There is a API to set how transparent objects are blended in WebViewer.
setTransparencyMode
View — HOOPS Communicator documentation
TransparencyMode
TransparencyMode — HOOPS Communicator documentation
1hwv.view.setTransparencyMode(Communicator.TransparencyMode.SingleLayer);

1hwv.view.setTransparencyMode(Communicator.TransparencyMode.Unsorted);
