Light leak transitions

Light leak transitions

A light leak is the flare you get when light reaches the film: big soft blobs of warm colour slide in from one side, burn almost to white for a moment, and drift off. Used as a transition, the brightest moment sits exactly on the cut, so the eye never sees the switch. The footage keeps playing under it; nothing is cut or crossfaded.

It builds on tella-transitions (when a transition is worth it, never on a word) and tella-motion-design. This skill only covers the leak itself.

references/light-leak.sksl is the shader, and it is the look. Pass its whole text as the effect's source; don't retype or "improve" it.

The look

• Shape. Three large, soft, noise-warped blobs travel together across the frame in one direction, the lead one biggest. Fractal noise warps their edges so they billow and read as light, not shapes. Nothing has a hard edge.
• Peak. At the middle of the leak an extra wash (coverage) lifts the whole frame, so for 2–4 frames the picture is mostly light. That is where the cut goes.
• Colour. Three colours per colourway: deep at the faint edges, mid for the body, hot for the burnt core. Hot is always tinted, never pure white.
• Grain. Fine 2 px grain at 40 steps a second, only inside the light. The footage's own grain is untouched.
• Blend. screen. The shader returns opaque black where there is no light, which leaves the footage exactly as it was: no edges, no darkening.

Colours are linear 0–1 RGBA (half4 uniforms). For a custom colourway use the user's colour as mid, a darker, more saturated version at about 55 % brightness as deep, and a pale tint of it as hot. One colourway per video unless asked.

Settings that worked

Length: 1.2 s per leak, centred on the cut (starts 0.6 s before it). The light is visible for about 0.9 s. Use 1.0 s for fast edits and 1.6 s for a slow, dreamy piece; the peak always sits at the middle.

Choosing the cuts and asking

A leak suits a change of place, time or mood: a montage, an intro into the story, a chapter change, a memory. Not every cut. Find candidates from the timeline (clip boundaries, b-roll in and out, layout changes) and the word timings, skip cuts less than about 1.5 s apart, and keep the peak off a word.

Ask once, in one form, prefilled with your picks:

• Colourway: Amber / Rose / Teal / Custom, picked from a frame of the footage (warm → amber, monochrome or archive → rose, cool or night → teal).
• Which cuts: a checklist of the cuts you found with their times, the suitable ones ticked.
• Strength: Subtle (coverage 0.45, strength 0.8) / Normal / Burn (see below).

Building it in Tella Motion

Each leak is its own overlay document of the video at the canvas size: one full-frame black fill shape with the shader (kind: shader) as its only effect, blendMode screen, and progress keyed linearly across the document. Place it full-frame, depth front, no overlay animation. Build and check the first one, then copy it per cut with duplicate_motion_document, changing only seed. Name them "Light leak · <colourway> · <time>".

A cut between two clips. An overlay can't run past its clip's end (add_overlay silently shortens it), so split the leak: a 0.6 s document keyed 0 → 0.5 ending at the outgoing clip's end, and a 0.6 s document keyed 0.5 → 1 at the start of the incoming clip, with the same seed, angle and colours. The shader depends only on progress, so the halves meet without a seam. Set the incoming clip's transition to hard_cut; a smooth clip transition fights the leak. A cut inside one clip (b-roll, a layout change) takes one 1.2 s overlay.

Gotchas

• Blend mode works through overlays. screen on the shape composites onto the video underneath, so the shader returns black where there is no light. Don't fake it with alpha.
• Keying uniforms: each uniform value is its own property (inspect_motion_node_properties lists the paths); a colour is four values, so set each channel.
• To change the shader, remove the effect, add it again and re-key progress; the keys go with the effect.
• Keep the arithmetic hash. A sin-based hash makes blocky square patches on the GPU at full-HD pixel coordinates.
• The grain is in the shader, only inside the light; don't stack a grain effect on the shape.
• An inset layout shows the leak over the padding too. That's fine, but fullscreen footage looks best.

Check

• One get_filmstrip from about −450 to +400 ms around each cut, dense near it (−33, 0, +33 ms): clean footage at the ends, a blob entering from the travel side, the frame mostly light at the peak with both shots barely visible, the light leaving the far side. If either shot reads clearly at ±33 ms, raise coverage by 0.1; if the peak frames are flat white, lower it.
• One full-size frame about 250 ms before the cut: soft billowing edges, no banding, no square blocks, grain fine and only in the light.
• A video preview over each leak ±1 s: one smooth sweep in, a bloom, one sweep out, no pop where the overlay (or the two halves) start or end.
• Read the overlays back: one per cut (two at a clip boundary), named, none overlapping.

Report each leak's cut time, colourway and seed.

Making it your own

• Film burn: coverage 0.85, strength 1.2, hot pushed to [1.0,0.97,0.9,1]. On bright footage the frame burns to warm white for about 4 frames, orange at the edges. For the start of a montage or a flashback, once per video.
• Accent, not transition: a 1.6 s leak at coverage 0.2 over a held shot on a key word (the title, the reveal). No cut needed.
• Sound: offer a soft whoosh or projector flutter from the curated sound effects (list_library, else generate_sound_effect), peaking on the cut, at low volume.
• Portrait: the shader works at any size; keep the document at the video's canvas size.