7,600 m across — an 8.8 km diagonal — so 288 × 42 (12 km) is what covers it from every station in the film.

The discharge is a light event, not a fill

A strike lasts three to five frames. That is not long enough to be a light going off — a fill that appears and vanishes inside a fifth of a second reads as a cut. So the raw strobe from flashNow runs through an envelope with an instantaneous attack and a half-second release, and everything downstream rides the envelope rather than the strobe: the scene's sun and ambient, the background gradient, the cloud tint, the bloom and the lens. M.flashLevel carries it, rewritten every frame.

Read M.flashLevel, not flashNow. Any slice responding to the storm's light — the grade blending toward Film.PAL.flash, the flood's specular, the score ducking — wants the envelope. The strobe is the channel; the envelope is what a camera and an ear see of it.

Three things happen on it:

Why the stop-down is not exposure. The exposure module has one compensation slot and the camera slice already owns it — Film.PAL carries ev per act and hands it over every frame. Two writers on one slot is a last-writer-wins bug that presents as a flicker nobody can find. Scaling the scene alone is also the better picture: exposure scales the finished image, discharge included, which stops the whole frame down uniformly and reads as a dim flash rather than a bright one in a dark world. M.baseEV and M.drivesExposure are the seam if a slice ever wants the dip routed through the exposure module after all; drivesExposure is false and the module is the camera slice's alone.

Why the bright pass cannot find the discharge by brightness. Nothing in this frame ever exceeds 1.0 — the tonemap is baked into the material shader, so the whole post chain runs on display-referred data (#6215). A threshold low enough to catch a lightning channel also catches sunlit salt crust, and bloom plus a lens flare at that threshold takes a plain act-I frame to uniform white. Hence the depth split, and hence every gain being driven from the film clock rather than from anything in the picture.

lensFx rides the envelope too, at a threshold of 0.94 with a nearly-closed flare — its halo and streak belong to the discharge alone. Its baseline is small rather than zero so the render feature stays resident between strikes instead of being released and rebuilt seventeen times. Grain is left at 0; that belongs to the grade.

Inspection

Three hooks, all inert in the film:

local Sky = require("greensky.sky")
Sky.hold = 61.0          -- pin the film clock: every slice sees one instant,
                         -- the camera stops on its station, and two captures
                         -- of the same moment are comparable
Sky.flashOverride = 0.95 -- force the discharge level without waiting for one
Sky.photoFrozen = true   -- stop this slice rewriting the bloom pass and the
                         -- lens every frame, so one property can be held at a
                         -- stated value and photographed on its own
Sky.report(61.0)         -- what the sky is doing at that film second, plus the
                         -- bloom pass's live status and the lens's settings

Sky.hold = nil releases the clock. Leave all three inert.

A module field is not a reliable channel here. After a hot reload — of this file, or of greensky/film, which it requires — the table require hands an execute is sometimes not the table the film is ticking, and a field write then takes, reads back as the value written, and reaches nothing. It is intermittent, it comes back on its own, and every read on the wrong table is plausible, so a probe reports a healthy sky while the world ignores it. So each hook is also carried as an attribute on the SKY anchor, which is scene state and belongs to no VM:

local sky = entity(entity.find("SKY").id)
sky.attribute.set("gs_hold", 63.5)   -- same as Sky.hold
sky.attribute.set("gs_flash", 1.0)   -- same as Sky.flashOverride
sky.attribute.set("gs_photo_frozen", 1)
sky.attribute.get("gs_flash_level")  -- the envelope, published every frame

Resolve the anchor by name at both ends. A scene reload rebuilds it, so an id cached before the reload names a different entity and puts the writer and the reader on two different attribute tables — which looks exactly like the hook doing nothing.

Clear a hook with sky.attribute.remove(name). set(name, nil) leaves the previous value in place, so a probe that "cleared" its override that way holds a discharge on the world for every other viewer until the anchor is rebuilt.