Reverse lookup

Reverse lookup answers “where does this file come from, and what are its tags?” from the file itself, without a URL. monloader walks a chain of sources you configure - exact hash searches on the boorus, image-similarity services, and optionally the local Hydrus PTR index - and folds what it finds back into monbooru.

You reach it two ways:

• From monbooru: the “fetch tags” button on an image asks monloader to find tags for a file monbooru already holds and merge them in. The monbooru-side view is in monbooru’s lookup guide.
• From monloader: paste a file’s md5 (bare or as md5:<hash>) into the add bar to import the booru post carrying that file, like any single-post download.

Every lookup runs as a job on the queue page, so its outcome is visible like any download.

The lookup chain

The chain is one ordered list of sources, asked top to bottom; the first source that answers wins. A source is one of two kinds:

• An exact md5 search on a booru that indexes file hashes. Precise, but it only matches the exact original bytes - a resized or re-encoded copy misses.
• A similarity service: danbooru’s IQDB instance (iqdb) or SauceNAO (saucenao). These find visually matching posts, including re-encodes, by uploading the image’s thumbnail (a few KB - never the original file) and answering with candidate posts and 0-100 scores. The best candidate at or above the min similarity floor (default 80%) is then fetched through gallery-dl like any pasted URL.

Out of the box the chain asks danbooru by exact md5 first, then IQDB, then SauceNAO, then gelbooru, rule34, e621, yandere, and konachan by exact md5.

A source that is down or refuses does not kill the lookup; its error is noted and the chain continues.

Editing the chain

The Settings lookup section is the one place the chain is configured. The edit-order dialog has two columns: drag a source between queried (asked top to bottom) and not queried. Each chain row has a test button that runs one real query with a built-in probe image, so “no match” is a successful round trip; the SauceNAO test also reports your remaining daily quota. The similarity floor is set in the same section (lookup.min_similarity in the TOML).

Credentials

A source missing its credential is skipped at query time, so it costs nothing until configured:

• iqdb uses the danbooru site credentials (username and api key from the danbooru row in sites): danbooru refuses anonymous IQDB uploads. The account must be at least Member level (email verified); danbooru denies uploads from restricted accounts even with a valid key.
• saucenao needs its own api key (free registration at saucenao.com; the free tier allows roughly 100 queries a day, and monloader backs off while the service reports its limit reached). Set the key via the key dialog in the lookup section.
• The exact md5 searches follow each site’s normal auth kind - gelbooru and rule34, for instance, need their api key and user id.

When a match cannot be fetched

A similarity service returns pointers, not tags, and sometimes the post it points at cannot be fetched - a missing credential, a deleted post. The best candidate is then still recorded as the image’s source, with no tags, and the queue row is marked source only. A confident hit outside the supported sites (an anime database, a Pixiv or Twitter page) gets the same treatment when no supported site matched: its URL is recorded as the source, labeled by its host.

When no source holds the file at all, the item ends hash_not_found, and its message lists every source asked and why it missed or was skipped. A similarity service that found nothing usable lists its closest candidates with their scores - under the floor, or on a site monloader cannot fetch - so a near-miss stays visible and you can judge whether the floor is too high.

Hash import specifics

A pasted hash carries no image to compare, so a hash import uses the chain’s exact-md5 sources only; the similarity services are skipped. A pasted sha256 is refused: boorus index the md5 of the original upload. sha256 is the hash the PTR backend keys on, and PTR lookups are driven from monbooru, not the add bar.

The PTR as a backend

With the Hydrus PTR index enabled, monloader can also answer from its local sha256-keyed copy of the PTR - offline, and covering files that were deleted from the boorus or never posted there. monbooru’s “fetch tags” button runs every enabled backend: the PTR first when it is on, then the online chain, with both results folded into one merge note. The add bar shows at a glance which lookups are available: the booru chain (hover to see the queried sources in order) and the sha256 lookup with its sync state.