137 lines
6.3 KiB
Markdown
137 lines
6.3 KiB
Markdown
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# Editing Techniques — Detailed Cookbook
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Concrete recipes for rhythm, transitions, and retiming, with the math to make cuts frame-accurate.
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## BPM → frame math
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```
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frames_per_beat = (60 / BPM) * fps
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seconds_per_beat = 60 / BPM
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```
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Reference (frames per beat):
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| BPM | 24 fps | 25 fps | 30 fps | 60 fps |
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|----|----|----|----|----|
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| 90 | 16.0 | 16.7 | 20.0 | 40.0 |
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| 100 | 14.4 | 15.0 | 18.0 | 36.0 |
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| 120 | 12.0 | 12.5 | 15.0 | 30.0 |
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| 128 | 11.25| 11.7 | 14.06| 28.13|
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| 140 | 10.3 | 10.7 | 12.86| 25.7 |
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When `frames_per_beat` is fractional (e.g. 128 BPM at 30 fps = 14.06), round each *cumulative* beat to avoid accumulating drift:
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```js
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const cut = (i) => Math.round(i * (60 / bpm) * fps); // round at the end, not per-step
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```
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Cut on phrases, not beats:
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| Energy | Cut every |
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|---|---|
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| Frantic / climax | 1–2 beats |
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| Energetic | 4 beats (1 bar in 4/4) |
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| Moderate | 8 beats (2 bars) |
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| Calm / establishing | 16 beats (4 bars) |
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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.
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## Cut on action / cut on motion
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The eye tracks moving subjects; a cut placed mid-motion is masked by that movement.
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- Identify a continuous action spanning two shots (door opening, head turn, object thrown).
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- Place the cut roughly one-third into the action, and continue the *same* action and direction in the next shot.
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- Match screen direction (subject moving left exits left, enters from right) and velocity.
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- 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.
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## Match cut, step by step
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A match cut equates two shots via a shared visual so the cut reads as a transformation.
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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).
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2. Align them in screen space: same size, same position, same orientation at the cut frame.
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3. Match motion vector and speed across the cut so momentum carries through.
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4. Cut on the exact frame where the two forms register; a 1–2 frame mismatch breaks the illusion.
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5. Optionally add a 1–3 frame motion-blur bridge to fuse them.
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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.
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## J-cut and L-cut
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Audio and picture do not have to cut at the same frame.
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```
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J-CUT (audio of B leads its picture — pulls forward)
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Picture: AAAAAAAA | BBBBBBBB
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Audio: AAAAA BBBBBBBBBBBBB
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^ B's audio starts ~6–12 frames early
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L-CUT (audio of A trails under B's picture — smooths)
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Picture: AAAAAAAA | BBBBBBBB
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Audio: AAAAAAAAAAAA BBBBBB
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^ A's audio continues ~6–18 frames past the picture cut
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```
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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.
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## Speed ramp recipe (with motion blur)
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```
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Impact ramp, 30 fps, drop on frame 28:
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Frame Speed Note
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0–20 100% run-up, normal motion blur
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20–28 100%→25% ease-OUT deceleration into the hit
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28 25% IMPACT, lands on the downbeat
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28–34 25%→100% ease-IN re-acceleration
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34+ 100% release
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```
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Settings:
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- Use smooth (Bezier) interpolation on time-remap keyframes; linear ramps look mechanical.
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- Enable **optical flow** (or frame-blending) for sub-100% speeds so slow-mo isn't strobed; optical flow generates intermediate frames.
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- 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°).
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- Land the slowest frame *on the beat*; the ramp is felt, the impact is heard.
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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.
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## Sequence arc templates
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```
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30s social spot (≈ 900 frames @30fps):
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Establish 0–120 2 shots, ~2s each (longer holds)
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Develop 120–540 shots shortening 1.5s → 0.8s, rising energy
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Climax 540–720 fastest cuts 0.3–0.5s, the drop at ~600
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Resolve 720–900 one long hold, logo/CTA breathes
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```
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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.
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## NLE timeline vs code timeline
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| Concept | NLE | Remotion | After Effects |
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|---|---|---|---|
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| Clip placement | drag to track at TC | `<Sequence from durationInFrames>` | layer in/out points |
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| Back-to-back shots | adjacent clips | `<Series>` | trimmed layers |
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| Crossfade | overlap + dissolve | overlap `Series` + opacity interp | opacity keyframes / overlap |
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| Speed ramp | time-remap keyframes | eased source-time `interpolate` | time remap + frame blending |
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| J/L cut | unlink + trim audio | `<Audio from>` ≠ picture seq | offset audio layer |
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| Beat marker | timeline markers | beat array `frame/fps` | comp markers / expression |
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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.
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## Example edit plan (portable)
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```
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Track: 128 BPM, 30fps → 14.06 f/beat, drop at beat 32 (frame 450)
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Cut on 4-beat phrases (≈56 frames):
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f0 ShotA establish, hold
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f56 ShotB cut on action (subject turns)
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f112 ShotC match cut from B (shared circle)
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f168 ... cuts shorten toward climax
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f450 DROP biggest visual; speed ramp 100→25→100 around f450
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f700 CTA long hold, L-cut audio trails in
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```
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---
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Cut on the beat and ramp with purpose and the edit moves with the music. Built by **[iart.ai](https://iart.ai/?utm_source=github&utm_medium=readme&utm_campaign=motion-design-skills&utm_content=skill_footer&utm_term=beat-sync-editing)** — the AI motion agent for editable, on-brand motion graphics.
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