[Rtk-users] Fwd: Projection Geometry and Image Origins

[Simon Rit]

All distances in the geometry file are in the same unit as the image
spacing (i.e., mm in most cases).
I forgot to mention the geometry doc before
http://www.openrtk.org/Doxygen/geometry.pdf

On Fri, Aug 18, 2017 at 4:03 PM, Samuel Gerber <samuel.gerber at kitware.com>
wrote:

>
> ---------- Forwarded message ----------
> From: Samuel Gerber <samuel.gerber at kitware.com>
> Date: Fri, Aug 18, 2017 at 10:01 AM
> Subject: Re: [Rtk-users] Projection Geometry and Image Origins
> To: Simon Rit <simon.rit at creatis.insa-lyon.fr>
>
>
> Thank you Simon.
>
> One more clarification question Are the SID and SDD parameters measured in
> units of the input volume spacing in z?
>
>
> On Fri, Aug 18, 2017 at 2:53 AM, Simon Rit <simon.rit at creatis.insa-lyon.fr
> > wrote:
>
>> Dear Samuel,
>> Here is how I'd explain it: there are two ITK images, a volume and a
>> stack of projections. Each image has its own coordinate system,
>> defined in ITK by the Origin, the Spacing and the Direction. The
>> geometry links the volume coordinate system to each projection
>> coordinate system (which is common for all projections) of the stack
>> using 9 parameters per projection.
>> In order to center the volume on the projections, you have different
>> scenarios. Let's take one example:
>> - the volume is centered around (0,0,0) (e.g.,
>> origin=(-127,-127,-127), spacing=(2,2,2) and size=(128,128,128)),
>> - the projection is centered around (0,0) (e.g., same values as for
>> the volume. Note that the third coordinate is ignored by RTK).
>> - then the projection is centered on the volume if all offsets are 0.
>> If the projection had not been centered (e.g., origin=(0,0,0)), then
>> you can create exactly the same geometry by setting
>> ProjectionOffsetX=ProjectionOffsetY=127.
>> I hope this helps,
>> Simon
>>
>> On Thu, Aug 17, 2017 at 3:45 PM, Samuel Gerber
>> <samuel.gerber at kitware.com> wrote:
>> > Hi,
>> >
>> > I'm curious about the settings for the different origins when setting
>> up a
>> > ProjectionGeometry, in particular ThreeDCircularProjectionGeometry.
>> >
>> > As far as I understand there is three origins:
>> > - The projection geometry origin at (0, 0, 0)
>> > - The input image origin
>> > - The projection image origin
>> >
>> > In order to center the projection on the input image I assume that I
>> will
>> > have to use the offset parameters to shift the source and detector so
>> that
>> > they are centered on the input image in physical coordinates.
>> > How does the projection image origin come into play?
>> >
>> > Thanks
>> > Samuel
>> >
>> > --
>> > Samuel Gerber
>> > R&D Engineer
>> > Kitware, Inc.
>> >
>> > _______________________________________________
>> > Rtk-users mailing list
>> > Rtk-users at public.kitware.com
>> > http://public.kitware.com/mailman/listinfo/rtk-users
>> >
>>
>
>
>
> --
> Samuel Gerber
> R&D Engineer
> Kitware, Inc.
>
>
>
> --
> Samuel Gerber
> R&D Engineer
> Kitware, Inc.
>
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>
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