MEL How-To #74

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How can I select all of the isoparms for a NURBS surface?

The trick is in the use of the "surfaceInfo" node. Please refer to the following How-To -- the first half of this explains the use of the surfaceInfo node:

How can I determine the world space coordinates of NURBS surface edit points?

Now it's simply a matter of querying the ‘.knotsU’ and ‘.knotsV’ attribute for the surfaceInfo node and building a selection array from these parametric values.

The following MEL script illustrates the technique:

global proc selectIsoparms()
  // Get all selected NURBS objects
  string $nurbs[] = `filterExpand -sm 10`;

  // Create a surfaceInfo node -- used to determine the UVs for the NURBS spans
  // a.k.a. the isoparms.
  string $surfaceInfo = `createNode surfaceInfo`;

  // The array of components to be selected when finished.
  string $isoSelect[];

  for ( $nurb in $nurbs )
    // Connect the surfaceInfo node to the current NURBS surface.
    connectAttr -f ( $nurb + ".worldSpace" ) ( $surfaceInfo + ".inputSurface" );

    // Get all the U isoparms
    float $isoU[] = getAttr ( $surfaceInfo + ".knotsU" );

    // Add each U isoparms to selection list
    for ( $iso in $isoU )
      string $parm = ( $nurb + ".u[" + $iso + "]" );
      $isoSelect[`size $isoSelect`] = $parm;

    // Get all the V isoparms
    float $isoV[] = getAttr ( $surfaceInfo + ".knotsV" );

    // Add each V isoparms to selection list
    for ( $iso in $isoV )
      string $parm = ( $nurb + ".v[" + $iso + "]" );
      $isoSelect[`size $isoSelect`] = $parm;


  // For some reason Maya barfs if I try to select directly.
  // So I trick it by using an `ls` on the array.
  select `ls $isoSelect`;

  // Delete the temporary surfaceInfo node.
  delete $surfaceInfo;

It wouldn't be hard to add a bit of logic to restrict the selection to only U or V isoparms, or for only border/internal isoparms.

Related How-To's

Wednesday, October 10, 2001