PDF3DStream stream, Element root) {
// 3D Stream and annotation are linked directly or over a 3D reference
String refGroupName = DOMUtil.getAttribute(root, "refgroup").trim();
if (refGroupName.length() > 0) {
// attach 3D stream over a 3DReference to the annotation
PDF3DReference ref;
// Map key contains reference group name and stream filename
String refKey = refGroupName + ";"
+ DOMUtil.getAttribute(root, "src", true);