[Gmsh] BooleanFragments output identification

Lionel Wilhelm wilhelm at ycoorsystems.com
Tue Nov 28 09:23:56 CET 2017

Dear all,

I do not really understand why the BoundingBox command does not give the 
expected results for "complex" volumes. I developped a small workaround 
consisting in looping over the volume boundary faces and each time 
evaluating the BoundingBox of the surface.

In case other might be interested I post the small piece of code below.



Macro VolumeBB
  //Piece of script that gives the bounding box of a volume
  //Inputs : vol  volume ID number  
  //Output : out  list with xmin,ymin,zmin,xmax,ymax,zmax
  //Reserved variable names :  q_iterate
  //                           j_iter
  //                           b
  //                           tmp     
  /* --------------------------------------------*/
  out = {};
  b() = Unique(Abs(Boundary {Volume {vol};} ));
  // loop over the boundary surfaces of the volume
  For q_iterate In {0:#b[]-1}
    tmp() = BoundingBox Surface{b[q_iterate]};
    If(q_iterate == 0)
      out[] = tmp [];
      // loop over Xmin, ymin, zmin
      For j_iter In {0:2}
        //out[j] = min(out[j], tmp[j])
        out[j_iter] = (out[j_iter] < tmp[j_iter]) ? out[j_iter] : 
      // loop over xmax, ymax, zmax
      For j_iter In {3:5}
        out[j_iter] = (out[j_iter] > tmp[j_iter]) ? out[j_iter] : 
    Delete tmp;
  Delete b;
  //Printf("vol has bounds : ", out());
  // ----------------------------------------------
  // End of piece of script
-----Original Message-----
 From: Lionel Wilhelm <wilhelm at ycoorsystems.com>
 To: David.Colignon at uliege.be, gmsh at onelab.info
 Date: Mon, 27 Nov 2017 12:01:07 +0100
 Subject: Re: [Gmsh] BooleanFragments output identification

Hi David,

 Thank you for your reply. Unfortunatly, the BoundingBox does not seem to
 work. At least not as I expected...
 As far as I understood, the BoundingBox command returns a list with
 [Xmin,Ymin,Zmin,Xmax,Ymax,Zmax]. I can then build a Box (volume) with
 the command Box{Xmin,Ymin,Zmin,Xmax-Xmin,Ymax-Ymin,Zmax-Zmin}.

 I attach a screenshot with the original volume and its computed bounding
 box [see VolumeOfInterest_withBB.png]. I also noticed that the yellow
 point representing the volume in the GUI is not even close to the center
 of the volume (which is supposed to be the case)[see VolumeOfInterest.png].

 Does anybody have a hint ?



 On 26.11.2017 19:46, David Colignon wrote:
 > Hi Lionel,
 > You could try:
 > http://gmsh.info//doc/texinfo/gmsh.html#Floating-point-expressions 
 > BoundingBox Point|Line|Surface|Volume { expression }
 > Regards,
 > Dave
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