Implementing Public Watermark Delivery — Keep Original Promo Frames Private

Public watermarks should be delivered by keeping original promotional frames private. Generate a watermarked derivative once and limit it to public cache. For an edtech platform that converts prompts into short promo videos, the poster image displayed in galleries is disposable but cacheable. The clean frame used to regenerate or re-edit the video is not.

If altering an object requires new creative work, it is a master; if it can be recreated from a master and recipe, it is a derivative. Separate storage namespaces and access policies should be used for masters and derivatives. Derive a public key from master ID, watermark recipe version, and output format. Write with create-only semantics and return immutable cache headers after the derivative exists.

Never make the master reachable through the public handler. Implement a private and public store interface. The private store allows reading masters with no public URL method, while the public store enables creating derivatives but not fetching masters. Use a deterministic publication step: decode the private image, draw the supplied watermark, encode JPEG bytes, and create a deterministic public object with a key based on the asset ID and recipe.