2. DVD Elements and Hierarchy
2.1. Cells
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...In the old days of VHS tape, a typical movie is recorded on a spool of tape to view from the beginning to the end in a sequential fashion. The movie consists of many individual sequential segments of video scenes or film clips. The only interaction one can have with a VHS tape is rewinding and forwarding to the scenes of interest and play. From the film editor's perspective, each individual scene is a basic building block, interconnecting with other scenes to produce the entire movie. In the DVD terminology, this individual scene is called a cell. Most if not all authoring packages require cells to contain a segment of video. The video may or may not have accompanied audio. From the DVD author's perspective, a cell is the smallest element of a DVD project. On each DVD, a cell is a unit of playback of real-time data and is uniquely identified by a set of numbers: cell id (from 1 to 255) and VOB id (from 1 to 65535). The usage of cell id and VOB id are explained in subsequent sections.

The DVD specifications require the video and its optional audio component to be recorded in specific data formats. A cell can be accessed randomly from any position of the DVD. The playback duration of a cell can be less than a second or several minutes long. At the end of the cell playback, each cell can request the DVD player to perform a specific task via a cell command. A cell command is a sequence of 8 bytes of binary data which can be any command provided by the DVD virtual command set. There are certain restrictions on cell commands depending upon the position of the cells. The DVD specifications provide a set of virtual machine (VM) commands which can instruct the DVD player to set, verify certain playback conditions or user options, and navigate or modify the playback sequence accordingly.

The presence of cell command is optional. This is how user interactivity is achieved. For example, a cell can play itself indefinitely until the user presses "stop" on the remote control by issuing a cell command to start the playback of itself. A cell command can request the DVD player to skip over a group of cells so that these video segments can never be played back even though they exist on the DVD. The possibilities are endless.

A cell can optionally have a mask and highlight layer to simulate the behavior of a push button in menu selection and the highlighting of certain display items for interactive navigation. Mask and highlight are complex topics and will be presented in details in the pages of cell architecture.

There are two types of cells: single-angle and multi-angle cells. Explanations of different types of cells are illustrated as following in the Unofficial DVD specifications Guide 2.2 ...