Hi all, so here's a follow up to my investigations;
I figured out why I had bad matches: I had to revert the last change I made
to my scripts, that removed TrZ optimization. Now I understand that the
optimizer needs to be able to tweak TrZ to adjust TrZ, FoV and resolution
together.
I'm back to good matches, but the trapezoidal distorsion is also back. So
far it has never been severe anymore (maybe I've been lucky though), but
it's noticeable enough that I prefer manual stitching when it happens :/
To rule out some slant in scanned images, I took an example of 4 scans that
demonstrate the issue when stitched together, rotated all of the 180° (no
rescan) and stitched them again that way;
And both time I get trapezoidal distorsion with left end taller than right
end, so it doesn't follow images content, it seems there's some bias in the
stitching process itself:
http://ks369561.kimsufi.com/~petchema/gfx/I121-19/I121-19.jpg
http://ks369561.kimsufi.com/~petchema/gfx/I121-19/I121-19r.jpg
I created a tar with all resources needed to reproduce the problem, if
somebody wants to investigate:
http://ks369561.kimsufi.com/~petchema/gfx/I121-19/I121-19.tgz
--
Pierre
Le samedi 6 août 2022 à 11:15:07 UTC+2, Pierre Pierre a écrit :
> Thank you both for suggestions, I wasn't sure how to best measure
> shearing, so I scanned a try square multiple times, changing position and
> orientation, then used scalar product to calculate angle, and only found a
> few thousandth of degree discrepancies, which must be below measurement
> errors. If shearing exists, it's probably very small.
>
> I added `--set x=0,y=0` to my `pto_var` call, it does seem to have the
> intended effect (limit transformations to translations and rotations)
> sometimes with visible output artefacts that are probably due to poor
> automatic selection of control points. I will see over time how often it'll
> require manual optimization, but detecting matching errors seems like an
> improvement.
>
> --
> Pierre
>
>
>
> Le vendredi 5 août 2022 à 20:16:22 UTC+2, [email protected] a écrit :
>
>> Hi Pierre, I would first check that both the pitch and yaw of all your
>> input photos is set to zero and that the output projection is set to
>> rectilinear (not equirectangular).
>>
>> Some scanners can produce quite strong 'shear' distortion, like a
>> parallelogram, if the head is not exactly perpendicular to the tracks.
>> Panotools/Hugin has g & t lens optimisation parameters that will correct
>> this. You only need one or the other depending if your shear is horizontal
>> or vertical.
>>
>> I *think* g & t optimisation is hidden under a Hugin 'expert user'
>> setting, I'm not at a computer at the moment so can't verify.
>>
>> --
>> Bruno
>>
>>
>> On Fri, 5 Aug 2022, 19:42 Pierre Pierre, wrote:
>>
>>> Hi all,
>>>
>>> First, some background, I'm currently trying to digitize all my father's
>>> paintings and drawings; For artworks somewhat larger than my scanner I
>>> needed stitching, and have been using hugin for that purpose for a few
>>> months now.
>>> Using a script heavily inspired by the one referenced in the "stiching
>>> scanned image" tutorial, or the script from Matthew Petroff (
>>> https://github.com/mpetroff/stitch-scanned-images) usually gives good
>>> results.
>>> The main issue I'm often facing is that the output image has some kind
>>> of trapezoidal distortion, from mild to pronounced: usually the left end is
>>> taller than the right end.
>>> I don't know why this happens, images overlap correctly and I can stitch
>>> the same images manually without too much effort (it's just a lot slower).
>>> My current hypothesis is that output uses a rectilinear projection, but
>>> I'd need a projection that's both rectilinear and conformal. I there a way
>>> to request such optimization, or to postprocess control points mapping to
>>> convert it to a conformal mapping?
>>> Thank you for your attention,
>>>
>>>
--
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