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Glossary

What is a foreign resource (and its fallback)?

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A foreign resource is a file whose media type is not one of EPUB's core media types, such as a TIFF image. EPUB 3.3 lets you include it only with a fallback, declared in the manifest with the fallback attribute or provided by HTML, that a reading system can show instead.

Why it matters when you convert a PDF to EPUB

When a converter copies files out of a PDF without checking their format, it can produce foreign resources by accident, and then the EPUB is invalid unless each one has a fallback. Converting everything to core media types removes the problem instead of patching it with fallback chains.

Fallback chains have their own rules: every item in the chain must exist in the manifest and the chain must not loop back on itself, which EPUBCheck reports as OPF-040 and OPF-045.

How EpubCC handles it: it never writes foreign resources. Images are re-encoded as JPEG or PNG when they are extracted, so its EPUBs have no fallback attributes at all.

Related terms

EPUBCheck messages about this

  • RSC-032 Fallback must be provided for foreign resources, but found none for resource "…" of type "…".
  • OPF-040 Fallback item with id "…" could not be found.
  • OPF-045 Encountered circular reference in fallback chain.

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FAQ

What is a foreign resource (and its fallback)?

A foreign resource is a file whose media type is not one of EPUB's core media types, such as a TIFF image. EPUB 3.3 lets you include it only with a fallback, declared in the manifest with the fallback attribute or provided by HTML, that a reading system can show instead.