xrf.portalNonEuclidian = function(v, opts){
  let { frag, mesh, model, camera, scene, renderer, THREE} = opts

    let toFrag = xrf.URI.parse( v.string )

    // turn plane into stencilplane 
    mesh.material = new THREE.MeshPhongMaterial({ color: 'green' });
    mesh.material.depthWrite   = false;
    mesh.material.stencilWrite = true;
    mesh.material.stencilRef   = xrf.portalNonEuclidian.stencilRef;
    mesh.material.stencilFunc  = THREE.AlwaysStencilFunc;
    mesh.material.stencilZPass = THREE.ReplaceStencilOp;
    //mesh.material.side         = THREE.DoubleSide  // *TODO* this requires flipping normals based on camera orientation
    mesh.portal = {
      stencilRef: xrf.portalNonEuclidian.stencilRef
    }

    let stencilPos = new xrf.THREE.Vector3()
    mesh.getWorldPosition(stencilPos)

    // allow objects to flip between original and stencil position (which puts them behind stencilplane)
    const addStencilFeature = (n) => { 
      n.stencil = (
        (pos,stencilPos, stencilMat, mat ) => (enabled) => {
          let sRef          = enabled ? mesh.portal.stencilRef : 0
          stencilMat.depthTest = false
          n.position.copy( enabled ? stencilPos : pos )
          n.material = enabled ? stencilMat : mat 
          xrf.portalNonEuclidian.selectStencil(n, sRef ) // disable depthtest of world-container (sky e.g.)
          n.traverse( (c) => !c.portal && (xrf.portalNonEuclidian.selectStencil(c,sRef)) )
        }
      )( n.position.clone(), stencilPos, n.material.clone(), n.material )
      return n
    }

    // collect related objects from XRWG to render inside stencilplane
    let objs = XRWG.match(mesh.name,0)
    if( objs.length == 0 ) return console.warn(`no objects are tagged with (portal)object name '${mesh.name}'`)
    objs = objs[0].nodes 
                  .filter( (n) => !n.portal ) // filter out (self)references to portals (prevent recursion)
                  .map(addStencilFeature)
    
    // add missing lights to make sure things get lit properly 
    xrf.scene.traverse( (n) => n.isLight && 
                               !objs.find( (o) => o.uuid == n.uuid ) && 
                               (objs.push(n)) 
    )

    // put it into a scene (without .add() because it reparents objects) so we can render it separately
    mesh.stencilObjects = new xrf.THREE.Scene()
    mesh.stencilObjects.children = objs 

    // *TODO* stencilize any tagged plane without material

    // enable the stencil-material of the stencil objects 
    const showPortal = (n,show) => {
      if( n.portal ) n.visible = show
      return true
    }
    mesh.onAfterRender = function(renderer, scene, camera, geometry, material, group ){
      mesh.stencilObjects.traverse( (n) => showPortal(n,false) && n.stencil && (n.stencil(true)) )
      renderer.autoClear        = false 
      renderer.autoClearStencil = false
      //renderer.sortObjects      = false
      renderer.render( mesh.stencilObjects, camera )
      //renderer.sortObjects      = true
      mesh.stencilObjects.traverse( (n) =>  showPortal(n,true) && n.stencil && (n.stencil(false)) )
    }

    xrf.portalNonEuclidian.stencilRef += 1 // each portal has unique stencil id
    console.log("enabling portal for object '${mesh.name}'`")
}

xrf.portalNonEuclidian.selectStencil = (n, stencilRef, depthTest) => {
  if( n.material ){
    n.material.stencilRef = stencilRef 
    n.material.stencilWrite = stencilRef 
    n.material.stencilFunc = xrf.THREE.EqualStencilFunc;
  }
  n.updateMatrixWorld(true)
}

xrf.portalNonEuclidian.stencilRef = 1