Zoom blur

Zoom blur

One treatment for a short stretch of the user's own footage: on the hit, the frame bursts into streaks that radiate from a point on the subject, the colour splitting red-outside, blue-inside at their ends, and under a second and a half later the clip is clean again. It sits over the clip as a full-frame Tella Motion overlay that filters whatever is beneath it, so it works on camera footage, b-roll or a screen recording in any layout. Audio is untouched. It is an accent: one or two per video.

Builds on tella-motion-design (timing to words, checking) and tella-camera-moves (pair it with a punch-in layout for more impact; it is not a camera move itself). For a rainbow edge split without the blur, a glow, or a ripple, this is the wrong skill.

Ask once

Work these out yourself and never ask:

• The hit: from get_transcript word timings, the stressed word of the line (an impact word, "and then…", a reveal, a number); with music, get_music_beats on the background music (or the clip's source); with no speech, the loudest transient in get_audio_levels (10 ms steps) or, from frames, the moment of action (a pop, a landing, a cut-in). Pick the strongest one in the clip the user means, or in the current selection.
• The subject's position at each key, from get_clip_frame at hit − 0.25 s, hit + 0.2 s and hit + 0.5 s.
• The canvas size, the clip and overlay times, and the video's flashes already there.

Then one ask_form, every field prefilled so submit alone builds it:

• Show the pick options as words only; no pictures are needed.
• No media to ask for: the effect uses the clip's own footage. If the stretch has no footage (a blank clip, a pure Tella Motion clip), say so with ask_question and offer "lay it over the next clip", "add a vintage library clip from list_library scope vintage as b-roll first", or "skip".
• Beat mode without background music or a range: ask with ask_question "Which stretch should hit on the beat?" with up to three suggested ranges from the transcript.
• Skipped form: build with the prefilled values.

Timing

Times are on the overlay's own clock (hit = the moment − the overlay's start; the overlay starts at least 0.9 s before the first hit and ends about 0.45 s after the last decay).

• The peak lands on the word's first syllable or the beat, not before it.
• Beat mode (untested): the same five-key envelope per hit, one overlay per clip the range covers (see "Across clips" below); each envelope must reach 0 before the next one's rise. Keep hits at least 1.5 s apart (at most two in any 3 s, for viewers sensitive to flashing); drop beats to get there, and shorten the length to fit.
• Never let a peak sit over a name, number or on-screen text the viewer has to read.

The look (from the example, 1920 × 1080)

• amount peak 0.7, shoulder 0.62, fringe 0.4, clear 0.04: a tight sharp spot and streaks that reach almost to the centre.
• center x 0.52 fixed; y keyed with easeInOutSine 0.47 at hit − 0.25 s → 0.36 at hit + 0.2 s → 0.30 at hit + 0.5 s, so the sharp spot rides up with the board as it rises. Follow whatever the subject does the same way: one key per position, about 0.3 s apart, only while the blur is visible.
• "The face": centre on the bridge of the nose. "Frame centre": 0.5, 0.5, no keys.
• At full strength the frame is mostly streaks for about 0.4 s; the subject must stay readable in the sharp spot, which is why the centre tracks it.

Building it

1. create_motion_document with the videoId, named Zoom blur: <what it hits>; canvas.set its duration to the overlay's. Show it with add_overlay motionDocumentId on the clip: point 0,0, dimensions the video size, depth: "front", transition: "hard_cut", no animation.
2. One node fx: addShape shape: "fill", color: "#00000000".
3. addEffect on fx: kind: "shader", backdrop: true, the source from references/shader.md as written, uniforms amount 0, center [0.52, 0.47] (or your point), fringe 0.4, clear 0.04.
4. Key with setKeyframes on the paths inspect_motion_node_properties lists (a backdrop has a doubled effect): field:common/effects/0/effect/effect/uniforms/amount/values/0 and …/center/values/1 (and /values/0 for x). Times in seconds on the document's clock.
5. Whoosh: list_library scope default sound effects, a short whoosh, add_sound_effect starting about 0.15 s before the hit, volume 0.3–0.5.

Across clips

An overlay belongs to one clip and can't span a cut, and a document plays in one place only. The moment and the range are on whole-video time; overlay times are on each clip's playback timeline (cuts applied). So:

• Beat mode (untested): split the range at clip boundaries from list_clips (each clip's start = the sum of the earlier clips' cut durations). Build one document and one overlay per clip, each covering only its part of the range, and convert every key to that document's own clock (video time − clip start − overlay start).
• A punch that straddles a cut (a beat or the one hit within a punch-length of a clip's end): part 1 on the outgoing clip, ending on its last frame, with the keys before the cut; part 2 on the incoming clip from 0, with the rest of the keys shifted back and a first key holding the envelope value (and center) in force at the cut, so the blur carries on without a step. If the hit is in the last clip, shorten the decay to end on its last frame instead.
• A hit less than 0.9 s into a clip: start the overlay at the clip's 0 and keep the keys on its clock; the rise may be shortened, never moved off the hit.

A second hit elsewhere: duplicate_motion_document (a document plays in one place only), then move the keys. Name them in order ("Zoom blur 02: landing").

Checking it

• get_filmstrip of the clip at the overlay start, hit − 0.3 s, the hit, hit + 0.2 s, the end of the decay and the overlay end: clean before and after, the peak on the moment, no pop at the overlay's edges.
• One get_clip_frame at the hit, 1920 wide: the subject sharp in the clear spot, streaks even all round, red outside and blue inside at the ends. A blur pulling from empty floor or sky means the centre keys are off.
• A 10 fps get_video_preview across the range: the ramp moves smoothly, no snap where a key jumps.
• Report each hit's time, what it lands on, the peak and the centre.

Gotchas

• The fill must stay clear and its blend mode normal: any other blend mode drops the backdrop and shows the footage unfiltered.
• addAdjustment inside an overlay only sees the overlay's own layers; the clear backdrop fill is what reaches the footage.
• A brand-new document refuses a raw shader insert through node.patch; use addEffect. A shader that fails to compile poisons the document's renders: rebuild in a fresh document.
• Changing the source means removeEffect, addEffect again, then re-keying.
• Keep amount × (1 + fringe) ≤ 1 (see references/shader.md), so with fringes on never push past 0.71.
• Built-in blur or fx.chromaticAberration can't make this: one is not radial, the other draws three ghosts and coloured edge bands.

Variations

• Subtle accent under a talking head: 50–60 %, fringes off, length 0.8 s, centre on the face.
• Stack with a punch-in layout on the same word for a harder hit.