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Pressline

Pre-rendering and hosting

Pressline never stores image bytes. It hot-links your Preview and validates your Printfile by reading its header with one ranged GET, so:

  • URLs are public and immutable. A Printfile URL for a (Design, Spec Hash) pair never changes content. Put files under a content-addressed key (the sample uses printfiles/<designId>/<specHash>.png) in R2, Vercel Blob, S3 or your own disk; serve Range requests if you can (Pressline reads at most 64 KiB, and expects a PNG’s IDAT to start inside it). Use one dot in the last path segment (<specHash>.png, never name..png): Printful’s upload refuses a two-dot filename.
  • Design IDs are unguessable: 8–128 characters of [A-Za-z0-9_-], random. Anyone with the ID can see the Preview.
  • Pre-render on finalize. Fetch GET /api/offers from the Pressline instance: every Offer and variant comes with its Printfile Spec. Render a Printfile per Spec while the Customer is still in your app and checkout never waits; a Spec you cannot render (aspect, budget) makes that Offer ineligible via offers.
  • Sync or 202. If a render takes longer than a request, answer 202 { retryAfterMs } and finish in the background; Pressline polls, and the Storefront tells the Customer to wait. Answering 200 synchronously is fine when rendering is fast.
  • Pressline verifies: exact size; 8-bit; non-interlaced; not palette-indexed; a declared color space that is not CMYK or grayscale; alpha per the Spec; a file under 200 MB; specHash echoed, Content-Type and size as declared. Anything else it can see but does not enforce it records as a Deviation and shows the Operator; a Deviation never blocks a sale.

The sample Engine does all of this in a few hundred lines.