HDi (formerly iHD) is Microsoft's implementation of the Advanced Content interactivity layer in HD DVD. It is used in the Xbox 360 HD DVD add-on as well as stand-alone HD DVD players.
Applications written for HDi are written using the XML dialect and ECMAScript, the latter of which is processed by the JScript engine when running on Microsoft Windows platforms. The HDi runtime exposes the APIs defined by the Advanced Content standard. It provides only a single-threaded programming model, though certain operations (such as network and persistent storage access) are executed as asynchronous operations.
An HD DVD movie, including the interactive functionality, is presented as an Advanced Content application, which is executed and rendered by the HDi runtime. The advanced content application consists of the playlist files (.xpl), subtitles (.xas), markup files (.xmu), and scripts (.js) in addition to the actual video, in a defined directory structure. The HDi runtime parses the markup and the scripts to execute the action. The playback of the video, along with its integration with the rest of the navigation system, is initiated from and controlled by script code.
The HDi runtime is responsible for the execution and final rendering of the movie playback and navigation application. The markup is parsed into a Document Object Model, which allows ECMAScript code to control and modify the UI layout during execution. By dynamically altering the layout of UI widgets is how animations and interactivity is achieved. The DOM and associated APIs is used to enable other scenarios such as pausing playback and replacing it with the navigation UI, or seeking to a certain area in the movie (used for either manual seek or seeking to bookmarks). For the rendering stack, it presents six planes (which are containers for graphics) that are layered in front of each other. The final image displayed is the composition of the images from the individual planes. The composition of the planes into the final image is handled by the HDi runtime. These rendering layers, from back to front, are:
Microsoft does not provide design tools for the development of HDi applications, though third parties have made such tools available. Because the components used by Advanced Content (and HDi) - XML, XSL-FO, XPath, ECMAScript - are widely used, any development tool supporting these can be used to develop HDi applications. However, Microsoft has made an HDi simulator available as a free download, as a part of the HD DVD Interactivity Jumpstart Kit, to let users author and debug HDi content on computers running Windows XP, although this is not intended as a full authoring tool nor a playback device.
HDi is not inherently limited to being used on optical media; it can be used on media delivered or streamed over the Internet or any other network. In fact, on October 4, 2007, Toshiba and Microsoft announced the creation of the Advanced Interactivity Consortium (AIC) to "extend and promote interactive experiences beyond optical media to new platforms."