[Rtk-users] Projection Geometry and Image Origins

[Simon Rit]

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.
>
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