music-video-gen/flow-state/EPIC-4.md

Epic 4 — the song tells a story

Epic 3 asked what is on screen. This one asks a question none of the existing layers can answer:

Why is this the last drop rather than the first one?

Nothing in the generator knows. And a viewer who cannot tell the difference is watching a loop with good production design.

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1. What is actually missing

The generator has four timescales, and ArcDriver's header lists them honestly: per frame, per shot, per section, whole song. Three of the four are cyclic or local. The fourth — the "whole song" one — turns out to be much thinner than it reads:

• _slowAxisFor is the only thing in the system that travels one way across the track. It moves one or two params per scene, journey = smoothstep(t/duration), in a direction picked by rng.bool(). It is real progression, and it is blind: it does not know where the drop is, it does not know a section from a boundary, and half the time it travels the wrong way for what the song is doing.
• paletteArc.underDrift — a bounded hue crawl, plus per-kind offsets.
• buildSlope — one bar of lookahead. Local by design.

Everything else that decides what a video looks like is keyed on section kind, and kinds recur:

assignRostersByKind()   roster per KIND — every drop cuts between the same visuals
KIND_ENERGY / biasFor() same energy, density, motion for every drop
rhythmFor(energy)       same cutting pattern for every section of that energy
derivePaletteArc()      kindHue.drop — the same hue offset at every drop
frameShot(style, ...)   framing from previous shot + energy; no notion of when

So the fourth drop is cast from the same roster as the first, biased to the same energy, cut at the same rate, tinted the same hue and framed by the same rule. The content of the video is a function of kind, and kind has no arrow of time in it. That is precisely a song structure without a story: recurrence without consequence.

This is not a bug in any of those modules. Kind-keying is what gave the video its identity (§assignRostersByKind), and it should stay. What is missing is the second coordinate.

Today a section is identified by what kind of thing it is. It should be identified by what kind of thing it is, and where in the story it sits.

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2. The proposal, in one paragraph

A new pure module, look/Story.js, runs once per track between segmentation and the look generator. It reads the section list and the summary, picks a plot the way directors.js picks a director, locates the song's key moments, and emits a small narrative state per section and per frame. Everything that currently keys on kind keys on (kind, story) instead. No scene changes, and — in the first slice — no new uniforms: the story acts by rewriting inputs the whole library already consumes, so it applies to all 61 scenes on day one.

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3. What a story is, here

Three layers, smallest to largest.

Position — where a section sits

Derived, not invented. From the sections segment.js already produces:

{
  index, kind,
  ordinal, ordinalOf,   // "the 3rd of 4 drops" — the single most useful missing fact
  act,                  // setup | development | turn | climax | resolution
  isMoment,             // one of the key moments below
}

ordinal alone unlocks most of the devices in §5, and costs a Map and a loop.

Moments — the frames the story turns on

Measured from the track, never imposed. A short list, and each one has a definition that falls out of data already on the FeatureTrack:

moment	definition
arrival	first section whose energy clears 0.6 × max — the video's first "here it is"
turn	largest energy fall between adjacent sections after the arrival
climax	max-energy section; ties broken toward the later one
resolution	first section after the climax whose energy stays below it

A two-section track collapses these onto each other, and that is fine — a track with no structure gets almost no story, which is the correct behaviour and not a degenerate case to defend against.

Plot — what the track does with them

One coherent narrative shape per track, chosen weighted-random with the audio tilting the odds, exactly the way pickDirector works, and for the same reason: a fixed mapping from measured features to narrative is how a library ends up with one story per genre.

plot	shape
emergence	almost nothing, then something. Reveal rises monotonically and stays.
escalation	each recurrence of a kind is further than the last. A ratchet, not a curve.
collapse	order → entropy. The climax is a breakdown of the thing, not a peak of it.
return	ABA. The outro rhymes with the intro, transformed by what happened between.
unveiling	the protagonist is withheld until the climax and then it is all there is.

Each plot is a set of curves over five story variables, evaluated per frame:

tension     0..1   how hard everything is pushed
reveal      0..1   how much of the song's identity has been shown
closeness   0..1   wide and distant → close and involved
population  0..1   sparse → crowded → alone
order       0..1   regular → broken (or the reverse; the plot decides the sign)

These are staged curves, not ramps: they hold flat inside a section and move at boundaries, with a step at the climax. That is what makes them read as a story rather than as a slow zoom — a story advances in scenes.

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4. Where it plugs in

Every one of these is an existing call site gaining an argument.

site	change
biasFor()	tension modulates energy/density within the kind's envelope, bounded to ±0.2, so a breakdown at high tension is still a breakdown
sampleValues(…, temperament)	extremity scaled by tension — the ratchet for escalation
assignRostersByKind()	roster stays per kind; which member plays becomes a story decision (§5.1)
buildStack()	overlay chance follows population, not just energy
rhythmFor()	later acts pick from faster patterns; the resolution gets a held shot
frameShot()	closeness biases the size draw
derivePaletteArc()	new 'narrative' mode keyed on act rather than on kind
ArcDriver._paramsAt()	journey comes from story.journeyAt(frame); the slow axis gets its sign from the plot, not from rng.bool()
ArcDriver.update()	passes a story-shifted personality clone, memoised on rounded reveal exactly as _paletteAt memoises on rounded shift

That last one is how the story reaches Epic 3's content registers without a new uniform: setPersonality is already called every frame, and the identity uniforms are derived from the personality object. Scaling notchDepth, hollow, inkOutline, posterize and latScaleSpread toward their full values as reveal rises makes the song's cast literally arrive over the course of the video.

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5. The devices, ranked by legibility per unit of work

5.1 The anchor is earned. Today roster[0] opens every section of its kind. Instead, reserve it: earlier drops play companions, and the anchor arrives at the climax. Same roster, same identity, and now the biggest visual in the video lands on the biggest moment. Roughly twenty lines.

5.2 Recapitulation. Under the return plot, the outro re-casts the intro's scene, with the story's parameters rather than the intro's. The oldest device in music video and the cheapest one here — the scene is already built and cached.

5.3 The ratchet. Under escalation, ordinal/ordinalOf scales temperament extremity and slow-axis travel per recurrence. The fourth drop is measurably further out than the first, on every param the scenes declare.

5.4 Reveal schedule. §4's identity scaling. Under unveiling the protagonist's u_cast* form is a near-circle until the climax, then snaps to full at a downbeat.

5.5 Punctuation. A budget of two or three single-use events for the entire video, spent at the moments in §3 — the only cut to black, the only symmetry-fold flip, the only feedback reset. Single-use is the whole point: a device used twice is a style, used once it is a moment. These are the only additions that need a flash-gate review.

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6. How to know if it worked

The existing instrument can measure this almost unmodified. Build the section×section descriptor distance matrix that checks/variety already knows how to produce, and ask three questions of it:

• Direction. Does distance correlate with |i−j| beyond what kind explains? Today this is ~0 by construction: the matrix is kind-blocked, all drops mutually near, and time is invisible. A story makes it a gradient. This is the headline number.
• Recurrence. For a repeated kind, is d(first, last) > d(first, middle)? That is the ratchet, and it is the one a viewer names as "it kept going somewhere".
• Coherence bound. Adjacent-section distance must stay under the existing ceiling. A story that maximises Direction by shuffling scenes is the failure mode, and this is the gate that catches it.

Plus: each punctuation fires exactly once, on a downbeat, under the flash limit; and every curve is a pure function of frame, so seek-exactness and the determinism grep hold.

A prediction, stated in advance: the seed-variety floor — how far a video travels from itself — will rise, because that is what progression is. Epic 3 spent its effort pushing that number down. Both are correct, and the instrument is what is wrong: it measures distance and calls it drift, with no way to tell wandering from travelling. The fix is one extra statistic, not a retreat from the feature — split the self-distance into an ordered component (monotone with time; a story) and an unordered residual (a shuffle). Ship that statistic before the feature, or the first honest measurement of Epic 4 will read as a regression and be argued about instead of read.

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7. Risks

The story overrides the song. A plot that declares a climax where the track is quiet is worse than no plot. Mitigation is structural: moments are found in the audio (§3), never placed by the seed, and tension is bounded inside the kind envelope so the quiet-kind coupling in directors.js — the one that keeps an intro off a strobing scene — still holds absolutely.

Every video tells the same story. The exact failure directors.js was written to fix. Same mitigation: five plots, weighted, with seeded curve parameters inside each.

It becomes a slow zoom. If the curves are smooth ramps, this is an effect, not a narrative. Staged curves with plateaus and a step at the climax are load-bearing, not a refinement.

Short tracks. Under three sections, most of this has nothing to work with. Degrade to the current behaviour explicitly rather than letting the curves do something arbitrary.

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8. The smallest experiment worth running first

Do not build five plots on a prediction. Three changes, no new uniforms, no scene edits:

1. Story.js with position and moments only — no plot templates, one hardcoded escalation curve.
2. Wire it to exactly two sites: the slow-axis sign/magnitude in _paramsAt, and the anchor reservation in §5.1.
3. Add the Direction statistic to the variety report and run the song bank.

The prediction is specific: Direction moves off zero and Coherence holds, while the between-song distance is unchanged — the story should differentiate a video from itself in time, and have no opinion about other songs. If Direction does not move, the story is not reaching the image and the rest of the epic is worth nothing until it does.

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11. What was built, and what it measured

Built, in look/Story.js plus one argument at each call site listed in §4:

• position (ordinal/ordinalOf/act), the four moments, five plots with seeded curves;
• the anchor is earned (§5.1), the ratchet on temperament (§5.3), recapitulation (§5.2), the reveal schedule on the identity uniforms (§5.4), story-driven cut rate, framing closeness, and a narrative palette-arc mode;
• the slow axis now takes its direction from the track rather than a per-scene coin flip, and its journey from the staged story curve;
• Phase 13 (checks/phase13.js, 8 checks, no GPU) and the direction statistic in the variety report (checks/variety/signature.js).

Punctuation (§5.5) was not built — it is the only part that needs a flash-gate review, and it is worth doing after the numbers below are understood rather than before.

The first direction measurement

checks.html?variety=1&library=0&seeds=6, one song, against the arcless single-scene reference:

floor      0.1697        direction   -0.14
observed   0.1594        arcless ref  0.01
ceiling    0.1996

The prediction in §8 was that direction moves off zero upward. It did not. Three readings, in the order they should be checked:

1. The recapitulation is fighting the statistic, by construction. Roughly two videos in five recap, and return — the plot most likely to — is an arch that comes back. Its first and last probes are deliberately similar, which is exactly what a negative rank correlation between time separation and distance means. The statistic as written cannot tell ABA from no story at all; it may need to be measured against the journey curve rather than against clock time.
2. Six seeds of one song is a small sample, and the probe count (5) makes each video's correlation rest on ten pairs.
3. The arc may not be reaching the image, which is the reading that matters and the one §8 was written to expose. If 1 and 2 are controlled for and direction stays at zero, the story is moving parameters that do not change the picture — the same failure the slow axis had before scenes declared slowAxis, and the fix would be the same: name the levers rather than guessing at them.

The no-story control arm (the same report with the story layer bypassed) did not complete — the run hung in the browser after the first arm, so the floor and separation figures above are not yet attributable to this work either way. That comparison is the next thing to run, and it should be run before any conclusion is drawn from the numbers.