PanoTools NG:
Re: Re: Sigma 8 F3.5 and its sharpness
Roger D. Williams 2007-May-16 07:13:27
On Wed, 16 May 2007 13:42:29 +0900, John Houghton
<#removed#> wrote:
> --- In #removed#, Paul F <fawley@...> wrote:
>>
>> some images only have 10 or so control points in odd places, others
>> have 30 or more...
>> what am I missing?
>>
>
> Calibrate your lens. Control points serve two main purposes. The
> first is to enable the optimizer to evaluate the lens parameters, and
> you need a good spread of points for this. The second purpose is is
> to align the images with each other, and you need only a few points
> per seam for this. So if you have already calibrated your lens, you
> only require a few intelligently placed points per seam. Place the
> points only on distant (stationary) features for which parallax isn't
> an issue. Resist the temptation to put points on nearby objects,
> i.e. don't try to use the optimizer to correct parallax errors.
I have been helped often enough by your great experience and advice,
John, to be reluctant to differ with you on anything, but I would say
that you need more than a few intelligently placed points per seam.
On panoramas made from hand-held shots and to a lesser extent from a
camera on a unipod, there are up to six degrees of freedom for the
relative camera movements between shots. Since it would take at least
two points to define each degree of freedom, that makes at least 12
points at each seam as an absolute mininum. In view of the warping
that is also going on, I would have thought double that to be no more
than a safe minimum.
I also find that quite serious stitching errors can occur for whatever
reason even where parallax is not an issue. Deleting erroneous control
points is fine where that's the cause, but adding control points can
quickly lead to much better seams. I find myself doing this all too
frequently...
Roger
--
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