0055. Exports take a default instead of asking, and effects can be seen

Status: accepted (export-defaults-and-visible-effects change, 23 October 2026). Revises decisions 0022 and 0026.

Context

Decision 0026 made a video of effects that repeat forever, which is every animate block without repeat, a usage error until a duration was given; a diagram without moving effects was refused the same way. The command line and the playground refused, VS Code asked in an input box, and an export profile without duration was export.missing-duration. The same kind of refusal blocked a transparent MP4, at the command line and in profiles (export.invalid-option), and a picture or video over the capture limit of 16384 pixels a side. On 2026-10-22 the owner hit the GIF question and stated the rule: never block an action for the sake of it; pick a sensible default and say so.

The same export showed a second problem. A headless run proved the GIF sound (distinct frames, the loop block, played by the browser) but its motion nearly invisible: decision 0022 coloured flow particles and pulse halos with the target’s own stroke, so a particle was a 5 px grey dot on a grey line and a halo took the pale #DCE5ED border of a box on a near-white background. A particle also drew above every relation, over the letters of its own label.

Decision

• A video without a duration lasts one loop of its effects. defaultMotionDuration(plan) in @meltemi/diagrams-motion: the least common multiple of the effects’ cycles after the longest delay, so a looping GIF is seamless (motionLoop); when that would pass 10 s, the longest cycle after that delay; 1 s for a diagram without moving effects. With the engine’s defaults (flow 1.2 s, dash and pulse 1 s, trace 1.5 s) any mix lasts 6 s. A given duration and the end of finite effects win. produceExport applies it for every host and profile, and export.missing-duration is gone.
• What an export chose is said, not hidden. ExportOutput.notices carries sentences in the interface language (the default duration, the longest cycle, a still diagram, the MP4 background, a clamped size). The command line prints them on stderr after Wrote, as <file>: <notice>; VS Code adds them to the message after the export and no longer asks; the playground adds them to its status line and shows the length an empty Duration field gives as its placeholder.
• A transparent MP4 takes the theme’s background, at the command line and in profiles, which compile without a diagnostic. In the browser, whose encoders have no alpha, a transparent WebM takes it too, and the notice says the command line keeps it.
• Over the limit, the largest that fits. A PNG is captured at the largest scale that keeps each side within 16384 pixels (the exact ratio, so the long side is the limit), a PDF page and a video size shrink keeping their proportions. Values outside an option’s range (--scale 1 to 8, --size 16 to 16384, a duration that does not parse) stay usage errors: that is validation of what was typed, not a block.
• Particles and halos take the accent. Flow particles, pulse halos and traces default to the theme’s accent; a moving dash keeps its line’s colour, since it animates the line. A particle is a disc of max(4, 1.5 × width) and rides inside its relation’s group under the label, in the frame and the CSS profiles alike; color still overrides.

Alternatives considered

• A fixed default length (5 s): it cuts most cycles mid-way, so the GIF jumps at its loop.
• One cycle of the longest effect: it cuts the others.
• A single-frame video for a still diagram: some players refuse to open it, and it differs per format.
• Switching a transparent MP4 to WebM: it changes the file the person asked for.
• Sizing particles in output pixels: the canvas and a 960 × 540 GIF would draw different pictures of one diagram.
• A contrast rule choosing stroke or accent per theme: the particles of one diagram would change colour with its palette.
• Structured notice codes for the hosts to word: nothing reacts to them; strings are enough.

Consequences

• No export of the product stops for a value it can pick; every chosen value is named.
• Animated pictures change: particles and halos are blue in the default theme. No stored snapshot draws an animated frame, and the canvas and player browser suites check the new geometry.
• A 6 s default makes larger files than a 2 s one; --duration and the Duration field take any length, and the cap bounds the default at 10 s.
• New public surface: defaultMotionDuration, motionLoop, DEFAULT_DURATION_CAP_MS and STILL_DURATION_MS in the motion package, ExportOutput and its notices in the editor package. The language stays 0.2; export.missing-duration and the editor’s refusals endless, still, transparentMp4, tooLarge and videoSide are gone.