[fprint] Merging together multiple scans of the same fingerprint?

[Alan Davidson]

Hi libfprint folks -

I've got a Digital Persona U.are.U 4500 fingerprint reader, and I'm trying
to migrate to libfprint from the closed source drivers I got from Digital
Persona themselves.

The driver chosen by libfprint is uru4000, which seems right. When I call
fp_dev_get_nr_enroll_stages on the device, it returns 1: a single scan is
enough to enroll a fingerprint. and indeed, I can scan my finger once and
get back a fp_print_data struct (through the callback passed to
fp_async_enroll_start).

However, the old closed source drivers had a way to capture a set of 4
scans of the same finger (what they called an "enrollment", as opposed to a
single-scan "capture") that were merged together to form a more robust,
reliable view of the finger. Is there a way to mirror that functionality
with libfprint, given that my device seems to use just 1 stage for an
enrollment?

The motivation here is that when someone registers their fingerprint, we
want to make sure we've got a high quality scan on file, so that they can
reliably match it later.

Thanks in advance for your help.

 - Alan
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