Video Hardware - Upgrading and Repairing PCs - page 802

Microsoft DirectX
Direct3D is part of Microsoft's comprehensive multimedia API, DirectX. Although the
most recent versions of DirectX (9.0c and up) provide support for higher-order surfaces
(converting 3D surfaces into curves), vertex shaders, and pixel shaders, significant differ-
ences exist between DirectX versions in how these operations are performed.
DirectX 9.0c uses separate pixel and vertex shaders to create 3D objects. Although Dir-
ectX 9.0c provides greater precision in data handling as well as support for more instruc-
tions, more textures, and more registers than its predecessors, its use of separate shaders
can still lead to slow 3D rendering when more pixels must be rendered than shaders, or
vice versa. Shader Model 3 (used by DirectX 9.0c) is simply a development of the split-
function design first developed for Shader Model 1 (used by DirectX 8.0) back in 2001,
adding support for more instructions and greater numerical accuracy.
DirectX 10, developed for Windows Vista, includes a completely rebuilt Direct3D ren-
dering engine with a brand-new shader design, Shader Model 4. Shader Model 4 adds a
geometry shader to the vertex shader and pixel shader design used in earlier shader mod-
els to improve the handling of real-time scene changes such as explosions. However, the
biggestsinglechangeinShaderModel4istheuseofunifiedshadersthatcanbeswitched
between vertex, pixel, and geometry shader operations on the fly, eliminating bottlenecks
and improving performance, no matter what types of 3D data exist in a scene.
Note
With the replacement of dedicated vertex and pixel shaders in the DirectX 10 3D rendering
pipeline, DirectX 10 GPUs are rated in terms of the number of stream processors on board.
Each stream processor performs vertex, geometry, and pixel shading as needed.
When you are comparing two otherwise-equal DirectX 10 GPUs (same GPU, memory size
and speed, motherboard and memory bus designs), the GPU with a larger number of stream
processors will be faster.
OtherarchitecturalchangesinDirectX10includeprocessoptimizationstoreducetheload
on the CPU. In a sample of different types of images rendered, DirectX 10 reduced the
command cycles by as much as 90% over DirectX 9.
DirectX 11 was originally developed for Windows 7 (and is also available for Windows
Vista) and adds several new features:
• Tessellation —Provides additional pipeline stages that increase the number of visible
polygons at runtime.
• Multithreaded rendering —Enables the execution of Direct3D commands on mul-
tiple processor cores.