From Marco Jez, added example that uses FBO extension to do prerendering.

examples/osgfbo/Renderbuffer.cpp

@@ -0,0 +1,53 @@
+#include "Renderbuffer.h"
+#include "FBOExtensions.h"
+
+using namespace osg;
+
+Renderbuffer::Renderbuffer()
+:    Object(),
+    _width(512),
+    _height(512),
+    _internalFormat(GL_DEPTH_COMPONENT24)
+{
+}
+
+Renderbuffer::Renderbuffer(int width, int height, GLenum internalFormat)
+:    Object(),
+    _width(width),
+    _height(height),
+    _internalFormat(internalFormat)
+{
+}
+
+Renderbuffer::Renderbuffer(const Renderbuffer &copy, const CopyOp &copyop)
+:    Object(copy, copyop),
+    _width(copy._width),
+    _height(copy._height),
+    _internalFormat(copy._internalFormat)
+{
+}
+
+GLuint Renderbuffer::getObjectID(unsigned int contextID, const FBOExtensions *ext) const
+{
+    GLuint &objectID = _objectID[contextID];
+
+    int &dirty = _dirty[contextID];
+
+    if (objectID == 0)
+    {
+        ext->glGenRenderbuffersEXT(1, &objectID);
+        if (objectID == 0) 
+            return 0;
+        dirty = 1;
+    }
+
+    if (dirty)
+    {
+        // bind and configure
+        ext->glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, objectID);
+        ext->glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, _internalFormat, _width, _height);
+        dirty = 0;
+    }
+
+    return objectID;
+}