Film grain technology
Another feature of HD DVD format, not available in the BD format, is the use of Film Grain technology, FGT. Film grain is perceived to be an important component of movie watching experience and many film makers chose specific film material because of their grain property. If DVD reproduction is to be true to the original intentions of the film makers, the film grain effect must be re-created. MPEG-4 AVC specifications include a standardised mechanism for transmitting film grain characteristics in the form of a sup- plementary enhanced information (SEI) message. HD DVD decodes this information and uses it to stimulate the original look of the film.
Interactivity
One of the main merits of both HD DVD and BD is enhanced interactivity and seamless connection to the internet. HD DVD use iHD Interactive Format while Blu-ray has opted for a format called BD-J. Unlike the DVD- video experience where a menu screen must first be selected by the user before the movie starts playing, the HD DVD and BD user is able to start a commentary from the main menu in the middle of the film without stop- ping it. Another enhanced feature is the ability to interact with the inter- net. For example a pre-recorded disc can be linked to a website to be constantly updated. Through this connection, studios can make additional content available to consumers, including clips, games, new releases and even enhanced subtitles.