[Insight-users] Question about RegularStepGradientDescentOptimizer

[Karthik Krishnan]

Hi Feng,

You can also try plotting the metric around the starting position with 
respect to the parametric space.

Set the metric with the initial transform and invoke
metric->GetValueAndDerivative( parameters, value, derivative );
and write the output into an image. You can do this with respect to the 
translation parameters, and use Paraview to throw contours and visualize 
the metric. This will tell you if you have local minima.

Alternatively you can write the derivative to the file and plot the 
deformation field using Paraview again (See "Visualizing deformation 
fields using Paraview") section of the Software Guide. This field will 
roughly indicate the direction the gradient descent optimizer will move in.

In general this will tell you if your metric is noisy or smooth. Its 
hard to converge using the GradientDescentOptimizer if your metric is noisy.

Also, if you are using any of the metrics derived from the 
HistogramImageToImageMetric, you can plot the joint histogram too.
See  Examples/Registration/ImageRegistrationHistogramPlotter.cxx
Please cvs update cause they were added two minutes ago.

Thanks
regards
karthik










Feng Qiao wrote:

> Hi,
> I tried to use the RegularStepGradientDescentOptimizer to register PET 
> images. When I monitor the registration progress, I found that the 
> measure value initially goes down during the first couple of 
> iterations, then it just kept increasing for many iterations (the 
> metric was set to be minimized). The increment for each step was not 
> that large though, and it did stop finally. The parameters I  used 
> was:  max step length =20, min step length = 0.01.
> I'm just wondering if this is normal...and if so why is the case? Any 
> help would be appreciated.
>
> Feng
>
>
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