[fprint] Aspect ratio of images is wrong?

[Martin Hejnfelt]

Hi Alan,

On Fri, 2016-09-23 at 14:23 -0400, Alan Davidson wrote:
> Hi libfprint folks -
> 
> I've got a U.are.U 4500 reader. I see in libfprint/drivers/uru4000.c
> that the image dimensions are hardcoded to 290x384 pixels. However,
> that doesn't appear to be the right aspect ratio for my hardware.
> Running fprint_demo, I can see 5 scans side-by-side by enrolling a
> print. If I rotate my finger 90 degrees between these scans, I see
> that the horizontal axis is stretched out compared to the vertical
> one (features that are left-to-right of each other in one image are
> noticeably further apart than those same features in another image
> where they're up-and-down). If I enroll my finger vertically and then
> scan it sideways, the scans don't match.
> 

I think this is what is mentioned on the (rather old) URU4000 drivers (
https://www.freedesktop.org/wiki/Software/fprint/libfprint/uru4000/)
page, which also mentions some anti distortion code to deal with this.

However isn't this a rather "thought up" issue, as the unit really
seems meant to be used in one direction only. I would however think it
would make a (small?) difference in minutiae detection if the finger is
not placed straight in the oval direction of the sensor pad.

> I've also been using the closed source drivers from DigitalPersona
> themselves, which do not have this problem: the distances between
> corresponding minutiae don't vary as I rotate my finger, and it has
> no trouble matching fingers that have been rotated. I believe the DP
> drivers think the resolution of the reader is 354x391 (found by
> comparing the bytes it records when you save fingerprints to a file
> with Slide 10 of [1]).
> 
> Has anyone else seen this? Is there a way to query the hardware to
> see what resolution images it thinks it records? How can I adjust the
> libfprint scans so they match fingerprints regardless of orientation?
> 

I am using the unit without issues in a 1:n scenario. I must admit I
have not tried rotating my finger. Question is if one could find the
anti-distortion code mentioned (or if it is already there, I have not
gone through the git history).

Regards,
Martin