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

2.2 Advanced Graphics Processing
2.2.2 GPGPU - General-Purpose Computation on the GPU
Programming the GPU for general purposes has several constraints. The techniques to
handle them are presented in this section and much more in detail in [Thompson2002;
Pharr2005; Nvidia2005].
2.2.2.1 Graphics Card/GPU Memory
The access to memory on GPUs is restricted to various kinds of indexed texture memory
lookups into one, two or three-dimensional banks of fast accessible texture memory.
The amount of memory has to be defined in advance through a texture of a certain
size. Each of its ”memory” element (texel) gets processed during one iteration. First
and foremost this means that the programmer has no opportunity to allocate memory
during the shader program’s execution or to use pointers. Nevertheless, these concepts
are generally not necessary since the complete data is represented by a stream and
interpreted by a stream-processor. Consequently, using memory on graphics hardware
in both, vertex and fragment shader will mostly look like the following algorithm:
1. define a texture of a certain size with the main application,
2. pass the texture-ID to the desired shader program,
3. define in the shader program what to do with each texture’s element,
4. tell the hardware when to use the shader in each rendered frame.
Using memory is always accompanied by two basic concepts:
• Gathering describes an operation which takes values from different memory
addresses to obtain one result.
• Scattering is the opposite an operation which stores several results in different
memory addresses at once.
On a GPU only the first concept is available because of the fixed defined output
address of the process data stream. However, it is possible to store a result in multiple
render targets at this predefined position. This implies that graphics hardware is able
to render calculated pixel values not only on screen but also in a dedicated buffer in the
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