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[Andre Massing <question101686@xxxxxxxxxxxxxxxxxxxxx>]

Question #101686 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/101686

Andre Massing proposed the following answer:

Anders Logg wrote:
> Question #101686 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/101686
> 
> Anders Logg proposed the following answer:
> On Sat, Feb 20, 2010 at 01:06:17AM -0000, Andre Massing wrote:
>> Question #101686 on DOLFIN changed:
>> https://answers.launchpad.net/dolfin/+question/101686
>>
>> Andre Massing proposed the following answer:
>>
>> Garth N. Wells wrote:
>>> Andre Massing wrote:
>>>> Question #101686 on DOLFIN changed:
>>>> https://answers.launchpad.net/dolfin/+question/101686
>>>>
>>>>     Status: Open => Answered
>>>>
>>>> Andre Massing proposed the following answer:
>>>> On Friday 19. February 2010 18.33.47 marc spiegelman wrote:
>>>>  > New question #101686 on DOLFIN:
>>>>  > https://answers.launchpad.net/dolfin/+question/101686
>>>>  >
>>>>  > Hi All,
>>>>  >    I've implemented a rather nifty semi-lagrangian advection scheme in
>>>>  >  dolfin that requires rapid checking to see if a point intersects a
>>>>  >  cell
>>>>  >  (without necessarily firing up the full intersection operator).  Pre
>>>>  >  0.9.7
>>>>  >  this was implemented as cell.intersects(point).
>>>>  >
>>>>  > Everything works fine in 0.9.6   with the new CGAL intersection
>>>>  > operator
>>>>  > replacing the GTS intersection operator (with about a 20% speedup).
>>>>
>>>> Did you only use the intersection between meshentities and points or did
>>>> you
>>>> use also the new mesh intersection operator? Just curious, because the
>>>> former
>>>> code for intersection between points og e.g. triangles did not change.
>>>>
>>>>  >  However, in 0.9.7 the code breaks because  cell.intersects() has been
>>>>  >  removed as a GTS dependency (actually, looking at
>>>>  >  TriangleCell.intersects(point), it looks like it simply reproduces
>>>>  >  GTS
>>>>  >  code but wasn't actually dependent on GTS)
>>>>
>>>> Yep that's true, the intersection functions was removed and there is a new
>>>> PrimitiveIntersector class in C++ with two static functions. These computes
>>>> the intersection between rather arbitrary mesh entities (exact or only
>>>> using
>>>> doubling precision). The functions are exposed as do_intersect and
>>>> do_intersect_exact in the python interface. But you are right, a point -
>>>> meshentity intersection is not provided. I attached a patch against the
>>>> recent
>>>> 0.9.7 release. Could you can apply to you sources and see whether it
>>>> works for
>>>> you?
>>>>
>>> Andre: Did you forget to attach it?
>> Hmm, I did not, at least in my sent box it also appeared with
>> attachment. Strange... resend it (CC to the DOLFIN-ml).
> 
> We also need to add back the simple interface
> 
>   bool MeshEntity::intersects(const Foo& foo) const;
> 
> where foo can be either a Point or another MeshEntity.

Of course we can do this. But before I removed it, we discussed it a bit 
and concluded that intersects should not be a member function of an 
entity (IIRC :), because it is a symmetric operation between geometric 
objects and calculation of intersection detection is dependent on both 
involved types.

> 
> do_intersect looks a bit strange to me. (But it might be good for the
> low-level interface that interacts with the CGAL templates.)

Why exactly? It takes two entities or an entity and a point and just 
returns a boolean value, stating, whether to entities "do intersect" :)
But what might be a better, more meaningful name? Just "intersects"?  Or 
what did you think of?


> 
> --
> Anders
>

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