[Rtk-users] bWithExternalHalfPixelBorder in RayBoxIntersectionImageFilter

[Simon Rit]

Dear Chao,
Thanks for the question and sorry for the delay in answering it. The
boolean allows to determine if the box goes from itk::Image::Origin to the
coordinate of the last pixel or whether you want to add a half voxel
border, see code here
<https://github.com/SimonRit/RTK/blob/master/src/rtkBoxShape.cxx#L147-L168>.
In SART, you want to divide the difference between the projection and the
measure by the projection of a volume filled with one. Our projectors
consider that the intensity is 0 before Origin and after the coordinate of
the last pixel so I think the current version is the correct one. The
rtkforwardprojectiontest.cxx
<https://github.com/SimonRit/RTK/blob/master/test/rtkforwardprojectiontest.cxx>
tests this and you can check in it that there is indeed no half pixel
border. So I believe this is the right thing to do, yes.
Best regards,
Simon

On Mon, Aug 19, 2019 at 10:15 PM Chao Wu <wuchao04 at gmail.com> wrote:

> L.S.,
>
> I have noticed a behaviour change in SART after commit 5dc1d78.
> In SARTConeBeamReconstructionFilter, m_RayBoxFilter->SetBoxFromImage(this->GetInput(0),
> false) attempts to pass false to bWithExternalHalfPixelBorder, although
> before this commit this variable was not used in
> RayBoxIntersectionImageFilter::SetBoxFromImage and then when
> rtk::BoxShape::SetBoxFromImage is called the default half pixel border flag
> was set to true. After this commit a false can finally be passed through.
> This gives slight different reconstruction results than before. I was just
> wondering which one is more correct, with or without the half pixel border
> for the volume used for the normalization. I tried to follow the external
> link of explanation given in the code of SetBoxFromImage but found it
> broken. Could anyone give a brief explanation? Thanks.
>
> Best regards,
> Chao
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