Hard hit transition

Hard hit transition

A one-second hit on a cut. Shot A starts to judder, the judder grows, and the frame pushes in, rolls and bends into the switch. Shot B arrives at full push, bent and rolled the other way, and shakes itself still. It is a treatment of the footage on both sides of a cut, not an overlay drawn over it: both shots go into one Tella Motion document so the shader can read either at any pixel. Use it for one big moment (a launch, a drop, the start of a sequence) or a few cuts of a montage, never on every cut of a talking video: three or four a minute at most. Builds on tella-transitions (when a transition is worth it, never over a word) and tella-motion-design; tella-audio-sync for beats.

Ask once

Work out yourself, never ask: the candidate cuts (every clip boundary and every jump cut in the selection or the video, from get_timeline with cuts and words), which one is strongest (a change of place or energy, a pause, the first word of a new section; never mid-word), whether the video has background music, and its beats (get_music_beats). The footage is the video's own: nothing to upload.

Then one ask_form, every field prefilled:

The shake runs on both axes, as in the example; there is no direction field (horizontal-only or vertical-only needs a shader change that hasn't been tested). If the user skips the form, use the prefilled values. If a chosen cut has footage under 640 px wide, say that the landing will look soft and suggest 60–70 % strength.

Timing

• The switch is cut × dur after the effect starts: at strength 70 % and above, 0.45 s into a 1.0 s hit; below, 0.30 s into 0.6 s. Call pre = cut × dur and post = (1 − cut) × dur: 0.45 s and 0.55 s at full strength, 0.30 s and 0.30 s below 70 %. The hit starts pre before the cut and ends post after it; every length below uses them.
• On the beat: put the switch on a downbeat (downbeatsMs, else beatsMs) by cutting the end of clip A or the start of clip B so the boundary lands there. Move it at most 0.3 s, never into a word, and skip beats in the track's first 2 s (it fades in).
• Without music, the cut stays put. Never start a hit inside a word on either side; if the pre before the cut holds the end of a word, use the 0.6 s timing.
• At least 1.5 s between two hits; none in the first or last 0.3 s of the video.

The look

Exactly the example (Skill example: Impact transitions (shake: hard hit)): canvas 1920×1080 at 30 fps, a 1.0 s shader on a full-frame black fill from 1.05 s to 2.05 s, switch at 1.50 s. Shader references/shake-cut.md, uniforms at 100 %:

dur 1, cut 0.45, amp 38, hz 14, push 0.25, bend 0.22, warp 0.35, roll 3, blur 1, split 6, flash 0.25, seed 4 (a new number for every hit: 4, 7, 9, …).

Strength s (0.3–1.0) multiplies amp, push, bend, warp, roll, split and flash; hz 14 and blur 1 stay. Below 0.7 use dur 0.6, cut 0.5, hz 12. Never go past 100 %: push above 0.3 or warp above 0.45 turns the landing to radial mush, and amp above 45 looks like a broken stabiliser.

progress is keyed 0 → 1 linear over the hit (the shader eases itself). The motion: A's envelope grows as t² into the switch, the shake starting at a quarter strength; B lands at full push mirrored (opposite bend and roll) and decays as (1−t)². An 8-tap blur over one frame never crosses the switch, so every frame is all A or all B. Shots are fit: "cover" with scale 1.03 (A) and 1.02 (B) so old film's thin black edge never repeats in the mirror tiling.

Building it

Pass the shader text from references/shake-cut.md whole as source; don't retype or "improve" it. Ticks are 254,016,000,000 a second.

Across a clip boundary (tested). A b-roll belongs to one clip, so the hit is two documents, one per clip, each on its own clip's clock:

1. Get each side's footage as a source: the clip's own upload if it is in list_library (private/workspace), else get_clip_preview (width 1920, fps 30) of the frames needed (clip A's last pre, clip B's first post), before adding any b-roll there, then create_source and PUT the file.
2. Per side, create_motion_document with videoId, then one batch: canvas.set duration = that part's length; addMedia nodeId: "shot" with the sourceId; node.patch {fit: "cover", sourceIn, audio: {assetId: "media.<sourceId>", gain: 0, mute: true}}; addShape nodeId: "hit", shape: "fill", #000000, full length; addEffect on hit, kind: "shader", inputs: {shotA: "shot", shotB: "shot"}, every uniform; setKeyframes on field:common/effects/0/effect/uniforms/progress/values/0 with keys [{at: 0, value: p0}, {at: len, value: p1}], easing: "linear". Clip A's part is pre long and runs progress 0 → cut; clip B's is post long and runs cut → 1. Binding one shot to both inputs works because the shader reads A only before the switch and B only after.
3. add_layout on clip A from its end − pre for pre, and on clip B from 0 for post, each transitionStyle: "hardCut", media: {type: "motion", documentId}.
4. A time-ranged b-roll makes a follow-on layout for the rest of the clip, which can carry a spring transition: list_layouts on both clips and update_layout every layout touching the hit to transitionStyle: "hardCut", or clean footage returns through a dip.

A jump cut inside one clip. One document, pre + post long, on that clip from pre before the cut to post after, the clip's rendered footage of that range as shot, progress 0 → 1; placed and hard-cut as in steps 3–4.

Between shots in a Tella Motion montage (the example). Add the shots last first so the earlier one draws on top. A runs to the hit's end, B starts at the hit's start; the shader shape spans the hit with inputs: {shotA, shotB}. Name each shape in order ("Hit 01").

Sound. add_sound_effect with presetId: impact-boom exactly on the switch (its hit is at 0 ms) at 1.0 × strength, on clip B at 0; deep-woosh at 0.4 starting 0.4 s before the switch, on clip A, 0.4 s long. Over music, halve both.

Checking it

• get_video_preview at 1280+, 30 fps, from 1 s before the switch to 1 s after; tile its frames. Every frame before the switch is A, every one after is B; the push builds over the last third, not on one frame; B lands bent the other way and settles in about 0.4 s; no empty border; no jump where a b-roll starts or ends (A continuous into the hit, B continuous out of it).
• Read the preview's loudness in 50 ms bins: the boom's peak lands within one bin of the switch.

Gotchas:

• A shader on a media node sees the source's own pixels, not the canvas: always put it on the separate full-frame shape.
• In a montage, if shot A outlasts the hit it pops back on top for a frame. Trim it to end with the hit.
• A preview taken after the b-roll is placed includes the hit itself; take the footage first.
• Footage at 320–480 px goes to mush at the landing. Stock clips sometimes carry a stray frame at their head: start B's sourceIn a few frames in.
• To change a uniform, setProperty on its field: path; to change the shader, remove the effect, add it again and re-key progress.
• Flashes: keep flash at 0.25 or less and no more than three hits in 3 s.

Variations

• Beat run: several hits at 50–60 % on consecutive downbeats of a montage, two bars apart.
• Vertical (9:16): the shader scales with the canvas; keep the same values.