[tiling] The GeoMatrix Tiling system

[Chuck Stein]

Greetings all,

The GeoMatrix(r) Globe tiling system is a very powerful system used in
virtual globe rendering systems.  This is the same tiling system implemented
by our GeoMatrix Toolkit SDK and used by ESRI in ArcGlobe, ViewTec in
TerrainViewGlobe, and other products including our free web-based GeoPlayer
viewer (www.geoplayer.com/gateways).  I have attached a pre-print of a paper
presented by Paul Hansen (GeoFusion President) at the 2nd International
Conference on Discrete Global Grids.

The GeoMatrix system uses a hybrid projection, Geographic between +45 and
-45 latitude, and a "square polar" projection in the polar areas.  The
square polar projection allows us to have a very efficient representation at
the poles while reducing distortion during rendering (texture mapping).
Please read the PDF for the details.  Beyond the projection details our tile
format includes:

+ fast lookup "database" (minimum-quad-tree - mqt) look-up file that
identifies tiles that exist in the dataset and whether they have full or
partial coverage by the data.

+ optional 1-bit in alpha channel indicating valid/invalid (transparent)
pixel data.  Full 8-bit alpha supported for blending of datasets.

+ support for numerous "texture" formats.  Since our renderer is using
texture mapping to display the imagery on the terrain, we support normal
texture formats including DXT texture compression formats.  These provide a
huge gain for the transmission phase and client side (reduced requirements
for texture memory and loading).

+ zlib (lossless) compression of each tile for further reduction in size for
transmission and loading.

+ the scheme supports arbitrarily sparse datasets (islanded).

Until now we have used our own GeoMatrix Tile Server and I am very
interested in getting GeoMatrix support from a WMS server.  GeoMatrix Tiles
can be specified simply with:

        + dataset name
        + globe face
        + resolution level
        + row
        + column

We currently load a reference file (XML metadata) and the MQT database file
first before requesting tiles.   This provides an optimized situation where
the client knows in advance where the tiles are.  An alternative is to
assume global coverage and just request tiles from a WMS.  The biggest
hurdles in implementing this in a WMS is getting support for the
square-polar projection, DXT compression formats, valid/invalid bit, MQT
file.  I am very interested in exploring how a WMS can server GeoMatrix
Tiles, there are many possibilities including having some of the work could
be done on the client side (although this would not be optimal).

I look forward to your comments.

Chuck

     Chuck Stein
   GeoFusion, Inc.
  www.geofusion.com
     831.662.2282
 stein @ geofusion.com 


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