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Idea Re: Trouble with G4SubtractionSolid -- bug? in CLHEP::HepRotation 

Forum: Geometry
Re: Question Trouble with G4SubtractionSolid -- pipe at angle to wall (Michael H. Kelsey)
Date: 15 Jan, 2011
From: Michael H. Kelsey <Michael H. Kelsey>

I believe that I have found a bug in the handling of compound rotations about the coordinate axes in CLHEP::HepRotation (== G4RotationMatrix). The details are below, but my question is this. Does GEANT4 have an official channel to communicate bugs to the CLHEP developers, or should I report this on my own, with a small test example?

On Sat, 15 Jan 2011 05:31:01 GMT, Michael H. Kelsey wrote:

> I am having trouble constructing a specific G4SubtractionSolid: a flat
> panel (scintillator veto) with a cylindrical hole drilled at an angle to
> the surface.

The attachment http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2011/01/15/13.25-77267-pentagon-hole_wrong.eps shows the problem: The hole you see should be along the viewpoint (X) axis, but instead it is perpendicular to the angled yellow panel.

To create the panel with a hole, I used:

   G4Box* panel = new G4Box("Panel", scintThick/2.,
                            sidePanelWidth/2., midPanelHeight/2.);
   G4Tubs* pipe = new G4Tubs("PipeHole", 0., radius, thick/2., 0, 360*deg);

   G4double phi = panelTransform.getRotation().phiX();

   G4RotationMatrix holerot;
   holerot.rotateY(90*deg).rotateZ(phi);

   G4ThreeVector holepos(0., holeY, holeZ);

   G4VSolid* panelWithHole =
     new G4SubtractionSolid("PanelWithHole", panel, pipe, &holerot, holepos);

The "holerot" construction should be clear: first rotateY(90*deg) is done, and second rotateZ(phi) is done). What I have discovered is that I get the correct result if I reverse those two rotations, that is,

   G4RotationMatrix holerot;
   holerot.rotateZ(phi).rotateY(90*deg);

Which says do the Z rotation first followed by the rotation about Y; see http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2011/01/15/13.37-94283-pentagon-hole_right.eps . Now the hole doesn't have visible sides, and the adjacent panel now has a nice piece chipped out of it.

If I generate my full detector, including the pipe coming through the hole, neither grid_test nor cylinder_test report any errors. The picture looks right (no overlapping colors), http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2011/01/15/13.43-40324-pentagon-pipe.eps .

I believe this reflects a bug in CLHEP::HepRotation. Compound rotations should be applied by matrix left multiplication, that is R(1+2) = R2 * R1, for arbitrary rotation matrices 1 and 2. This is how the rotateX/Y/Z operations are documented in CLHEP/Vector/Rotation.h.

My result above suggests that HepRotation has actually implemented the compounding by right multiplication; that is rotateX(), rotateY(), and rotateZ() are implemented as R' = R * rotateX, etc., instead of as R' = rotateX * R. I just checked this by explicit multiplication: left multiply shows up wrong, right multiply shows up correct.

I know this isn't a GEANT4 issue. As I asked above, though, does GEANT4 have an "official" way to communicate such bugs back to CLHEP, or should I do so on my own?

-- Michael Kelsey

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1 None: Re: Trouble with G4SubtractionSolid -- bug? in CLHEP::HepRotation   (Tom Roberts - 15 Jan, 2011)
(_ None: Re: Trouble with G4SubtractionSolid -- bug? in CLHEP::HepRotation   (Mike Kelsey - 15 Jan, 2011)
(_ None: Re: Trouble with G4SubtractionSolid -- bug? in CLHEP::HepRotation   (Joseph Perl - 15 Jan, 2011)
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