4.9. Menus in video DVD
A menu is the result of combining the functions of three layers which perform the following functions:

1. A display layer which contains a still picture or video clip. The contents of the display layer can be placed in one or more cells within a PGC. For performance reason, this layer usually comprises of only one cell.
A still picture is usually placed in a cell with infinite cell's still time. For motion video background or still picture with audio, the cell which contains the video or audio executes a looping cell command to repeat the display of the video or audio (at the end of the cell). The LinkPGN command should be used to minimize any noticeable delay or discontinuity produced by the required looping back.

2. A picture mask layer which usually is a black-and-white or up to 4-color background image. In simple black-and-white masks, the positions of black pixels in the mask are used to display colors for the action of selection or activation of a particular item on the menu. For more information on how the menu colors are organized, see menu color and mask.

3. A highlight layer defines the position of an enclosed area of up to 36 buttons per display menu. When a button (or highlight area) is selected (or when its defined area is referred to), the portion of the mask layer enclosed in this rectangular area is used to decide what colors to display. Each button may contain a single DVD command defined in the navigation packs of the associated VTSM domain (VTS_nn_0.VOB where nn is the VTS number). Highlight information (coordinates of the rectangular button area) is also stored in these navigation packs. The mask layer data is treated as a subpicture stream in the VTS_nn_0.VOB file.

Menus provide interactions and allow a user to move between titles and other menus in a DVD. There are two types of top-level menus in a movie DVD:

The title menu appears any time the user presses the Title key on the remote control. There can be only one title menu (per language unit) per disc. The title menu resides in the Video Manager (VMG) and is designed to provide top-down access to all video title set menus and titles on the DVD disc. If the DVD has only one major title, the title menu can be made to be the same as the root menu.

A root menu appears when the user presses the Menu key on the remote control. Each root menu resides in a video title set (VTS) and is designed to provide top-down access to menus, titles, chapters, and other menus within the VTS when it is playing. One root menu is available for each language defined in the VTS.