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Figure 58 |
The Volume plot uses a visualization technique known as volume-rendering, which assigns color and opacity values to a range of data values. The colors and opacities are collectively known as a volume transfer function. The volume transfer function determines the colors of the plot and which parts are visible. The Volume plot uses three types of volume-rendering to visualize data.
The first volume rendering method, hardware-accelerated splatting, resamples the entire database onto a small rectilinear grid and then, at each node in the grid, draws a small textured polygon. The polygon gets its colors and opacity from the transfer function. This method is fast due to its use of graphics hardware but it can require a large number of points in the resampled mesh to look accurate.
Like the first volume rendering method, the second method, hardware-accelerated 3D texturing, resamples the entire database onto a small rectilinear grid. Once the data has been resampled, it is converted into a 3D texture using the Volume plot's volume-transfer function and gets loaded into the video card's texture memory. The Volume plot then draws a set of planes that are perpendicular to the view vector from back to front, with each plane getting the pre-loaded texture mapped onto it. The resulting image is very crisp and captures details not evident when the splatting method is used.
The third volume-rendering technique, called ray-casting, used by the Volume plot is not hardware accelerated. In ray-casting, a ray is followed in reverse from the computer screen into the dataset. As a ray progresses through the dataset, sample points are taken and the sample values are used to determine a color and opacity value for the sample point. Each sample point along the ray is composited to form a final color for the screen pixel. Rays are traced from closest to farthest to allow for early ray termination which stops the sampling process when the pixel opacity gets above a certain threshold. This method of volume-rendering yields superior pictures at the cost of speed and memory use.
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Figure 59 |