Iso Plots: Controlling the Attributes of Iso Mode Plots
Iso Plots: Controlling the attributes of ISO mode plots
By default ISO plots are opaque, but this can obscure internal detail so it is possible to make them transparent.
Transparency on graphics hardware is not perfect and, moreover, can be slow to render. So two alternative methods of displaying transparent ISO contours are provided.
Here is the same cantilever as above, rendered as a default (opaque) ISO plot: 
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This is the same model using transparent "stippled" contours, with the lowest values (blue) rendered as opaque. 
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And here is the same image again, rendered using "Alpha blending". The quality of the transparency is much better although it would be slower to draw with a large model. However ... 
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... using Alpha transparency does not always give the "right" answers. Here is the same image, this time with the high values opaque, and it is clear that the lower colours are not visible when they are behind the higher ones. This is because "Alpha blending" in the hardware is not, in itself, always a totally reliable way of producing realistic transparency because the results depend on the order in which facets are drawn. It may be necessary to revert to stippling, which will always give "correct" precedence of facets - albeit with a poorer spatial resolution - to get acceptable results. 
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bit versus 16 bit Plot Resolution
8 bit gives a coarser result leading to a "bumpier" plot that is usually acceptable for structural data.
16 bit gives a smoother result that looks better, and is better suited to the
sort of eulerian data used in Volume III type analyses. 16 bit plots use twice
as much memory, which is why they are not the default for structural data plots.
You can change these defaults via the preferences:
d3plot*struct_iso_resolution: 8 or 16
d3plot*vol3_iso_resolution: 8 or 16