# GDML xtru definition

I am trying to understand the GDML xtru definition.
Page 24 - http://lcgapp.cern.ch/project/simu/framework/GDML/doc/GDMLmanual.pdf
specifically xOffset, yOffset, scalingFactor

Does the xOffsets mean I should add the xOffset to the corresponding x coordinate of the blueprint polygon when specifying its z coordinate,
similarly with yOffset.

How do I apply the scalingFactor? To the zPosition value ? if yes seems strange, why not just use the zPosition.

I note that the definitions of G4ExtrudedSolid at http://geant4-userdoc.web.cern.ch/geant4-userdoc/UsersGuides/ForApplicationDeveloper/html/Detector/Geometry/geomSolids.html seems to be different from the GDML xtru parameters.

An extuded solid is constructed by moving a polygone along a polyline. We can also say, that an extruded solid is given by a sequence of polygons.

The polyline nodes are defined by (xOffset, yOffset, zPosition).

During move the polygone remains always parallel to the XY-plane, however it can be scaled. Scaling is defined by scalingFactor at each zPosition. It is applied only to xy-coornates of the polygon

To calculate the xy-coordinates of the vertices of the polygone at given zPosition it is required

• first, apply scalingFactor to the xy-coordinates of the original polygon;
• then, shift the vertices by (xOffset, yOffset);

Thanks, makes sense now.