[Insight-users] Gradient Difference Metric

[Aviv Hurvitz]

Hi Dan,

Thanks, I've been using ITK for a while now and I didn't know about this
feature of optimizers.

All the metrics I know in ITK go "down". There was no special documentation,
so that's what I expected. For comparison, the Mutual Information metric,
which is something you want to maximize, is actually minimized by the
framework.

I think people shouldn't use the metric like you suggest. Defaults are
important if you want the registration metric to be modular
("plug-and-play").

Please try to imagine the typical user experience with this metric. It is
EXTREMELY frustrating, because you have to play around with many
optimization parameters before you either debug it or give up. Who knows how
many users went down the path Sven and I went before giving up? I know of at
least two more users who gave up.

So for future reference, I still classify this bug as really "huge".

Kind regards,

- Aviv

On Thu, Jun 26, 2008 at 9:20 AM, Dan Mueller <dan.muel at gmail.com> wrote:

> Hi Aviv,
>
> 2008/6/25 Aviv Hurvitz <aviv.hurvitz at gmail.com>:
> > In short, the GradientDifferenceImageToImageMetric is bad! It has one
> huge
> > bug and a few other debatable problems. The huge bug is that it returns
> the
> > MAXIMUM value when the registered images are perfectly aligned, whereas
> it
> > should return the MINIMUM. That's right, this metric actually pushes the
> > moving image away from the right result.
> >
> > I wonder how this came to be? My guess is that the original developer was
> > using it in a different framework, where registration involved
> maximizing. I
> > Googled a bit and the few reports on this metric were either that it
> doesn't
> > work or that it *maybe* works, so I guess it  checks out.
>
> For future reference, this is not really a "huge" bug. Optimizers can
> be configured to either minimize or maximize a metric; simply call
> optimizer->MinimizeOn() or optimizer->MinimizeOff() depending on your
> chosen metric.
>
> HTH
>
> Regards, Dan
>
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