🚀 go-pugleaf

In article <1992Dec30.150955.18683@netnews.whoi.edu> rsignell@crusty.er.usgs.gov (Richard P. Signell) writes:
>Is there a way to tell isosurface to ignore missing or special
>values in a 3D grid?  Setting missing values to zero or some
>other value does *not* have the desired affect.
	You might try putting your data into a UCD.  You will have to
	partition your data space into 3-D cells ( hexahedrons,tetrahedrons,
	pyramids, prisms) such that none of the nonexistent data points
	is included in a cell. Note that it must be a partition---no
	overlap is allowed.  The ucd iso module should then do the right thing.

	By the way, the problem you describe is rather common with the 
	AVSfield data type and one of the reasons why the UCD type is needed.
	Any time that the domain of definition of the function (the points 
	at which functional values occur) is topologically "complex" it 
	cannot be represented well with an AVSfield.  AVSfields assume that 
	the domain of definition is 2N-fold connected (where N is the 
	dimensionality of the space) and that linear interpolation can be done
	between neighboring points.  To make this more concrete, AVS fields
	in 3D assume that there are 6 neighbor points to point(i,j,k) 
	{point(i,j,k+1),point(i,j,k-1), . . . point(i+1,j,k),point(i-1,j,k)}
	It also assumes this same neighbor relation for functional values 
	and assumes that linear interpolation can generate the values 
	between two points (e.g. data(i,j,k) and data(i,j,k+1)).
	Your functions do not fit this assumption.  As you pointed out some 
	other packages take care of topological problems like yours by 
	allowing a complex data domain to be "gridded over" by a nice 2N-fold 
	grid where "illegal" values are used to indicate "holes".
	This can be a problem since it is conceivable that any value you
	may choose to be illegal may exist in some data sets and it can
	also be wasteful of storage since the grid must be the bounding box 
	of the data set.  In AVS the use of UCDs to model complex topologies
	is the preferred method.

	I hope this answers your question.  Feel free to contact me via
	email at the address below if you have any further questions.

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Steve Gardner					Convex Computer Corporation
Software Engineer				3000 Waterview Parkway
Scientific Visualization Applications		P.O. Box 833851
email: gardner@convex.com			Richardson, Texas 75083-3851
Phone: 214-497-4539  				Fax: 214-497-4500
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