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Lsqnonlin optimization unexpected behavior

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OD on 11 May 2021
Answered: Alan Weiss on 11 May 2021
I am experimenting with lsqnonlin, first with a very simple problem: make lsqnonlin find translation params. I have an "original" image I, appy translations and then try to make lsqnonlin find them: results of lsqnonlin are in txy_sol_lsqnonlin.
The problem is that it almost immediately stops and always gives me useless results i.e. the same init values that I give it + maybe 0.5 or something like if it can't do more than a few steps and immediately stop searching anymore... Please help me find the problem.
The full code is :
I = phantom(256);
I_tr = imtranslate(I,[tx ty]);
fun = @(txy) [reshape( imtranslate(I,[txy(1) txy(2)])-I_tr,[],1) ];
tx_init = 0;
ty_init = 0;
[txy_sol_lsqnonlin,resnorm,residual,exitflag,output] = lsqnonlin(fun,[tx_init, ty_init],[],[],lsqnonlinoptions);
% the Output in Matlab console is:
% First-Order Norm of
% Iteration Func-count Residual optimality Lambda step
% 0 3 5058.12 410 0.01
% 1 6 4752.75 58.7 0.001 0.572352
% 2 9 4738.31 1.21 0.0001 0.177726
% 3 12 4738.3 0.00463 1e-05 0.00236374
% 4 15 4738.3 2.41e-05 1e-06 1.27094e-05
% So it seems clear that it thinks it has reached an optimum but why?
  1 Comment
OD on 11 May 2021
I tried to change tolerance values with 'TolX' but it does not seem to be related to this ...

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Answers (1)

Alan Weiss
Alan Weiss on 11 May 2021
lsqnonlin is a gradient-based solver. It first attempts to estimate the local gradient by small finite difference steps. If your function is locally constant, such as a step function, then lsqnonlin sees zero gradient and stops.
To have your function not look like a step function, perhaps you can interpolate it using interp2, for example.
Alan Weiss
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