[Rtk-users] projection geometry offset

[Moritz Schaar]

Dear RTK users,

I am struggling with understanding RTK's projection geometry.
My measurement setup is that source and detector are fixed and the object is on a rotating plate.
The rotation axis has a slight horizontal offset as it can be seen in projections measured with 180° angular shift.

My assumptions based on http://www.openrtk.org/Doxygen/DocGeo3D.html now are:

1)      The isocenter is fix and in my case the center of the rotation axis of the plate.

2)      The documentation says that "The source position is defined with respect to the isocenter with three parameters".

So I should apply the estimated horizontal offset as SourceOffsetX.

3)      The documentation also says that "The detector position is defined with respect to the source with three parameters"

Does this imply that SourceOffsetX also has an impact on the detector position, e.g. the detector origin (0,0)?

Because this would be contrary to the next sentence of the documentation:

"(ProjectionOffsetX,ProjectionOffsetY,SourceToIsocenterDistance-SourceToDetectorDistance) are the coordinates of the detector origin (0,0) in the rotated coordinated system."

So either the detector is also positioned with respect to the isocenter, then the coordinates are (ProjectionOffsetX,ProjectionOffsetY,SourceToIsocenterDistance-SourceToDetectorDistance)

or respect to the source and should have (ProjectionOffsetX+ SourceOffsetX,ProjectionOffsetY+SourceOffsetY,SourceToIsocenterDistance-SourceToDetectorDistance)

Is this correct or did I miss something?
Basically it comes down to the question: Do I need to compensate for the horizontal offset of the rotation axis in ProjectionOffsetX as well or is it already handled by setting SourceOffsetX?


Best,
Moritz

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