@corerus/chorus-web

Reference web renderer for Caption with Intention manifests. DOM plus variable-font axes, drives off any <video>.

npm install @corerus/chorus-web

import { CwiRenderer } from '@corerus/chorus-web';

const renderer = new CwiRenderer(document.getElementById('captions'));
renderer.load(manifest);
renderer.bind(document.querySelector('video'));   // or observe() + seek() yourself

Roboto Flex is required. The intonation layer animates its wght and wdth axes; a static font silently loses that entire dimension.

Two things it gets right that are easy to get wrong

No reflow. Each line is laid out once at each word's volume-derived size, so the white read-ahead text already occupies its final geometry and the 15% onset pop is a pure transform. Animating font-size instead reflows the whole line on every word.

Frame-relative sizing. Type size resolves against the video frame, not the DOM element, so letterboxed and pillarboxed playback still yields the spec's 5% baseline.

API

• new CwiRenderer(container, options?)
• load(manifest) — swap the caption track
• bind(media) — observe the element and drive from its clock
• observe(media) — track its frame box only; you own the timebase
• seek(seconds) — render one instant; cheap enough to call per frame
• setOptions(patch) / getOptions() — override spec constants
• destroy()

Honours prefers-reduced-motion.

About the design system

This package implements the Caption with Intention design system (V1.0, 2025.1), created by FCB Chicago with the Chicago Hearing Society. This toolchain is MIT; the design system is not this project's to license. Its specification PDF is marked All Rights Reserved and the system is marked ©, despite widespread "open source" framing in press coverage — there is no licence file or repository anywhere upstream. Seek written clarification from [email protected] before commercial deployment.

Roboto Flex, which the system requires, is separately available under the SIL Open Font License.