Engine developer guide

An Engine is your app: whatever makes the image a customer wants printed, be it a template editor, a generative model, or a drawing tool. Pressline does not care how the image happens. It cares that you can answer three questions about a design and host the files you produce.

The division is deliberate and load-bearing. The Engine owns pixels; Pressline never renders, decodes or stores image bytes. It reads your file’s header to check it matches the Spec, then hands the provider your URL. That is why your files must stay reachable and unchanged for as long as an order might reference them.

• The protocol — the three endpoints, the Printfile Spec Pressline derives from the provider, and the Spec Hash that identifies exactly one file. Start here.
• Pre-rendering and hosting — why rendering at Finalize beats rendering at checkout, what Pressline validates when it fetches your file, and the rules your URLs must obey.
• Rendering with @pressline/render — an optional helper that produces a file satisfying a Spec: exact dimensions, DPI stamped, sRGB declared, alpha per the rule. Works on Node and on Workers.
• The Printfile format in detail — if you render with your own tools instead: every chunk and marker the file must carry, and what Pressline checks before payment.
• Checking your Engine — run the conformance suite against your Engine before you wire it to anything, Preflight the files it writes before you host them, and list the Offers and Specs an instance will ask you for.
• AI-assisted Engines — what changes when a render takes thirty seconds instead of thirty milliseconds.

If you want a worked example rather than a specification, the repository’s apps/sample-engine is a small, complete Engine built to be copied.