Thread View: comp.graphics.avs
3 messages
3 total messages
Started by Richard P. Signell
Wed, 30 Dec 1992 15:09
Handling of "missing" or "special values"
Author: Richard P. Signell
Date: Wed, 30 Dec 1992 15:09
Date: Wed, 30 Dec 1992 15:09
30 lines
2148 bytes
2148 bytes
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. The problem of missing values arises all the time in oceanography, since we are often working with data on some sort of a latitude, longitude and depth grid, but we only have data in ocean grid cells. In land grid cells, we have no data, and for graphical techniques like contouring and isosurfaces, we want these points to be ignored. Most contouring packages (e.g. ISM, NCAR graphics) allow you to define a special value, say 1.e35, that indicates to the application that the point is to be ignored. The contouring routine recognized special values, and no contours are drawn in any box of the grid with a special value at one or more of its four corners. This capability, however, does not seem to exist in AVS. Am I missing something fundamental here? The simple technique of just manipulating the data field by setting all the values to zero or some other number does not work, since contour or isosurface will try to draw contours or isosurfaces between the "good" data points and the "missing" data points. I believe the modules themselves (isosurface, contour to geom ,etc) would have to recognize "special values" and act apppropriately to handle this problem. I would love to here what other people have to say about this problem.... -- Rich Signell | rsignell@crusty.er.usgs.gov U.S. Geological Survey | (508) 457-2229 | FAX (508) 457-2310 Quissett Campus | " When marriage is outlawed, Woods Hole, MA 02543 | only outlaws will have inlaws. "
Re: Handling of "missing" or "special values"
Author: Steve Gardner
Date: Wed, 30 Dec 1992 16:42
Date: Wed, 30 Dec 1992 16:42
43 lines
3360 bytes
3360 bytes
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.
==============================================================================
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
==============================================================================
Re: Handling of "missing" or "special values"
Author: lakerb
Date: Wed, 30 Dec 1992 18:14
Date: Wed, 30 Dec 1992 18:14
16 lines
1029 bytes
1029 bytes
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. > One trick we considered (topo map with some corners missing) was to break the grid domain into several rectilinear 2-D fields. We would then assign the appropriate coordinates to each data point and combine the several 2-D views in a Geometry Viewer. We ended up re-gridding the data into a spreadsheet and filling in the missing corners by hand ... Rob Lake BP Research
Thread Navigation
This is a paginated view of messages in the thread with full content displayed inline.
Messages are displayed in chronological order, with the original post highlighted in green.
Use pagination controls to navigate through all messages in large threads.
Back to All Threads