[Insight-users] confused behavior of itk image origin with FlipImageFilter

[kdsfinger at gmail.com]

On 12/24/06, Peter Cech <pcech at vision.ee.ethz.ch> wrote:
> On Sun, Dec 24, 2006 at 03:34:01 -0500, kdsfinger at gmail.com wrote:
> > hi, there
> > To make things simple, let's define two 4x2 (2D) images: img0 and
> > img1. Both with spacing 1mm on x and y direction.
> > img0 has pixel value
> > 1 2 3 4
> > 5 6 7 8
> > img0 has origin (0,0).
> > img1 is produced by a FlipImageFilter with img0 as input and flip on x
> > axis, thus img1 has pixel value
> > 4 3 2 1 // We call this configuration
> > 8 7 6 5 // confA.
> > After flipping of img0, img1 has origin (-3, 0). (Why?)
>
> This is the correct default behavior. It flips (mirrors) the image about
> a hyperplane defined by equation "flip_axis == 0". Here it is x==0,
> which corresponds to flipping/mirroring about axis y.
>
> You can get an alternative behaviour, that flips the image without
> moving it, by calling FlipAboutOriginOff().
> http://www.itk.org/Doxygen/html/classitk_1_1FlipImageFilter.html#itk_1_1FlipImageFilterz609_0
>
> > Experiment 1:
> > I output img1 and it shows as confA.
> > Later, I set the origin of img1 to (0,0) and output img1, it still
> > shows as confA.
> >
> > Experiment 2:
> > I resample img1 with identity transformation and output the result, it
> > shows as confA.
> > Later, I set the origin of img1 to (0,0) and resample img1 with
> > identity transformation and output the result, it still shows as
> > confA.
> >
> > Here is the code for resampling:
> > --------------------------
> >  resampler->SetTransform( identityTransform );
> >  resampler->SetInput( img1 );
> >  resampler->SetSize(   img1->GetLargestPossibleRegion().GetSize() );
> >  resampler->SetOutputOrigin( img1->GetOrigin() );
> >  resampler->SetOutputSpacing( img1->GetSpacing() );
> >  resampler->SetDefaultPixelValue( 100 );
> > ---------------------------
> >
> > My question is: If I change the origin of an image and resample it
> > based on the new origin, the result must be different from the
> > original one (image shifted). Why in this case, with the origin
> > changed, there is no difference for the output? What is the exact
> > defination of origin and how to use it correctly? I do read the
> > softwareguide but still can't explain why.
>
>
> Origin is always coordinate of a pixel with index [0,0] in physical
> space. This means, that changing origin is equivalent to translating the
> image in physical space. Your above experiment your input image was
> already shifted, so the output will be identical to input.
>
> I hope this will help you.
>
> Peter
>

hi, Peter
Thanks for reply and I aware that since the output origin is set the
same as input origin, it just compensate for the shift occured to the
input origin.
But I am still not sure about the FlipAboutOriginOn() /
FlipAboutOriginOff(). Now I block the
// resampler->SetOutputOrigin( img1->GetOrigin() );
and I tried FlipAboutOriginOn() / FlipAboutOriginOff() setting for the
flipfilter, but the outputs or the resampler are the same. Both
flipped image has origin (-3,0) and displayed the same by translation
-3 along x axis. Thanks for your comments.
zl2k