845GL datasheet(128/193 Pages) INTEL

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Functional Description

128

Intel® 82845G/82845GL/82845GV GMCH Datasheet

4.4.2

Video Engine

4.4.2.1

Hardware Motion Compensation

The Motion Compensation (MC) process consists of reconstructing a new picture by predicting

(either forward, backward, or bidirectionally) the resulting pixel colors from one or more reference

pictures. The GMCH receives the video stream and implements Motion Compensation and

subsequent steps in hardware. Performing Motion Compensation in hardware reduces the

processor demand of software-based MPEG-2 decoding, and thus improves system performance.

The Motion Compensation functionality is overloaded onto the texture cache and texture filter. The

texture cache is used to typically access the data in the reconstruction of the frames and the filter is

used in the actual motion compensation process. To support this overloaded functionality the

texture cache additionally supports the following input formats:

• YUV420 planar

Sub-Picture Support

Sub-picture is used for two purposes; one is Subtitles for movie captions, etc. which are

superimposed on a main picture, and another is for Menus to provide some visual operation

environments for the user of the player.

A DVD allows movie subtitles to be recorded as Sub-pictures. On a DVD disc, it is called

“Subtitle” because it has been prepared for storing captions. Since the disc can have a maximum of

32 tracks for Subtitles, they can be used for various applications; for example, as Subtitles in

different languages or other information to be displayed.

There are two kinds of Menus, the System Menus and other In-Title Menus. First, the System

Menus are displayed and operated at startup of or during the playback of the disc or from the stop

state. Second, In-Title menus can be programmed as a combination of Sub-picture and Highlight

commands to be displayed during playback of the disc.

The GMCH supports sub-picture for DVD and DBS by mixing the two video streams via alpha

blending. Unlike color keying, alpha blending provides a softer effect and each pixel that is

displayed is a composite between the two video stream pixels. The GMCH can utilize four methods

when dealing with sub-pictures. The flexibility enables the GMCH to work with all sub-picture

formats.

4.4.2.2

Planes

A plane consists of a rectangular shaped image that has characteristics such as source, size,

position, method, and format. These planes get attached to source surfaces, which are rectangular

memory surfaces with a similar set of characteristics. They are also associated with a particular

destination pipe.

4.4.2.3

Cursor Plane

The cursor plane is one of the simplest display planes. With a few exceptions, it has a fixed size of

64x64 and a fixed Z-order (top). In legacy modes, the cursor can cause the display data below it to

be inverted.

845GL Datasheet(PDF) 128 Page - Intel Corporation

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