
Brooktree
229
3D GRAPHICS ACCELERATOR
Pixel Rendering Engine
L2166_A
Bt2166
Graphics/Video Controller
Translucency
This is the affect of blending a pixel that is to be displayed with a second pixel val-
ue which could be a value currently on the screen or a xed color.
Transparency
Texture maps can be used in such a way that they have ‘holes’ in them. When a
transparent part of a texture map is detected no pixel is written out so that the pre-
vious value in the screen map is left unchanged.
Alpha Mapping
It is useful to dene textures which also contain translucency, this known as an al-
pha map. A special type of mode is used on the texture to accommodate this. This
translucency data is blended with the texture color.
Light Sourced Texture
Mapping
Flat shading lighting effects can be applied to textures. The illumination model is
applied to the whole of the texture on the surface of the polygon.
Gouraud Shaded Texture
Mapping
Gouraud shading effects can additionally be applied to textures. The illumination
model is applied in different increments across the span.
Perspective Texture Mapping
When a texture recedes into the distance perspective is applied to it. Without prop-
er perspective correction applied to the co-ordinates of the texture, artefacts occur
on the texture closest to the view point.
Bilinear/Trilinear Interpolated
Texture Mapping
When displaying texture mapped polygons close to the viewpoint, one 'texel' in the
texture map is mapped to many screen pixels. The polygon thus takes on a very
'blocky' appearance. Bilinear or trilinear interpolation is used to smooth out such
artefacts, by interpolating the values in the texture map so that it appears more de-
tailed.
Z Buffering
When rendering 3-D scenes, some portions of polygons may not be visible. There
are two types of cases where this effect occurs. In the rst, some polygons which
dene the surface are facing away from the viewpoint - they are on the back surface
of an object. In the second, two objects may overlap or interpenetrate each other.
One solution to the problem of which polygons (and parts of polygons) to ren-
der is to use a 'Z buffer'. When a polygon is rendered, at the same time that a color
is written to the screen buffer, a value which represents the distance of that point on
the polygon from the viewpoint (the Z co-ordinate) is written to a corresponding
position in a Z buffer.
Through the use of the Z buffer objects that are not obscured by other objects re-
main on the screen.
Dithering
Shading effects sometimes produce contouring affects on a polygon. Through the
use of random dithering on the RGB values, before output to the screen memory,
this contouring effect is reduced.
Line Drawing
Lines drawing is accomplished by making use of PRE’s ability to draw trapezoids.