Re: [Algorithms] OBBs

[Pierre T. <p.t...@wa...>]

I'm afraid I don't get it.

You told us about the matrix made of absolute values. That one actually is
in the RAPID source, in the obb_disjoint function. And you said that taking
the absolute values was too optimistic. So, why would it work within the
RAPID code, if it doesn't work in theory according to you ?

Confusing.

----- Original Message -----
From: Kent Quirk <ken...@co...>
To: <gda...@li...>
Sent: Sunday, August 06, 2000 7:19 AM
Subject: Re: [Algorithms] OBBs


> Because of the legal use restrictions I did not read the RAPID code, but
> reconstructed my own code from the original paper. I was extremely
> careful, and had the same problems described here -- certain
> orientations of bounding boxes that were clearly intersecting would be
> reported as disjoint. I even reconstructed it in a spreadsheet.
>
> I have not gone back to see if I could make it work.
>
> If you used the RAPID source, I believe it works properly.
>
> Kent
>
>
> Pierre Terdiman wrote:
> >
> > Are you sure about the fact it's invalid ? I would be very interested to
> > know if this is true or not. This is weird because I've used Rapid
without
> > any problem.
> >
> > ----- Original Message -----
> > From: Kent Quirk <ken...@co...>
> > To: <gda...@li...>
> > Sent: Friday, July 21, 2000 2:33 PM
> > Subject: Re: [Algorithms] OBBs
> >
> > > Nik...@ao... wrote:
> > > > I recently finished writing a collision detection system for my
engine
> > (using
> > > > OBBs); however I have been having trouble with the system not always
> > > > detecting collisions.  I tested all of the functions separately to
> > confirm
> > > > their accuracy; yet this brought no avail.  I decided to work
backwards
> > to
> > > > find the bug, and I used a function from the RAPID library which
tests
> > for
> > > > disjointedness between two boxes.  Here is the description of the
> > function:
> > > >
> > > > This is a test between two boxes, box A and box B.  It is assumed
that
> > > > the coordinate system is aligned and centered on box A.  The 3x3
> > > > matrix B specifies box B's orientation with respect to box A.
> > > > Specifically, the columns of B are the basis vectors (axis vectors)
of
> > > > box B.  The center of box B is located at the vector T.  The
> > > > dimensions of box B are given in the array b.  The orientation and
> > > > placement of box A, in this coordinate system, are the identity
matrix
> > > > and zero vector, respectively, so they need not be specified.  The
> > > > dimensions of box A are given in array a.
> > > >     obb_disjoint(double B[3][3], double T[3], double a[3], double
b[3]);
> > > >
> > > > I set up a situation in Max using two boxes, in which the collision
> > detection
> > > > failed, and I then entered the box data in by hand.  The result was
that
> > > > after putting box 2 in box 1's coordinate system, the matrix B was a
128
> > > > degree rotation around the Z axis (or Max's Y axis).  The location
of
> > box 2
> > > > in relation to box 1 was:
> > > >     T[0]= -5.853f;
> > > >     T[1]= -2.173f;
> > > >     T[2]= 3.842f;
> > > > And the dimensions (half-lengths) were:
> > > >     Bd[0]= 2;  Bd[1]= 4; Bd[2]= 4.5;
> > > >     Ad[0]= 4;  Ad[1]= 1; Ad[2]= 7.5;
> > > > If you set this situation up in Max, it is obvious that the two
boxes
> > > > intersect (make sure to switch the Y & Z coordinates because of the
> > > > difference in D3D and Max's CS, and multiply the half-lengths by 2
to
> > get the
> > > > actual dimensions).  Yet, RAPID's function reports that the boxes
are
> > > > disjoint.  Any help would be appreciated.
> > >
> > > I too struggled with this for a long time.
> > >
> > > After I had given up, and gone to another collision system, someone
else
> > > reported that RAPID's OBB intersection algorithm (as documented) is
too
> > > optimistic in its use of absolute value. There is a matrix in the code
> > > that gets built as the absolute values of another matrix, thereby
saving
> > > some steps...but that's an invalid savings.  Sadly, I can't find the
> > > reference. But I hope this points you in the right direction, or
perhaps
> > > someone else has a better memory.
> > >
> > > Kent
> > >
> > > --
> >
> -----------------------------------------------------------------------
> > > Kent Quirk                   | CogniToy: Intelligent toys...
> > > Game Designer                |           for intelligent minds.
> > > ken...@co...      | http://www.cognitoy.com/
> > >
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> --
> -----------------------------------------------------------------------
> Kent Quirk                   | CogniToy: Intelligent toys...
> Game Architect               |           for intelligent minds.
> ken...@co...      | http://www.cognitoy.com/
> _____________________________|_________________________________________
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