regularly spaced coordinates

gabor@hermes.chpc.utexas.edu
Tue, 24 Mar 92 17:24:02 CST 

Dear Gurus,

  I cannot stand adding my opinion to this topic. I suggest we
approach the question carefully and look around before fixing this
problem locally. Quick fire-fighting work always turns out to be
painful.

  I have been working around metafile and data base formats defined on
many conceptual levels of computer graphics. As I have found, most of
the data formats have serious problems with implicit coordinate
definition. The issue of storage of regularly gridded data is present
everywhere in visualization; and it has not been soved, not even much
addressed.
  Scientic databases, like -so far- NetCDF and HDF simply omitted the
problem. On the level of 3D graphics systems PHIGS, GKS-3D, Dore, do
not support implicit coordinate definition. The situation is a little
bit better in 2D: PostScript and the character encoding of CGM support
"incremental coordinate mode."  Instead of specifying the whole
coordinate for each element in a list of coordinates, you specify only
the increment (or decrement) from the previous element of the list. It
is more, then nothing, anyway.

  Harry Jentner said he would not like to have assumptions. Me either.
But netCDF already does have one. It is assumed all data are given
over rectangular grid.  (I am speaking about three dimension for
simplicity. In higher dimensions the problem is analogous.) Distances
between every two gridlines along a dimension are not necessarily
equal. But the quadrliaterals were garanteed rectangular.)

  What Harry suggests is a way to indicate if the given grid in one
dimension (that may happen to be TIME) is equally spaced. But what
about the other functions? What if a dimension is logarithmical or
derived with any other function?

  And further, data points are not necessarily given over a
rectangular grid. The grid could be just genarally quadrilateral, or
may be triangular as often is. Or what about a general polygonal grid?
Will we introduce more and more assumptions?  What we need is to think
over the whole issue, introduce a CONCEPT instead of AD-HOC attribute
or whatever solutions. I'd be be glad to take part in this work any
time.

                       		Gabor Fichtinger

				Scientific Visualization Group,
				Center for High Performance Computing
				The University of Texas System
				Balcones Research Center, 1.154 CMS
				10100 Burnet Road, Austin, TX, 78758-4497
				Ph.  : (512) 471 2409
				Ph.  : 1-800-262-2472/2409 (toll free)
				Fax  : (512) 471 2445	
				Email: gabor@chpc.utexas.edu
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