[ITK Community] [Insight-users] Problem with translation scale and optimizer

[Luis Ibanez]

Hi Zoro,

Thanks for the detailed description of what you are observing.

Please note that the optimizer scales are multiplied by the
step length and in turn they are multiplied by the Gradient of
the metric.

Your estimation of :

maximum step length=0.05 means

maximum rotation = 0.05 rad = 9 degree

maximum translation = 0.05 mm (very small).

are correct as maximum valued.


They must still be multiplied by the gradient, and if it turns
out that the metric derivatives are small, the image may
end up not moving much.

As a consequence, you may simply have to use a larger
step length.

You shouldn't need to modify the image spacing.
(such modification will bring you other headaches).


Use the scales only to compensate the relative differences
between rotation and scaling.

A good initial value is to use the diagonal of the image
(in physical units) as the ratio between rotational scales
and the translational scales.

Then use the maximum step length the get the image
moving.

You should see the image moving in the first few (5 to 10)
iterations.

It is not uncommon to use step lengths = 1.0 or even 10.0


     Thanks


           Luis
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