Biomedical Engineering Reference
In-Depth Information
OpenGL-compatible video cards are designed to accelerate specific graphics procedures. These cards
have their own high-performance microprocessor, high-speed video RAM, and support programs
stored in firmware that can be accessed through software drivers. Image data communicated to the
video card from the workstation through the main bus are buffered and processed according to the
type of content. Content such as windows, buttons, and icons common to the Windows interface is
sent to the 2D graphics coprocessor that contains instructions in firmware optimized for rendering
these objects. These and other graphical interface elements are rendered to the Video RAM, which is
configured as a frame buffer. In this way, the processor can be rendering the next frame or image
while simultaneously driving the monitor with the current image, allowing a high-speed refresh rate
that eliminates flicker.
In a similar way, 3D structures are rendered at high-speed by the dedicated 3D graphics
coprocessor. The processor is optimized for making the many calculations required for rendering 3D
images—including smoothing image edges, shading, applying texture maps to objects, providing
perspective corrections, and mapping polygons over wireframe skeletons—thereby freeing the
computer's main CPU(s) to do other tasks. High-end, specialized workstations from Sun, Silicon
Graphics, DEC, and other manufacturers often use proprietary graphic support hardware that may
not be OpenGL-compatible. Programs that work on these computers must use special video driver
software in order to take full advantage of proprietary high-performance hardware.
Returning to the discussion of visualization program architectures, both Web-enabled and stand-
alone applications receive high marks for performance, for the same reasons cited for extend browser
applications. Web-enabled programs such as PyMol and stand-alone applications such as RasMol are
designed to take advantage of local RAM and CPU power, as well as OpenGL-compatible video cards.
Because updates to programs must be downloaded from the Web, maintenance is an issue, and it's
up to the user to maintain the latest version of the program and keep associated drivers up-to-date.
Portability is also an issue for Web-enabled and stand-alone programs because they have to be
installed on every workstation that may be used for rendering. In addition, stand-alone applications
tend to have a fixed user interface that can't be easily modified. In contrast, others, such as Chimera
from the Computer Graphics Laboratory, University of California, San Francisco, can be heavily
modified and integrated into other programs.
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