The philosophy of data manipulation within RTVR is object-oriented. One of the objects within the currently displayed volume is selected to be the ``active'' object. The selection is done by clicking on the object within the rendered image, or by selecting it through the GUI. From practical experience, it appears, that direct interaction with an object within the 3D scene is the most efficient way of working with data. Especially expert users prefer this way of interacting, compared to using standard GUI components. The most important properties of the active object can be changed by pressing one of the mouse buttons and dragging. Transfer function shape (contrast), opacity, and color can be changed directly within the 3D view. Furthermore, camera position, light source position, and zoom factor can be set within the view. The mapping of mouse actions to parameter changes is performed by an InteractionHandler component, which can be adapted to meet the needs of specific applications (for example to implement stream line integration from the position of a mouse click for a flow-visualization application).
As a supplement to parameter manipulation within the rendered view, all parameters of the active object can be adjusted using GUI components which are automatically generated by RTVR. This parameter panel allows an explicit selection of the active object and adjustment of it's parameters, and can (but does not have to) be used within any application which utilizes RTVR for visualization. A real-time screen capture of an interactive visualization session, which was performed by a viewer based on the RTVR library, can be downloaded from web page http://www.vrvis.at/vis/research/rtvr/.