# film The clock, the shot table and the palettes of GREEN SKY. Every other slice talks to this module and to nothing else. ## The clock A fixed **1/24 s per rendered frame**, ignoring the real frame delta. The film is therefore byte-identical on every run whatever the frame rate, so it can be recorded a frame at a time and retimed on the host. Film.FPS 24 Film.FIXED_DT 1/24 Film.DURATION 122 seconds `scenes/main.scene`'s entrypoint calls `Film.build()` once on load and `Film.tick()` once per frame. There is no cutscene, no coroutine timeline and no `task.wait` chain anywhere in the demo. ## The shot table `Film.SHOTS` — eighteen entries, `{ n, seconds, act, name, brief }`. `seconds` is the only timing authority in the film; `greensky/camera` owns where each set-up stands. Shot numbers are stable and are never renumbered, because every slice keys off them. Film.shotAt(t) -> the shot on screen at t, and seconds into it Film.shotStart(n) -> when shot n begins ## Registering a slice local Film = require("greensky.film") local M = {} function M.build() end -- once, on scene load function M.update(c) end -- every frame Film.register("pan", M, 10) -- lower order ticks first The context `c` every update receives: | field | meaning | |---|---| | `t` / `u` | film seconds, and 0..1 across the whole film | | `dt` | always `FIXED_DT` | | `frame` | integer frame counter from 0 | | `shot` / `shotT` / `shotU` | the shot on screen, and progress through it | | `cut` | true on the first frame of a new shot | | `act` | 1, 2 or 3 | | `storm` / `flood` / `clear` | the three progressions, 0..1 | `Film.arcAt(t)` gives those last three at any time, so a slice can ask what act III looks like without waiting for it. ## Two palettes, and they are not the same thing **`Film.PALETTE`** is the **light** — what the sun, the sky and the ambient are doing in the world, per act, plus the discharge. `greensky/sky` reads it every frame. **`Film.PAL`** is the **photograph taken of that light** — the response of the camera's negative, per act. `greensky/camera` reads it every frame and writes it into the `gs_grade` post-process pass. Its fields are `ev` (exposure compensation in stops), `exposure`, `contrast`, `pivot`, `saturation`, `vignette`, `vigInner`, `grain`, and the ASC-CDL triplets `lift` (offset, moves the blacks), `gain` (slope, the whites) and `gamma` (power, the mids). Both carry `dry`, `storm`, `clear` and `flash`, and both are interpolated by the same `c.storm` / `c.clear` ramps, so the light and the photograph of it always agree about which act it is. `Film.mixGrade(a, b, x)` mixes two `Film.PAL` entries. Every `gain` in `Film.PAL` is at or below 1 on purpose: the post chain is display-referred and hard-clipped at 1.0, so a gain above 1 clips the image rather than opening it up. Level lives in `ev`, in stops. ## The site `Film.SITE` — the numbers more than one slice needs about the place: pan radius, runway length and width, hangar position, cloud base and top, the cloud volume's horizontal extent, where the storm enters and leaves, and how deep the flood gets. Everything must live inside 8 km of every camera station, because the far plane is hard-fixed at 8000. ## Curves Film.ramp(t, a, b) smoothstep, 0 before a and 1 after b Film.lerp(a, b, x) Film.mix3(a, b, x) mix two {r, g, b} Film.clamp01(x) ## Files film.module/init.luau this module ../design/spec.md read this before changing any of it ../design/shotlist.md the eighteen shots as written

# gliders Three sailplanes over the salt pan, and the whole 122 seconds of flying they do. KILO is the lead ship the film keeps returning to; BRAVO and CHARLIE are already airborne when the film opens and join KILO for the last pass. They are the demo's only scale reference — a 15-metre wing against a 4.5-kilometre cloud. ## What it provides ```lua local Gliders = require("greensky.gliders") Gliders.SHIPS -- { "KILO", "BRAVO", "CHARLIE" } Gliders.poseAt(ship, t) -- -> position {x,y,z}, forward {x,y,z} Gliders.poseFull(ship, t) -- + speed, yaw, pitch, bank, rates, load Gliders.posAt(ship, t) -- -> x, y, z Gliders.tipAt(ship, side, t) -- a wingtip, side = -1 port / +1 starboard ``` Every one of them is a **pure function of film time**. They answer for any `t` in `0 .. Film.DURATION`, they read no entity, and they do not care whether the slice has been ticked — which is what lets the camera slice call `poseAt` from inside its own update to frame and ride a ship. `greensky/camera` uses this for shots 3, 9 and 12. ## How a ship is animated One authored key table per ship, evaluated as a monotone cubic Hermite spline. Everything else is a derivative of it, so there is no second animation to keep in sync and no way for the attitude to disagree with the path: | what | from | |---|---| | forward | normalised `dp/dt` | | pitch | `asin(forward.y)` | | bank | `atan(speed * dyaw/dt / g)` — a real coordinated turn | | ailerons | `dbank/dt`, opposite sides | | elevator | `dpitch/dt` | | rudder | `dyaw/dt` | | wing flex | load factor `1/cos(bank)` | | wingtip vapour | load factor, gated by humidity | Rates are read over a 0.75 s window. A cubic spline's second derivative ripples inside every segment, so a turn rate read at velocity resolution makes a steady thermal turn look like a shiver; 0.75 s is also about how long an aircraft takes to change bank, so the smoothing is the physics rather than a cover for it. On top of the path — never in it — the cell throws the ships about from the gust front to the last of the rain, and the pilots work against it. The route the camera reads stays exactly where it was authored. ## Where they are | film time | KILO | |---|---| | 0 – 6.4 | parked at the west end of the strip, nose east, one wingtip a hand off the concrete | | 6.4 – 12.4 | the ground roll: 130 m of concrete, wing level by t = 10.4 | | 12.4 | rotation and lift-off | | 12.4 – 17.5 | climb-out, easing right off the centreline | | 17.5 – 39.2 | the thermal: a 110 m circle at 30 m/s, 42 degrees of bank, climbing 12 m/s | | 39.2 – 55 | east along the gust front, through the 700 m cloud base at exactly t = 55 | | 55 – 76 | inside the mass, climbing to 1,370 m, then turning north and coming down | | 76 – 99 | out into the rain, descending across the pan | | 99 – 106 | a hard diving turn, wings coming level pointing due south | | 106 – 122 | the low pass: 68 m/s, 90 m off the flood, all three ships in line | BRAVO and CHARLIE fly their own thermals — wider and tighter respectively, at their own altitudes — until KILO leaves the turn at t = 39.2. All three exit on the same heading, and from there the wingmen hold a station on KILO that eases into their slot in the final line. ## The airframe Twelve entities under each ship's root, built nose along **+Z**, span along **±X**, up along **+Y**. Fuselage, nose, canopy, two wing panels with 3.5 degrees of dihedral, two ailerons, a tailplane, a fin, a rudder, and a wingtip entity each side carrying a `TrailRenderer` for the vapour. The root's rotation is `yaw(+Y) * pitch(+X) * roll(+Z)`, composed with `Transform.quatFromAxisAngle` and `Transform.quatMul`. This pass is a greybox: one flat material, no textures, silhouette only.

# 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: | what | how it moves | |---|---| | **The storm deck** | a `VolumetricSky` (`v5`) volume, y 700–5200, AABB half-extent 3800. Its whole preset table is rebuilt and reassigned every frame: coverage runs 0.47 sparse → 0.11 solid → 0.42 breaking up, the base drops from 1450 m to 700 m and the top rises to 5200 m, `anvilBias` flattens to 0.95, and `windSpeed` carries the mass bodily east. | | **Lightning** | 17 discrete events on the film clock, 3–5 frames each. Every one drives BOTH the cloud preset's sun (swung low and sideways, cold, ~44×) AND the scene's directional light (down out of the base, cold, ~23×) AND the ambient — so the cloud volume and the ground agree about where the light came from. Each event calls `score.thunder(distance, energy)` on its first frame. | | **The sun** | one directional `Light` plus the separate sun inside the cloud preset, both driven from one bearing/elevation pair that crosses from 292°/58° to 295°/9.5°. Colour and intensity blend `Film.PALETTE` dry → storm → clear. | | **Rain shafts** | six dark columns hanging under the cloud base, drifting east at 21 m/s and lengthening from 430 m down to 60 m above the pan as the cell matures. Gone by 1:53. | | **The rainbow** | three arcs of boxes about the antisolar point at bearing 115° — a primary at the 42° cone, a secondary at 51°, and a third lying over onto the flood. Absent until 1:41, full by 1:52, held to the end. | 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: | | | |---|---| | **The world lights up** | The cloud preset's sun goes to `PALETTE.flash.sunI` (46, inside the raymarch's own tone mapping, where it is what makes the mass read as lit from within). The scene's directional and ambient go to `PALETTE.flash.lightI` / `.ambI` — a **completely different budget**, because the frame the camera takes is display-referred and clipped at 1.0. A scene light far above the act-I sun does not photograph as brighter, it photographs as white with nothing left in it. 6.5 and 0.9 sit a stop and a half over hot act-I noon and about three and a half stops over the storm they interrupt, and the violence comes from the second number. | | **The deck blooms** | `gs_bloom`, this slice's own post-process pass at priority 500 (after the cloud composite at 5). Its bright pass weights every tap by the **depth attachment** rather than by luminance alone: sky reads depth exactly 0, so `sky_gain` scales the storm deck's contribution and `ground_gain` scales the pan's and the flood's. A tap's gain is decided by where the tap stands, not by the pixel it lands on, so the mass glows outward past its own silhouette. Baseline is a faint airglow on the deck's shoulders and nothing on the ground; at full discharge the threshold drops, both gains come up and the halo widens. | | **The frame stops down** | `gs_bloom`'s `scene_gain`, at `2^(-1.2 · flashLevel)`. It scales the SCENE and not the bloom added to it, so the world darkens around a discharge while the discharge does not — which is what a frame stopping down for a light source looks like. | **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: ```luau 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: ```luau 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.