82815 GMCH
R
Datasheet
145
4.8.2.
3D Engine
The 3D engine of the GMCH has been architected as a deep pipeline, where performance is maximized
by allowing each stage of the pipeline to simultaneously operate on different primitives or portions of the
same primitive. The GMCH supports perspective-correct texture mapping, bilinear, trilinear, and
anisotropic MIP map filtering, Gouraud shading, alpha-blending, colorkeying and chromakeying, full
color specular shading, fogging and Z Buffering. These features can be independently enabled or
disabled via a set of 3D instructions. In addition, the GMCH supports a Dynamic Video Memory
Technology (D.V.M.T.) that allows the entire 3D rendering process to take place in system memory;
thus, alleviating the need for the display cache.
The main blocks of the pipeline are the Setup Engine, Scan Converter, Texture Pipeline, and Color
Calculator block. A typical programming sequence would be to send instructions to set the state of the
pipeline followed by rendering instructions containing 3D primitive vertex data.
4.8.3.
Buffers
The 2D, 3D, and video capabilities of the GMCH provide control over a variety of graphics buffers that
can be implemented either in display cache or system memory. To aid the rendering process, the display
cache of the GMCH contains two hardware buffers—the Front Buffer (display buffer) and the Back
Buffer (rendering buffer). The image being drawn is not visible until the scene is complete and the back
buffer made visible (via an instruction) or copied to the front buffer (via a 2D BLT operation). By
rendering to one and displaying from the other, the possibility of image tearing is removed. This also
speeds up the display process over a single buffer.
The 3D pipeline of the GMCH operates on the Back Buffer and the Z Buffer. The pixels’ 16-bit
(or 15-bit) RGB colors are stored in the back buffer. The Z-buffer can be used to store 16-bit depth
values or 5-bit “destination alpha” values. The instruction set of the GMCH provides a variety of controls
for the buffers (e.g., initializing, flip, clear, etc.).
Figure 9. Data Flow for the 3D Pipeline
Notes:
1. Frame Buffer = Front and Back Buffers
3d_pipe2.vsd
Depth Buffer
(Z-Buffer)
Display Cache
System Memory
Textures
Instructions
and Data
GMCH
Interface
Primitives
Pixels
Mapping
Engine
Color
Calculator
Rasterize
Setup
Discard
(Back Face
Culling)
Frame Buffer
GMCH Graphics Pipeline
(Conceptual Representation)