[Numpy-discussion] Solving a NLLSQ Problem by Pieces?
Wayne Watson
sierra_mtnview@sbcglobal....
Sun Jun 27 09:29:29 CDT 2010
Thanks for the info. Where do I find hugin? The paper I'm looking is
from 1987. It is not clear what actual method he used. Probably
something popular of those times. Things have changed. :-)
Yes, scipy might be a better place to post this, and I will. Hmmm, I
think I have had problems finding a connection to a scipy mail list.
Astropy is lightly traveled, but I'll give it a shot.
On 6/26/2010 10:32 AM, Anne Archibald wrote:
> The basic problem with nonlinear least squares fitting, as with other
> nonlinear minimization problems, is that the standard algorithms find
> only a local minimum. It's easy to miss the global minimum and instead
> settle on a local minimum that is in fact a horrible fit.
>
> To deal with this, there are global optimization techniques -
> simulated annealing, genetic algorithms, and what have you (including
> the simplest, explore the whole space with a "dense enough" grid then
> fine-tune the best one with a local optimizer) - but they're
> computationally very expensive and not always reliable. So when it's
> possible to use domain-specific knowledge to make sure what you're
> settling on is the real minimum, this is a better solution.
>
> In this specific case, as in many optimization problems, the problem
> can be viewed as a series of nested approximations. The crudest
> approximation might even be linear in some cases; but the point is,
> you solve the first approximation first because it has fewer
> parameters and a solution space you understand better (e.g. maybe you
> can be sure it only has one local minimum). Then because your
> approximations are by assumption nested, adding more parameters
> complicates the space you're solving over, but you can be reasonably
> confident that you're "close" to the right solution already. (If your
> approximations are "orthogonal" in the right sense, you can even fix
> the parameters from previous stages and optimize only the new ones; be
> careful with this, though.)
>
> This approach is a very good way to incorporate domain-specific
> knowledge into your code, but you need to parameterize your problem as
> a series of nested approximations, and if it turns out the corrections
> are not small you can still get yourself into trouble. (Or, for that
> matter, if the initial solution space is complex enough you can still
> get yourself in trouble. Or if you're not careful your solver can take
> your sensible initial guess at some stage and zip off into never-never
> land instead of exploring "nearby".)
>
> If you're interested in how other people solve this particular
> problem, you could take a look at the open-source panorama stitcher
> "hugin", which fits for a potentially very large number of parameters,
> including a camera model.
>
> To bring this back nearer to on-topic, you will naturally not find
> domain-specific knowledge built into scipy or numpy, but you will find
> various local and global optimizers, some of which are specialized for
> the case of least-squares. So if you wanted to implement this sort of
> thing with scipy, you could write the domain-specific code yourself
> and simply call into one of scipy's optimizers. You could also look at
> OpenOpt, a scikit containing a number of global optimizers.
>
> Anne
> P.S. This question would be better suited to scipy-user or astropy
> than numpy-discussion. -A
>
> On 26 June 2010 13:12, Wayne Watson<sierra_mtnview@sbcglobal.net> wrote:
>
>> The context here is astronomy and optics. The real point is in the last
>> paragraph.
>>
>> I'm looking at a paper that deals with 5 NL (nonlinear) equations and 8
>> unknown parameters.
>> A. a=a0+arctan((y-y0)/(x-x0)
>> B. z=V*r+S*e**(D*r)
>> r=sqrt((x-x0)**2+(y-y0)**2)
>> and
>> C. cos(z)=cos(u)*cos(z)-sin(u)*sin(ep)*cos(b)
>> sin(a-E) = sin(b)*sin(u)/sin(z)
>>
>>
>> He's trying to estimate parameters of a fisheye lens which has taken
>> star images on a photo plate. For example, x0,y0 is the offset of the
>> center of projection from the zenith (camera not pointing straight up in
>> the sky.) Eq. 2 expresses some nonlinearity in the lens.
>>
>> a0, xo, y0, V, S, D, ep, and E are the parameters. It looks like he uses
>> gradient descent (NLLSQ is nonlinear least squares in Subject.), and
>> takes each equation in turn using the parameter values from the
>> preceding one in the next, B. He provides reasonable initial estimates.
>>
>> A final step uses all eight parameters. He re-examines ep and E, and
>> assigns new estimates. For all (star positions) on the photo plate, he
>> minimizes SUM (Fi**2*Gi) using values from the step for A and B, except
>> for x0,y0. He then does some more dithering, which I'll skip.
>>
>> What I've presented is probably a bit difficult to understand without a
>> good optics understanding, but my question is something like is this
>> commonly done to solve a system of NLLSQ? It looks a bit wild. I guess
>> if one knows his subject well, then bringing some "extra" knowledge to
>> the process helps. As I understand it, he solves parameters in A, then
>> uses them in B, and so on. I guess that's a reasonable way to do it.
>>
>> --
>> Wayne Watson (Watson Adventures, Prop., Nevada City, CA)
>>
>> (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
>> Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
>>
>> Air travel safety Plus Three/Minus 8 rule. Eighty %
>> of crashes take place in the first 3 minutes and
>> last 8 minutes. Survival chances are good in you're
>> paying attention. No hard drinks prior to those
>> periods. No sleeping pills either. Sit within 5 rows
>> of an exit door. --The Survivors Club, Ben Sherwood
>>
>>
>> Web Page:<www.speckledwithstars.net/>
>>
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>>
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--
Wayne Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
Air travel safety Plus Three/Minus 8 rule. Eighty %
of crashes take place in the first 3 minutes and
last 8 minutes. Survival chances are good in you're
paying attention. No hard drinks prior to those
periods. No sleeping pills either. Sit within 5 rows
of an exit door. --The Survivors Club, Ben Sherwood
Web Page:<www.speckledwithstars.net/>
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