[Insight-users] numerical problem after point transformation?

[Stefanie Nowak]

Hallo,

in my algorithm, I use an inital seedpoint for my computations. After 
setting the seedpoint in our application, I cut the volume with a region 
of interest and compute an axis through it.
Next I resample the volume with an affine transform that consists of a 
translation of the middle of the cutted volume to the origin, a rotation 
around the computed axis with an defined angle and a back translation. 
Now I want to know at which position the inital seedpoint is in the 
resampled volume.

For that I convert the seedpoint from index coordinates to world 
coordinates with the image information of the volume that was the input 
of the resample filter:
for(int i = 0; i < 3; i++)
{
          point[i] = inputResampleOrigin[i] + seedPoint[i] * 
inputResampleSpacing[i];
}

Then I take the transform that was used as input in the resample filter:
PointType resultPoint = transform->TransformPoint(point);

And transform the computed point back to index coordinates with the 
image information taken from the result of the resampling.
for(int i = 0; i < 3; i++)
{
    seedOutput[i] = int(0.5 + (resultPoint[i] - 
originAfterResampling[i]) / spacingAfterResampling[i]);
}

When I now mark the position of the transformed seedpoint in the 
resampled volume as a black dot, the seedpoint is not at the place where 
I initially set it.
To be sure about that, I marked the inital seedpoint in the dataset with 
a white dot. I need the transformed seed point for further computations 
and it is important that it lies in the anatomical region where I set 
it. You can see a picture of it in the attachment

Has anybody an idea where my mistake is? Is it a rounding error or 
numerical instabil?

Any help is appreciated!

Thanks,
Stefanie

-- 

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Dipl.-Inform. Stefanie Nowak

Fraunhofer Institute for Computer Graphics
Cognitive Computing and Medical Imaging (A7)
Fraunhoferstr. 5, 64283 Darmstadt, Germany
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email:    stefanie.nowak at igd.fraunhofer.de
http://a7www.igd.fhg.de/persons/snowak/

phone:	+49 6151 - 155 520
fax:    +49 6151 - 155 480
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