Master's Thesis - Studierstube Augmented Reality Project - Graz ...

2.2 Advanced Graphics Processing
programming features available since Shader Model 3.0 1 and the increased processing
speed have been used. This is because of the desired compatibility to currently available
systems in clinical routine. The next section outlines only the basic concepts of
shader programming since Shader Model 3.0.
Currently three main shader languages exist:
• Cg which stands for ”C for graphics” and is developed by Nvidia,
• GLSL stands for ”Graphics Library Shader Language” and is provided for
OpenGL,
• HLSL stands for ”High Level Shader Language” and applies to DirectX 2 based
applications.
They are all C-like programming languages and are of the same value. The implementation
in chapter 5 concentrates on GLSL. However the following techniques can be
implemented with all other languages as well.
In general two types of shader programs can be distinguished. Firstly a so-called
vertex program executed by the vertex processing unit is at least able to define a new
position for each given vertex position processed for each frame with any parameters as
input. Secondly a so-called fragment program processes all pixel candidates which were
produced between the vertices by a rasterization unit. The output of a fragment shader
will always be a RGBA color value. The output of such shader does not necessarily need
to be rendered to a display device. Instead it can be written into a buffer or texture
for further use. Furthermore the implicit float arithmetic of graphic microprocessors
allows calculations for general purposes.
A benefit - in contrast to CPU processing
- will be achieved if graphics operations are performed on multiple input data and
vectors data types. Since graphics hardware matches these constraints it is also called
a Single Instruction Multiple Data (SIMD) Processor. A perfect example for such a
SIMD operation will be the update mechanisms on millions of particles moving through
a flow velocity field as described in section 2.1.1.2.
1 Shader Model 3.0 (SM 3.0) was developed by Nvidia with the GeForce 6x-series in 2004.
2 http://www.gamesforwindows.com/en-US/AboutGFW/Pages/DirectX10.aspx
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