I tried to use dat.GUI in the following three.js example. I just did the following code changes to add a GUI to adjust mesh opacity. var loader=new THREE.VTKLoader(); loader.load ("models/vtk/bunny.vtk", function(geom){ var mesh = new THREE.Mesh(geom, material ); mesh.doubleSided=true; mesh.position.setY(-0.09); scene.add( mesh ); var gui = new dat.GUI(); var view = this; view.Opacity = 0.2; var m