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## Volume plot

Figure 58

This plot, shown in figure, uses both color and transparency to visualize 3D scalar fields. The values in the data range have associated color and opacity values that allow parts of the dataset to become partially or completely transparent. This plot captures internal details while keeping the whole dataset at least partially visible.

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.

Figure 59

The Volume plot attributes window, shown in figure, is divided into three main areas. The top Color area sets the colors that go along with the plot's data values. The Opacity area sets the opacity for the plot's data values. The bottom area contains controls which set the level of detail used to draw the plot.