cycle-atlas/.agents/skills/beat-sync-editing/references/editing-techniques.md

Editing Techniques — Detailed Cookbook

Concrete recipes for rhythm, transitions, and retiming, with the math to make cuts frame-accurate.

BPM → frame math

frames_per_beat = (60 / BPM) * fps
seconds_per_beat = 60 / BPM

Reference (frames per beat):

BPM	24 fps	25 fps	30 fps	60 fps
90	16.0	16.7	20.0	40.0
100	14.4	15.0	18.0	36.0
120	12.0	12.5	15.0	30.0
128	11.25	11.7	14.06	28.13
140	10.3	10.7	12.86	25.7

When frames_per_beat is fractional (e.g. 128 BPM at 30 fps = 14.06), round each cumulative beat to avoid accumulating drift:

const cut = (i) => Math.round(i * (60 / bpm) * fps); // round at the end, not per-step

Cut on phrases, not beats:

Energy	Cut every
Frantic / climax	1–2 beats
Energetic	4 beats (1 bar in 4/4)
Moderate	8 beats (2 bars)
Calm / establishing	16 beats (4 bars)

Detect BPM offline (aubio, web-audio-beat-detector, or tap-tempo) and also capture the offset of beat 1 — the grid rarely starts at 0:00.

Cut on action / cut on motion

The eye tracks moving subjects; a cut placed mid-motion is masked by that movement.

• Identify a continuous action spanning two shots (door opening, head turn, object thrown).
• Place the cut roughly one-third into the action, and continue the same action and direction in the next shot.
• Match screen direction (subject moving left exits left, enters from right) and velocity.
• In motion graphics: cut while a shape is mid-travel (transform velocity high), not when it has settled — the motion blur and momentum hide the splice.

Match cut, step by step

A match cut equates two shots via a shared visual so the cut reads as a transformation.

1. Find or design a shape/position/motion shared by the end of shot A and the start of shot B (a round clock → a round wheel; an arm wiping frame → next scene revealed).
2. Align them in screen space: same size, same position, same orientation at the cut frame.
3. Match motion vector and speed across the cut so momentum carries through.
4. Cut on the exact frame where the two forms register; a 1–2 frame mismatch breaks the illusion.
5. Optionally add a 1–3 frame motion-blur bridge to fuse them.

Common forms: graphic match (shape), motion match (direction/velocity), audio match (a sound bridges the cut), and the "invisible wipe" where a foreground object wipes the frame and the next shot is revealed behind it.

J-cut and L-cut

Audio and picture do not have to cut at the same frame.

J-CUT  (audio of B leads its picture — pulls forward)
  Picture:  AAAAAAAA | BBBBBBBB
  Audio:    AAAAA BBBBBBBBBBBBB
                  ^ B's audio starts ~6–12 frames early

L-CUT  (audio of A trails under B's picture — smooths)
  Picture:  AAAAAAAA | BBBBBBBB
  Audio:    AAAAAAAAAAAA BBBBBB
                       ^ A's audio continues ~6–18 frames past the picture cut

Typical offsets: dialogue J-cuts 4–12 frames; ambient L-cuts 12–24 frames. In an NLE, unlink audio/video and trim only the audio edge. In a code timeline, place the <Audio> for shot B with a from earlier than B's picture sequence.

Speed ramp recipe (with motion blur)

Impact ramp, 30 fps, drop on frame 28:
  Frame   Speed     Note
  0–20    100%      run-up, normal motion blur
  20–28   100%→25%  ease-OUT deceleration into the hit
  28      25%       IMPACT, lands on the downbeat
  28–34   25%→100%  ease-IN re-acceleration
  34+     100%      release

Settings:

• Use smooth (Bezier) interpolation on time-remap keyframes; linear ramps look mechanical.
• Enable optical flow (or frame-blending) for sub-100% speeds so slow-mo isn't strobed; optical flow generates intermediate frames.
• Keep motion-blur shutter consistent across the ramp — if the source has motion blur, the slow section keeps it; if synthetic, hold a constant shutter angle (e.g. 180°).
• Land the slowest frame on the beat; the ramp is felt, the impact is heard.

In AE: enable time remapping, keyframe the source time, ease the keyframes, set frame blending to Pixel Motion. In Remotion: drive a remapped source time via eased interpolate and keep effects deterministic.

Sequence arc templates

30s social spot (≈ 900 frames @30fps):
  Establish  0–120    2 shots, ~2s each (longer holds)
  Develop    120–540  shots shortening 1.5s → 0.8s, rising energy
  Climax     540–720  fastest cuts 0.3–0.5s, the drop at ~600
  Resolve    720–900  one long hold, logo/CTA breathes

Plot average shot length as a curve: high at the start, descending to the climax, then a single long shot to release. Energy is the derivative of cut frequency — accelerate cuts to build, decelerate to land.

NLE timeline vs code timeline

Concept	NLE	Remotion	After Effects
Clip placement	drag to track at TC	<Sequence from durationInFrames>	layer in/out points
Back-to-back shots	adjacent clips	<Series>	trimmed layers
Crossfade	overlap + dissolve	overlap Series + opacity interp	opacity keyframes / overlap
Speed ramp	time-remap keyframes	eased source-time interpolate	time remap + frame blending
J/L cut	unlink + trim audio	<Audio from> ≠ picture seq	offset audio layer
Beat marker	timeline markers	beat array frame/fps	comp markers / expression

To port an edit plan: express each cut as a frame number (from the BPM math), each clip as from/duration, each transition as an overlap, and each retime as an eased time-remap. The plan is renderer-agnostic; only the placement syntax differs.

Example edit plan (portable)

Track: 128 BPM, 30fps → 14.06 f/beat, drop at beat 32 (frame 450)
Cut on 4-beat phrases (≈56 frames):
  f0    ShotA  establish, hold
  f56   ShotB  cut on action (subject turns)
  f112  ShotC  match cut from B (shared circle)
  f168  ...    cuts shorten toward climax
  f450  DROP   biggest visual; speed ramp 100→25→100 around f450
  f700  CTA    long hold, L-cut audio trails in

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Cut on the beat and ramp with purpose and the edit moves with the music. Built by iart.ai — the AI motion agent for editable, on-brand motion graphics.