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50,000 Tetrahedrons

February 9th, 2009

After some tweaking (including getting re-aquainted with the dot product) the generative code is faster and the geo is 25% lighter… in fact they’re not really tetrahedrons anymore. Actually, each of them is as much as it ever was, but more aren’t as many as they used to be.

Using that logic, I can reliably get up to about 50,000 iterations before Maya starts getting cranky. It only takes a couple of minutes, and here’s what I get:

MEL code follows:

//--start--

// find the center of a face
proc vector centerOfFace(string $face) {
  // get locations of all the face's vertices
  float $pos[] = `xform -q -ws -t ($face)`;
  // get length of the array: # vertices * 3
  int $posSize = size($pos);
  // put the average of all vectors into 1st vector 
  for ($i = 0; $i < 3; $i++){
    for ($j = $i + 3; $j < $posSize; $j += 3){
      $pos[$i] += $pos[$j];
    }
    $pos[$i] /= ($posSize / 3);
  }
  return <<$pos[0], $pos[1], $pos[2]>>;
}

//get angle between two vertices with respect to a third point
proc float getAngle(string $vertA, string $vertB, vector $origin) {
  // get location of both vertices
  vector $aLoc = `xform -q -ws -t $vertA`;
  vector $bLoc = `xform -q -ws -t $vertB`;
  // normalize locations to the face center
  vector $aOrigin = <<$aLoc.x - $origin.x, $aLoc.y - $origin.y, $aLoc.z - $origin.z>>; 
  vector $bOrigin = <<$bLoc.x - $origin.x, $bLoc.y - $origin.y, $bLoc.z - $origin.z>>; 
  // use law of cosines via dot product
  float $dot = dotProduct( $aOrigin, $bOrigin, 1 );
  float $angle = rad_to_deg(acos($dot));
  //round answer down to two decimal places
  return (floor(($angle+.005)*100))/100;
}

proc doit() {
// make a regular tetrahedron and select it
select `polyCone -r 1 -h 1.414214 -sx 3`;
// move its pivot point to the bottom face
xform -rp 0 -0.707107 0;
// move to origin and freeze xforms
xform -t 0 0.707107 0;
makeIdentity -apply true -t 1;

//delete the bottom face to lighten the load
$old = `ls -sl`;
$selectedFace = ( $old[0] + ".f[0]" );
delete $selectedFace;

//I'm leaving the safety on, you can add more zeros if you like
for ($i=1; $i<500; ++$i) {
  // copy cone
  $new = `duplicate $old[0]`;
  // if ($i != 1) {parent -w $new;} // move to world space

  // pick a random face on $old that isn't the base
  int $r = rand(3)+1;
  $selectedFace = ( $old[0] + ".f[" + $r + "]" );

  // move $new to selected face
  vector $fc = centerOfFace($selectedFace);
  xform -a -t ($fc.x) ($fc.y) ($fc.z) $new[0];

  // aim $new at face with a temporary normal constraint
  string $tmpConst[] = `normalConstraint -aim 0 1 0 $selectedFace $new`;
  delete $tmpConst[0];

  // pick a vertex on each pyramid
  $newVertex = $new[0]+".vtx[0]"; // 0 is on the base
  $oldVertex = $old[0]+".vtx[3]"; // 3 is the tip

  // get angle between two vertices
  $angle = getAngle($newVertex, $oldVertex, $fc);

  if ($angle > 0) { // if not already aligned
    // rotate $new to align the corners
    xform -r -os -ro 0 $angle 0 $new[0];
    // check the result
    $angle2 = getAngle($newVertex, $oldVertex, $fc);
    // if still not aligned, go the other way twice
    if ($angle2 > 0) {
      $angle3 = $angle * -2;
      xform -r -os -ro 0 $angle3 0 $new[0];
    }
  }

  $old = $new;
}
}
doit();
print("done!");

//--end---
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2 Responses to “50,000 Tetrahedrons”

  1. Ethan Says:

    This continues to look like magic to me.

    Sexy magic, mind you. But still: magic.

  2. zoomy Says:

    Not just sexy magic, son — sexy logic magic. I call it: “slagic.”

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