![](http://datasheet.mmic.net.cn/180000/BT2166AHF_datasheet_11281796/BT2166AHF_32.png)
Brooktree
3
PRODUCT DESCRIPTION
Introduction
L2166_A
Bt2166
Graphics/Video Controller
Perspective-correct texture mapping is supported with the use of bilinear or
trilinear ltering with subtexel accuracy to improve the appearance of the texture-
mapped surfaces. Filtering avoids rendering 3D objects with a ‘blocky’ appearance
which results from the use of lower performance, simple point sampling. Texture-
mapping performance is further enhanced with the Bt2166 through support of
bilinear MIP mapping, which uses a selection of images with predened versions
of the same texture map at varying resolutions. The appropriate texture map
resolution is chosen to provide the closest match with the size of the target triangle
to be rendered. Since the lower resolution textures are pre-ltered, 3D rendering
performance and frame buffer bandwidth are increased and the quality of the
texturing is enhanced.
The Bt2166 supports both transparency and translucency of texture maps. Tex-
ture transparency enables higher levels of detail by allowing complicated objects
to be mapped onto transparent polygons e.g. the texture may have holes through
which the underlying background should be visible for realistic effect. A translu-
cent texture can also be applied by alpha blending the texture at varying levels of
translucence with an underlying object or scene, enabling realistic fog or smoke ef-
fects.
Palletized textures are supported in addition to 16-bit and 24-bit RGB textures.
Compared to 16-bit RGB texturing, palletized texturing reduces the memory re-
quirements for texture maps by up to 75%. Accordingly, a larger number of tex-
tures can be stored in off-screen memory, and the memory bandwidth requirements
for manipulation of texture data can be greatly reduced. With this technique a 4-bit
(16 color) or 8-bit (256 color) Color Lookup Table (CLUT) is assigned to each tex-
ture in a scene. Typically, a pallet of 16 or 256 colors is more than sufcient to ac-
curately render any given texture with 16-bit or 24-bit color resolution. Hence,
palletized textures enable the use of more textures and make available additional
memory bandwidth for the rendering engine, thereby providing enhanced realism
and rendering performance.
The alternatives to using palletized color texturing are to either sacrice render-
ing performance using 16-bit RGB textures or to achieve reasonable rendering per-
formance at the expense of reduced realism in the texture color representation. The
Bt2166 can read 4-, 8- and 16-bit textures packed into a 64-bit word. Support of
packed texture formats provides texture caching by grouping texels which are spa-
tially adjacent. This will improve the rendering engine throughput when bilinear
interpolation is being used by limiting the number of required memory accesses to
retrieve texture data.
When rendering objects, the Bt2166 rendering engine calculates and stores a Z
or depth value for each pixel in the display to support occlusion of surfaces on ob-
scured objects. Z-buffering is supported in resolutions of 8, 16, or 32 bits/pixel.
The Bt2166 also supports texture sorting when the Z-buffering algorithm is being
used to further optimize texture caching performance.