IDML in, the page InDesign would set, out

The Paged Annual is a 134-page document built by driving the real editor. Every page is rendered twice — once by paged, once by InDesign from the same IDML — and the two rasters are compared for colour distance (ΔE2000) and structure (SSIM). The numbers below are that comparison, including the pages that fail it.

The engine repository carries its own gate as well: a licence-clear set of generated fixtures, each with a reference PDF exported by InDesign and thresholds for mean ΔE2000, p99 ΔE2000 and SSIM. Rendering changes are gated on them, and a threshold is tightened after a fix — never loosened to make a failure pass.

Composition ends in a display list — one per page, made of paths, images, gradients and interned glyph outlines. Three backends consume it unchanged: tiny-skia on the CPU, the reference for the fidelity gate; Vello on WebGPU, for every interactive frame in the editor and the viewer; and a PDF writer for print. Nothing is laid out twice, so nothing can disagree.

The pipeline runs in four tiers. Laid-out paragraphs are cached under a hash of their content, unchanged stories are cheap to rebuild, and a text edit in one story reuses the pages it did not touch. That is what keeps an InDesign-class editor responsive inside a browser tab.

IDML is InDesign's open interchange format: a ZIP of XML parts. The designmap names every spread, master and story; the resources carry the swatches, the styles and the preferences. paged reads the parts that decide the page and says plainly which ones it leaves to the host — fonts are supplied, never guessed.

The engine names everything it holds — the document, a page, a frame, a swatch, a style, a table cell, a text range — in one address grammar with 30 forms, and selectors find them the way CSS finds elements. Every property has a typed schema; 403 of them are published in the capability catalog. A single set writes any of them, through the operation that already owns it, as one undo step.

Every change to a document is an operation: typed, serialisable and invertible. A drag in the editor, a line of a script and a command on the terminal all become the same operations, so they all undo the same way. Operations batch: a batch is atomic, costs one rebuild and lands as one undo step, however many objects it touches.

Snapping lives in the engine too: moves, resizes and path edits snap to every visible point and outline, the page, guides and the grid through one resolver in the engine, not in the user interface.

PDF export is the third backend over the same display list, written directly with pdf-writer. It is not a screenshot in a PDF wrapper: what was text is text, what was vector is vector, and the colour stays in the space the designer chose.

pageddrives the same typed door as the editor's worker, so it opens .paged containers as well as IDML and inherits fonts, colour management, PDF export, scripting and undo from the editor's own code path. Every operation in the catalog is a verb through paged invoke; a headless JSON session speaks the protocol line by line.