[Insight-users] Problem with translation scale and optimizer

[zoro]

Dear Luis and everyone,

My colleagues found a problem when setting translation scale and optimizer
in itk. We do understand translation scale's role (to balance dynamic range
of translation and rotation) but when apply with optimizer, the situation is
very different.

The situation is very simple. We have two volumes differed only by
translation about 100mm. When we used rigid registration with regular step
gradient descent optimizer and setting maximum step length = 0.05, the
registered result was not good. In fact, moving volume almost didn't move. 

So in our considers, setting maximum step length=0.05 means maximum rotation
= 0.05 rad = 9 degree (normally) but maximum translation = 0.05 mm (very
small). Moving volume will not move to fixed volume with the number of
iterations < 1000.

I wonders that how to set maximum step length of rotation and translation
separately to avoid this situation. In fact, we shrank voxel spacing and set
all translation scale = 1 to avoid this problem.



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