What it is

Pentimento is a labelled corpus for measuring steganalysis: clean photographs paired with copies of themselves that carry a hidden payload.

Steganalysis is the business of looking at a picture and deciding whether somebody hid a message inside it. To measure how well a detector does that, you need images where you already know the answer, and you need a lot of them.

Why another one

The corpora this field runs on cannot be redistributed.

Corpus	What it permits
BOSSbase	No readable licence survives. The organisers claimed rights and required assent before download; mirrors advertising CC0, MIT and Apache are all wrong
ALASKA2	CC BY-NC-ND. The no-derivatives term forbids distributing anything built from it, which is exactly what a stego image is

Both are fine to measure on. Neither can be republished, so a reader who wants to check a published result cannot obtain the data it was measured on.

Pentimento is built from photographs whose licences permit exactly that, and each file's own terms travel with it.

What it is not

It is not a benchmark leaderboard. It is data; what you measure with it is up to you.

It is not comparable to BOSSbase. See Limitations, which is the first thing to read before quoting a number from it.