[Insight-users] Rigid registration testing and reproducibility

[Blezek, Daniel J.]

Just 2 cents here.

1) For a linear registration, you could expect the registration results
to be ~0.1 degrees and ~0.1 mm cross platform.  The major source of this
problem is differences in floating point representations under different
compilers/hardware.  I shouldn't worry about this, it'd be difficult for
a human to see.  For something like BSplines, you might have a bit more
error.

2) A very simple way to do this is to do a forward transformation using
one transform (ground truth), then the inverse transform of the newly
calculated transform.  I think this is called Target Registration Error
by Fitzpatrick and Co., but you should look it up.  This is where you
need to decide on your tolerance.  As Karthik mentioned, only simple ITK
transforms have an inverse, which is really a shame.  They are so
useful, even if they are numeric.

2a) I suppose you could forward transform the same point using two
different transforms and see how far apart they are.  This seems
reasonable, but you'd have to sample a bunch of points to account for
rotation, and transform centers, etc.  And you'd only get a distance
measure, not a rotational measure.

For transforms with an inverse, you can do what you are asking, and it
would be a valuable contribution to ITK, but it's not general, as not
all transforms support an inverse.  And you could always test the
transform you care about...

Incoherent as usual,
-dan

-----Original Message-----
From: insight-users-bounces at itk.org
[mailto:insight-users-bounces at itk.org] On Behalf Of Andriy Fedorov
Sent: Tuesday, January 27, 2009 6:18 PM
To: ITK Users
Cc: Miller, James V (GE, Research)
Subject: [Insight-users] Rigid registration testing and reproducibility

Hi,

I would like to do regression tests of rigid registration with real
images, and compare the result with the baseline transform. Here are two
related questions.

1) ITK registration experts, could you speculate on what is the normal
variability in the rigid registration results run with the same
parameters, metric initialized with the same seed, when run across
different platforms? What could be the sources of this variability,
given the same initialization and same parameters?

2) If I understand correctly, the current testing of registration that
comes with ITK generates a synthetic image with known transform
parameters, which are compared with the registration-derived transforms.

The testing framework implemented in itkTestMain.h allows to compare
pixelwise two images, but this does not seem to be practical for
registration regression testing, because of the variability in the
registration results I mentioned earlier. Accounting for this
variability using tolerance values of just 1 pixel hugely increases the
test runtime, but in my experience, the comparison may still fail.

Would it not make sense to augment itkTestMain.h with the capability to
compare not only images, but transforms? Is this a valid feature
request, or am I missing something?

Thanks

Andriy Fedorov
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