[hugin-ptx] Surprising difficulty

[paul womack]

I've been using Hugin for a goodly while now, and was recently asked
by someone how they should make a high res scan of an old school room poster
they'd bought.

They owned a DSLR with a standard kit lens, so the obvious
answer (to me) was to shoot a multirow pano, and stitch.

Since he's never done such a thing, I though I'd send him
a "how to".

I quickly rigged up my camera, pinned a broadsheet centrepage to a board,
and shot 6 images, lens around 130mm equiv.

By shooting from some distance, and using a long lens,
I minimise parallax issues; in any case the subject is 2D.
The long lens also gains me resolution.

So far, so dull.

I then imported the 2 row, 3 column images into Hugin,
made a few control points between the relevant pairs,
and optimised.

Lovely (I used a pano head...)

Average error 0.6, std dev .37, max 1.7.

The stitch is lovely, but I didn't have the camera "square on",
so the resulting newspaper had a perspective distortion,
which I removed in Gimp.

Then I thought - hang on,  removing (or adding...) perspective distortion
is exactly what Hugin is for, and doing it inside Hugin will save a sampling
step, thus improving quality in the resulting image.

http://hugin.sourceforge.net/tutorials/perspective/en.shtml

So I added some horizontal and vertical line control points to the data set,
set along the newspaper's edges.

And it would NOT optimise cleanly, nor was the resulting stitch
"square".

I find this odd, since (surely...) a set which will optimise "together"
should also be optimisable under a shared perspective transform
(the deskew)

I would truly welcome advise.

The "skewed" and "with lines" projects are attached.

   BugBear

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poster_line.pto
Description: application/ptoptimizer-script

poster_orig.pto
Description: application/ptoptimizer-script