Slicer Dicer is an interactive data exploration tool for fast visual access to volume data or any complex data in three or more dimensions. It is used for analysis, interpretation, and documentation of the data viewed and manipulated with various tools. The primary tools are transparency functions, block tools, cutout tools, rendering tools, lighting models, and animation.

Figure 18 illustrates the use of some tools for surface rendering in the study of an MRI data set. First, the transparency function was adjusted so that all values less than the air-skin threshold were transparent. Then, the block tool was used to define a large block encompassing the entire head. Next, the cutout tool was used to create two cutouts: one to remove a corner of the face, and the other to remove a central portion of the skull. Finally, the side-facing slice tool

FIGURE 20 MRI head data. Three frames from a Slicer Dicer animation featuring a moving axial slice.

was used to create an orthogonal slice in a particular coronal plane passing through the ear. Shaded isosurfaces that appear throughout the block represent the exterior boundary of opaque data.

Volume rendering provides an alternative, more holistic view of a data volume, as shown in Fig. 19, where an isosurface and cutout are defined allowing the exterior and interior portions of the skull to be seen. Also included are volume renderings in which the entire volume has been projected onto three orthogonal background planes, called projected volumes in Slicer Dicer. An image pixel in a projected volume is simply the maximal value along the perpendicular line of sight through the volume.

The Slicer Dicer lighting model includes both directional and ambient (nondirectional) light. The directional source can be placed anywhere using a special 2D controller. Distinct reflection coefficients control the amounts of directional and ambient light reflected from any surface. The shading parameter can be specified independently on three types of surfaces: isosurfaces, planar faces of slices, blocks, and cutouts, and background surfaces. These choices help to obtain a proper balance between the light used to indicate data variations, via a color or gray-scale mapping, and the light used to reveal surface shapes via shading.

For generating animations, almost any parameter can be animated, including the positions of objects (slices, blocks, etc.), "temporal" dimensions (for data sets defined in four or more dimensions), rotation angles, transparency, and others. To generate an animation, the user has to first "slice and dice" to create a useful visualization for the data set. Then an animation mode and output file format is selected. Figure 20 shows three frames from a Slicer Dicer animation featuring a moving slice. This example illustrates how both dynamic and static objects can be included in an animation. In this case, the static object is a block occupying the right half of the data volume. For both the block and the moving slice, the transparency function is adjusted so that all values less than the air-skin threshold are transparent.

Slicer Dicer supports more than 45 standard image formats, including TIFF, BMP, TGA, DICOM, JPG, HDF, netCDF, and AVI, and can also import user-defined data sets.

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