Glossary of Terms Used for Digital Video Devices
The following terms are used in describing the MCI commands for digital video devices:
Client Rectangle: The rectangular area used by the window to render motion video or still images.
Device Name: The name used to identify the device after it is opened. Typically it is the filename corresponding to a digital-video resource or an alias.
Device Specific: An aspect of the command set left to the discretion of the device developer. When a feature is outside the definition of the command set, it is referred to as device specific.
Device Instance: Each open command creates a device-driver instance and associates a device name with it. The device name is used in subsequent commands to reference the device-driver instance. The state of a device-driver instance is the net effect of all the commands sent to a device with a given device name. The device driver instance, and all state information associated with it, ceases to exist after the device is closed.
External Input: A digital-video device might be connected to a source of analog video and/or audio data. The result of digitizing this data is called the external input.
Frame Buffer: The frame buffer is an off-screen area containing the pixels that can be copied to the display. Commands in the digital video command set affect which pixels are updated as video is played or digitized, and how pixels are copied from the frame buffer to the visible display.
Frame Directory: Digital video encoding uses techniques such as interframe encoding and variable length encoding that can make starting playback at an arbitrary frame difficult. For algorithms and file structures that do not support uniquely identifiable image-start codes, auxiliary information associated with the file can make random access more efficient. When such auxiliary information is provided, it is called a frame directory. A possible method of generating the frame directory might involve post processing the media file. The format of the frame directory and how it is associated with the media file is device dependent. Whether or not a file has an associated frame directory affects the semantics of the various commands. These effects are described in the command overview and with the affected commands.
Presentation Source: The sound and pictures presented to the user can come from either the workspace or the external input. The presentation source is the selected origin for the sound and pictures. The user presentation is also conditioned by whether audio and/or video output have been enabled, which pixels of the frame buffer are enabled for update (if any), and what (if anything) is currently in the workspace or connected to the external input.
Track: The media can be divided into a series of tracks. These function like chapters in a book to break up a long file into sections. Most digital-video devices do not support tracks. For these drivers, the result of status queries about track information assume the media consists of exactly one track.
Unsaved Data: After recording, newly created data exists in the workspace. This data is discarded if commands such as close, load, record, and reserve are used before the data is saved. In some circumstances it is appropriate to warn the user before performing these commands. The device driver should keep track of whether there is unsaved data in the workspace and let the application detect this situation by using the status device name unsaved command.
Window: Video or images are displayed within the client area of a window. If the application does not specify a window, or specifies the default window, the device driver selects the window appearance and how it responds to user actions. Alternatively, an application might create its own window, and pass its handle to the device driver. In this case the application controls the appearance of the window and how it responds to user actions. The device driver can subclass this window and provide additional or alternative actions to those specified by the application's window procedure.
Workspace: Every device instance has an associated workspace containing a working copy of the multimedia data. Commands such as record, load, and paste place new multimedia data in the workspace. (Record compresses analog video and/or audio data into the workspace.) Commands such as play present the data in the workspace to the user.