|Message: Re: Mapping points on a surface to indices||Not Logged In (login)|
Click on the Forum title, e.g. on the "Forums by Category" page, to read a sequence of postings to the Forum and its threads all in one page. If you are only interested in one thread or the thread following a specific posting, click the thread or the posting, which takes you to a smaller page, which contains only the part you are interested in and may be easier to navigate.
Messages are "chained" if there are only replies at the first level, i.e. 1/1.html, 1/1/1.html etc. In case of "chained" messages the message number is replaced by the icon and there is no indentation.
Inline: Display the subject line only or also the text of the posting(s); for the choice "All" the "Outline" choices are switched off.
|1||0||1||no text / full text of posting|
|2||1||All||text for level 1 only / text for All postings|
Outline: Choose the depth of the posting thread, successive toggle controls provide increasing detail.
|1||2||1||2 levels / 1 level (original posting)|
|2||3||2||3 levels / 2 levels|
|3||3||All||3 levels / all levels (all postings)|
Dear Giuseppe, Your request and potential application sound unusual and interesting. I am not sure I have understood, though, the need for the mapping - nor how it would be coupled with the Geant4 solids. Our solid classes offer certain functionality tailored for tracking particles along straight line tracks (like photon rays), DistanceToIn (point) an estimate of the distance to the nearest point from outside or on the surface ( or ToOut from inside or on the surface) DistanceToIn (point, direction ) for the distance along a ray SurfaceNormal( point ) .. etc but also include a few auxiliary methods to do other things: Estimate Surface Area Estimate Volume Create Random Point on Surface which are of use for calculating a few other properties. As you gathered, there is no (general) concept of a mapping between a two-dimensional index and points on the surface of a volume. This approach is not close to the algorithms utilized for intersections and other methods. And it is not simple to see how to create one - without someone undertaking much development. The approach could potentially have been used for some aspects of the more complicated solid types, NURBS and other BREPS primarily. Yet it was not. The BREPS module (and some capabilities in Faceted Solids and Tesselated Solids in the solids/specific directory) provide more general soids than the CSG solids, but its implementation relies on cycling through bounding boxes for each surface patch (or part), and intersecting only relevant surface patches. And it does not include any mapping. The existing solids capability does not include NURBS. In particular the method CreateNURBS in G4VSolid was meant for graphics and I understand that it is historical (obsolete). Could you please explain somewhat more about what you are seeking to do using this mapping? It could be that there would be other capabilities of the Geant4 solids that you could utilize. Best regards, John Apostolakis On May 26, 2008, at 10:38 PM, Giuseppe Vacanti wrote: > *** Discussion title: Geometry > Email replies to PublicHyperNews@slac.stanford.edu must include: > In-Reply-To: <"/geometry/908"@geant4-hn.slac.stanford.edu> > Subject: ...change this to be about your reply. > > Hello, I am writing an X-ray tracer in GEANT4. I am at the point > where I > would like to map points on the surface of a solid to two indices so > that local surface properties can be looked up on a point-by-point > basis. > > This smells like NURBS ((u,v)<->(x,y,z) mapping), but I understand > that > NURBS are not really implemented (by the way, can the base class > G4NURBS > be used?). > > I have looked into BREPS, which seem to offer a more complete set of > solids (at least those I need), but I do not see a way to do what I > want > (probably there isn't one). > > Does anyone have any suggestions/pointers or working on an application > with similar needs? > > Thanks, Giuseppe
|Inline Depth:||Outline Depth:||Add message:|