[Rtk-users] Rotation before and during rktprojections

[Simon Rit]

Hi Vincent,
Everything you did is correct but rtkfdk always applies the ramp filter
along the first dimension and does not check if this corresponds to the
direction orthogonal to the rotation axis. For your case, it is probably
not so difficult to adapt the code. When there is a small rotation, it is
debatable whether one should ignore the rotation for the ramp filter,
resample the image, or recalculate the filter in 2D (see our recent
publication here <https://doi.org/10.1088/1361-6560/ab7875> which has not
been implemented in RTK). I have opened an issue on github
<https://github.com/SimonRit/RTK/issues/335>, hopefully we'll find time to
fix this in the future (at least raise an exception above a parametrable
rotation threshold).
Best regards,
Simon

On Wed, Apr 15, 2020 at 7:00 PM vincent <vl at xris.eu> wrote:

> Hi RTK community,
>
> I hope everyone is doing well in those hard times.
>
> I have a panel that is rotated by 90° on the CT system.  I used to
> rotate the images back in the right orientation before building the
> sinogram with rtkprojections, but then I thought I might give a try at
> the "newdirection" option.  Using the matrix [0 1 0 ; 1 0 0 ; 0 0 1]
> gives the expected results.  So far so good. But when I try to
> reconstruct the latter with the same geometric parameters than for the
> pre-rotated projections, I obtain a very blurry result (vs a sharp one
> in the first case).  I am obviously missing something, and I would
> suspect I am not giving the right information to the rtkfdk
> application.  So far, here is what I have tried:
>
> -keep the projections in the initial orientation, but give the
> "newdirection" option to the rtkfdk application,
>
> -specify the "newdirection" in both rtkprojections and rtkfdk,
>
> -turn to you for advice !
>
> I thank you very much in advance for any help you might be able to give.
>
> Best regards,
>
> Vincent
>
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