Selecting Suitable Thresholds

The concepts behind generating a surface are similar in nature to creating a contour plot on two-dimensional data. A threshold value, or range of threshold values, must be selected from the data that will be used to define the surface of interest.

The main Tiling Parameters panel depicts the dataset volume rendered using the thresholds specified on the slider bar. Adjusting either end of the threshold bar erodes away portions of the data and the three-dimensional surface bounded by those thresholds appears. The rendering may be "rotated" using the sliders on the left, right, and bottom sides of the display window.

The image of the surface displayed when toying with the threshold values is only an approximation of the final product. The reduced volume visualization algorithm used to create the image in the render window is not used in the calculation of the three-dimensional surface, and therefore cannot be relied upon to predict the final outcome of the surface building procedure. The surface extraction algorithms described in the next section define the actual polygons that make up the surface.

Do it yourself...
1.	Note the appearance of the dataset as a solid block of data in the volume visualization/rendering pane of the Tiling Parameters panel. The default threshold selects everything for inclusion in the surface. (The surface appears as a solid only if the thresholds have not been adjusted through the use of the View:Intensities... control panel. Otherwise the threshold values are preserved and a structural surface may have appeared.)
2.	Adjust the minimum and maximum threshold values, inspecting the revealed surface in the render window after each change.
3.	Rotate the volume visualization using the sliders surrounding the image pane. Observe the dataset in a variety of orientations to ensure suitable threshold selection.
4.	Set the minimum threshold to 121 and the maximum to 255. These are the values used to create the examples in this technical report.

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