The cover is the first image reached by EPUB 3's cover-image property,
EPUB 2's <meta name="cover"> (by id, or by address where a converter wrote
one), the guide's cover page and the first <img> or SVG <image> on it, and
last any manifest image whose id or address says cover. A route that names
something unusable leaves a note and the next is asked.
Addresses are resolved against the document they are written in, with the
fragment off and percent-escapes undone; one that climbs out of the archive
or carries a scheme is not followed. Every member is read through a bound,
checked against the size the archive declares and enforced on the read
itself, so an understated entry stops at the cap too.
The media type is read off the bytes, now WebP as well, and a manifest that
disagrees is a note; bytes that are not an image are no cover. MAX_COVER
(16 MiB) is the cap for both formats, so a Kindle cover record larger than
that is left out as well.
A Kindle book's title, authors, publisher, description, ISBN, ASIN,
subjects, date and language come from the EXTH block, with the full name,
the Palm database name and the header's locale as fallbacks. The cover
image (EXTH 201, or the thumbnail) is returned as Metadata::cover with its
media type; it stays out of the JSON record. Every offset is bounds-checked:
a damaged header gives an empty record with a note, a broken EXTH block
keeps what came before the break, and an encrypted book is still read and
noted as drm.
Metadata gains description, which the EPUB reader now fills from
dc:description as well.
Fixtures are a story and cover written for the tests (CC0), converted with
calibre to MOBI 6 and AZW3, plus no-EXTH, DRM-flagged and truncated copies.
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