Re: [Algorithms] OBBs

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

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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