heygen-com/cut-the-curve

Cut the Curve — the technique catalog

Five SEAM techniques, one principle: cut at peak velocity, match direction and speed on both sides of the cut — plus the two in-scene techniques (§6 arrivals, §7 slides). The seam LAW — vector law, the current, the ledger, the Seam Gate — lives in motion-doctrine; read it first. This skill is the parameters and mechanics. All GSAP code templates (worker + registry): examples/gsap-implementation.md.

Catalog

#	Technique	Scope	Axis	Use for
1	Zoom-Through (forward)	Within-scene text swap	Z, toward viewer	progressing deeper into the same thought
2	Inverse Zoom-Through	Arrival / payoff beat	Z, away from viewer	something bigger lands
3	Cut the Curve	Between scenes	X / Y	the default boundary, the film's current
4	Waterfall Cut	Text-to-text seam	X, per-word	word-level handoff between big-text beats
5	Rack-Focus Blur-Cut	Same-surface state swap	X / Y / Z	the one cut you want SEEN — a DSLR focus-pull flourish
6	Waterfall Entry	In-scene ARRIVAL (no seam)	Y, from below	title cards, segment openers, list intros
7	Nudge Curve	In-scene group slide (no seam)	X / Y	repositioning a composed group to make room

Z direction is a sign

"Same axis" is not enough on Z — the sign of d(scale)/dt must match across the cut:

Z vector	Exit scale	Entry scale	Variant
Push (forward)	growing 1 → 1.2	growing 0.75 → 1	zoom-through
Pull (back)	shrinking 1 → 0.8	shrinking 1.25 → 1	inverse zoom-through

Banned mirrors: a receding exit answered by a grow-from-small entry (pull flips to push — the common one, since grow-from-small is the default element entrance), and a push exit answered by an oversized retraction. This binds the incoming scene's OWN entrances during the seam window (cut + ~0.5s), not just the wrapper tween: hold the incoming frame composed, or author its entrance to match the sign. Verify per Seam Gate rule 7.

Blur logic (all Z variants)

Subject	Peak blur	Why
Text-scale (headline, word group)	10px	20px smears letterforms — the cut reads as a glitch, not speed
Full-frame surface (window, card, screenshot)	18–20px	lighter blur on a big surface reads as a rendering hiccup

Same peak blur on both sides at the swap frame. Blur the WRAPPER, never children.

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1. Zoom-Through (forward)

Z-axis velocity-matched cut; never both texts visible. Everything GROWS: the outgoing text accelerates toward camera, a hard swap hides at peak blur, the incoming text keeps growing into the focal plane. Headlines and short phrases only. Total ≈ 0.4s.

Phase	Scale	Blur	Opacity	Ease	Duration
Exit	1 → 1.2	0 → 10px	1 → 0.15	power3.in (opacity: separate none tween)	0.2s
Cut (tl.set)	in: 0.75	10px	out: 0 / in: 0.15	—	—
Entry	0.75 → 1	10 → 0px	0.15 → 1	expo.out	0.5s

Exit opacity MUST be its own linear tween — power3.in holds opacity near 1 too long. On entry all properties share expo.out.

2. Inverse Zoom-Through (backward)

The pull-back mirror: the outgoing element RECEDES; the incoming arrives OVERSIZED (as if just behind camera) and retracts into the focal plane. Everything SHRINKS. Spend on ARRIVAL/payoff beats — a payoff line, a giant reply, a held end-state — never ordinary boundaries. Total ≈ 0.7s (30% exit / 70% entry).

Phase	Scale	Blur	Opacity	Ease	Duration
Exit	1 → 0.8	0 → 10px	1 → 0.15	power3.in (opacity: separate none tween)	~0.2s
Cut (tl.set)	in: 1.25	10px	out: 0 / in: 0.15	—	—
Entry	1.25 → 1	10 → 0px	0.15 → 1	expo.out	~0.5s

Blur is 10px text-scale; 18–20px only when both sides are full-bleed surfaces.

Sign discipline: the incoming scene arrives as a composed frame inside the retracting wrapper — no grow-from-small intro in the seam window. Staged entrances happen after the retraction settles, or start ≥1 and retract.

3. Cut the Curve (default scene boundary)

X/Y velocity-matched cut — the default for ALL scene-to-scene boundaries, in the film's current, not an accent. The outgoing hero accelerates in one direction, the cut lands mid-motion, the incoming hero continues the SAME direction and decelerates. Total ≈ 0.6s; directions LEFT / RIGHT / UP / DOWN (default LEFT).

Partial travel: ~12% of frame (≈230px at 1920) — never full off-screen moves.

Direction	Exit	Entry start → end
Leftward	x: 0 → −230	x: +230 → 0
Rightward	x: 0 → +230	x: −230 → 0
Upward	y: 0 → −230	y: +230 → 0
Downward	y: 0 → +230	y: −230 → 0

Mechanics:

• Mirrored eases: exit power4.in + entry power4.out, same distance and duration — the two halves of one power4.inOut, so velocity matches exactly at the cut.
• The fade trick: exit opacity completes at ~25–30% of its travel (fade ≈ 0.18–0.3s vs motion 0.3–0.34s); entry ignites at ~0.35 opacity mid-path. Time the last fading element to die right at the cut — a gap where nothing moves reads as dead air.
• Exit 0.2–0.4s; entry ≥ exit. Optional blur 8–10px.
• Stage ground: #root must be opaque (background: var(--canvas-deep, var(--canvas, #000))) — the mid-window cut opens a summed-opacity < 1 window that flashes white otherwise (see seam-craft).

push-slide exists but violates partial-travel and mid-motion phase; prefer cut-the-curve.

4. Waterfall Cut (word-by-word cut-the-curve)

Cut-the-curve at WORD granularity — the strongest leftward cut for text-to-text seams. Outgoing words ramp out on their own curves; incoming words cascade in mid-flight — a wave the eye rides across the seam.

Scope: worker-authored inside one multi-beat comp (stacked full-frame .beat layers), NOT a registry/injector type — it tweens word spans, not clip wrappers. The boundary into and out of the text-beat block still gets a normal registry transition. Does not count against the 2–3 transition budget.

Parameter	Value	Why
Travel	±230px (~12% frame)	partial travel + velocity > full-frame push
Exit	0.34s power4.in	the acceleration IS the cut
Exit fade	0.18s, starts with x	word gone by ~25–30% of travel — no smear
Exit stagger	+0.022s reading order	the line peels, not a block slide
Entry	0.3s power4.out	back half of the composite — velocity match
Entry start opacity	0.35	mid-path ignition; binary 0→1 pops
Entry gaps	0.05s × 0.84 decay	accelerating cascade, resolves composed

Rules:

• One direction per chain, riding the current. Inverse zoom is the chain's ARRIVAL beat only.
• Pre-set all words to x: +230, opacity: 0 at build time — immediateRender: false alone leaves un-started words visible at rest.
• A short first beat may exit whole-line: its fade ends ~0.02s before the cut so it is still streaking when the next words ignite — no dead gap.
• Transform/opacity only (seek-safe); opaque stage ground applies.

5. Rack-Focus Blur-Cut (the visible cut)

The one variant where the cut is SEEN: a defocus blur SPIKE hides a single-frame hard swap — a handheld-DSLR focus-pull. Use as an occasional flourish for a state swap of the SAME surface within one visual theme; never the default boundary.

Differences from the others: outgoing stays FULLY OPAQUE until the cut (the blur hides the swap — no early fade); eases power2.in / power2.out (soft optics, not momentum).

Rules:

• Fire only at a narrative beat, ≤ once per ~8s; never mid-caption or during a hold.
• Cut at PEAK blur (≥6px; peak 8–12px, ≤16–18px max) — swapping on the way up shows the cut.
• A subtle scale (~1.06 lens-breathing) sells it as optics.
• Same direction on both sides — the vector law still holds. Entry ≥ exit duration.
• Blur the wrapper; never blur + opacity in one tween on one element (headless compositing bug); never blur a <video> directly (wrap it).

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6. Waterfall Entry (in-scene arrival — not a seam)

Staggered ARRIVAL cascade: words/elements whip in from below (one consistent direction), each starting before the previous settles — an accelerating wave that resolves into a composed layout. Title cards, segment openers, list/feature intros. The seam sibling is §4; do not mix their rules:

	§6 Entry (arrival)	§4 Waterfall Cut (seam)
Opacity	BINARY 0→1 via tl.set at entry — never fade	ignites at 0.35 mid-path — the fade IS the velocity trick
Axis default	Y, from below	X, riding the current
Outgoing side	none	words ramp out on mirrored power4.in

Choreography:

• Overlap, don't queue — next element starts within ±2 frames of the previous settling; gaps SHRINK across the cascade; the last element snaps.
• Velocity varies by weight — heavy/anchor elements travel further and longer; light words/punctuation snap in tight:

Parameter	Anchor/heavy	Normal word	Light/punctuation
Y offset	60–80px	40–50px	30–48px
Duration	0.16–0.20s	0.13–0.16s	0.10–0.13s
Overlap	0–2f gap	1f overlap	1–2f overlap

• Ease power4.out (expo.out for extra snap); never .inOut on an entry.
• One direction per cascade.
• Split the FINAL word into fragments to extend the climax; fragments travel further.
• Post-settle, the group usually slides to make room for the next beat — that's §7.

7. Nudge Curve (in-scene group slide — not a seam)

Slow-fast-slow repositioning of a composed group (word rows, card stacks, lists) to reveal content or make room. No single built-in ease produces it — power4.inOut smacks to a stop. Chain three tweens on one property:

Phase	Ease	Distance	Time	Feel
1 ramp-in	power3.in	~10%	~20%	barely moves — motion registers, no jolt
2 burst	none (linear)	~65%	~18%	~2× average px/frame — purposeful
3 tail	power4.out	~25%	~62%	decaying creep to rest — kills the smack

Rules:

• The tail is ≥3× the ramp-in in TIME. If it still smacks: extend the tail's time (not distance) or use power5.out.
• Phase 2 stays linear — easing it loses the burst contrast.
• Reveal new content DURING phase 2 — the burst masks its appearance.
• Same ratios vertical; scale distances proportionally, keep the time ratios.

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Choosing a Variant

	Zoom-Through	Inverse Zoom	Cut the Curve	Waterfall Cut
Scope	Within-scene text swap	Arrival/payoff beat	Between scenes	Text-to-text seam
Z sign / axis	growing (push)	shrinking (pull)	X / Y	X, per-word
Travel/scale	1→1.2, then 0.75→1	1→0.8, then 1.25→1	±230px	±230px
Peak blur	10px text / 18–20 full-frame	10px text / 18–20 full-frame	8–10px optional	none
Eases	power3.in / expo.out	power3.in / expo.out	power4.in / power4.out	power4.in / power4.out
Feel	progressing through	arriving at	carried sideways	a wave across the seam

Anti-Patterns

Don't	Instead
Two texts visible during a zoom-through	Hard cut at blur peak, one text at a time
20px blur on text-scale subjects	10px text; 18–20px only full-frame
Inverse-zoom exit → grow-from-small entry (or push → oversized retraction)	Match the scale-velocity SIGN; verify at cut±0.1s
Incoming comp's own scale-up intro under a Z-seam wrapper tween	Arrive composed; stage entrances after the seam settles or match the sign
Mismatched blur/opacity at the swap	Identical values at the cut frame
Gentle entry easing (power2.out)	Mirror the exit: power4.out / expo.out
Full off-screen exits/entries	Partial travel (~12%) + early fade
.inOut eases on either side of a cut	Mirrored power4.in / power4.out
Lone element fading long before its cut	Fade ends ~0.02s before the cut, or word-cascade
Equal gaps across a waterfall cascade	Shrink gaps ×0.84 per word
Zoom-through on body text	Headlines and short phrases only
Scene cuts without cut-the-curve	It is the default boundary
Consecutive boundaries in opposing directions	One current; reserved vectors spent on meaning
Unpainted #root behind a mid-window cut	Opaque stage ground
Queued entries (each waits for the previous to settle)	Overlap ±1–2 frames — the cascade is a wave, not a queue
Same offset/duration for every cascade element	Vary by weight: anchors travel further, punctuation snaps
Gradual opacity fade on a §6 arrival	Binary 0→1 via tl.set — fading fights the snap (seam cuts fade; arrivals don't)
Single ease for a group slide (power4.inOut, slow())	The §7 three-phase chain
Nudge tail shorter than 3× the ramp-in	Extend the tail's TIME, not its distance

Code

All GSAP templates — worker-authored versions, registry gsap_templates, the combined cut-the-curve + zoom, waterfall DOM/CSS/JS, rack-focus — live in examples/gsap-implementation.md.