8.5 Graphic User Interface

The primary goal of the HAVi user interface is to offer users an easy to use operating environment. The HAVi architecture allows users to control devices through familiar means, such as via the front panel or via the buttons of a remote controller. In addition, the HAVi architecture allows device manufacturers to specify GUIs that can be rendered on a range of displays varying from text-only to high-level graphical displays. The GUI need not appear on the device itself, it may be displayed on another device and the display device may potentially be from another manufacturer.

8.5.1 Level 1 GUI

The Level 1 GUI, called the DDI, is intended for IAVs. DDI elements can be loaded from a DDI target, typically a DCM, and displayed by a DDI controller. The DDI controller retrieves the DDI elements via the DCM rather than directly from the SDD data, so it is possible that the DCM itself is the source of DDI elements. The DDI controller generates HAVi messages in response to user input, and it also responds to HAVi messages sent by the DCM as a result of changes in device state. This communication is called the DDI protocol.

It should be emphasized that the DDI controller does not understand what happens as a result of issuing or responding to a control message. The DDI protocol involves only abstractions of user interface elements and user actions and is independent of any particular device semantics. Therefore, it is possible for a DDI controller to handle new device functions which were not known at the time of DDI controller implementation.

The DDI controller cannot provide guarantees over the graphical rendition of DDI elements actually presented to the user, as their representation may be changed due to lack of display screen space or other controller resource limitations applications can create different representations using the DDI elements as hints.

8.5.2 Level 2 GUI

The MHP, OCAP, and DASE iTV architectures adopted the HAVi L2 GUI specifications (rather than the complete specification) [MHP],[OCAP],[DASE]. The HAVi L2 GUI, which is adopted by most iTV standards and the subject of this chapter, consists of Java APIs that may be provided by FAVs. The Java APIs used for implementing a HAVi L2 GUI are based on a subset of Java AWT 1.1 and the following extensions:

support for different pixel aspect ratios, screen aspect ratios, and screen sizes
support for alpha blending and video/image layering
support for remote control input
support for a set of visual interface components patterned after the features offered by the Level 1 DDI elements