[gdal-dev] libkml GroundOverlay support

[Clarke, Trevor]

I am in need of support for GroundOverlay loading from KML and I've been
prototyping some ideas. The biggest issue is that GroundOverlays are
conceptually raster data in a primarily vector format so it's unclear
how OGR/GDAL should handle this. It's useful to have relational
information and bounding rectangle information coming back from OGR but
OGR shouldn't load the raster data, that's obviously a GDAL job.

For background, here's now GroundOverlays are specified. A GroundOverlay
element has to have a LatLonBox to specify the bounding rectangle for
the imagery and it also needs to have a URL specifying the source of the
imagery. This can be a web url, a file url or a relative path (if it's
in a KMZ). In addition, descriptive metadata and a DrawOrder can be
specified. Defined behavior when an image URL isn't specified or can't
be loaded is to draw a rectangle with the specified (or default) color
and style information.

I propose the following behavior. In OGR, the GroundOverlay is added as
a polygon feature specifying the bounding box and style/draw information
for the "can't load" display mode. Draw order, and URL are attached as
fields as well as a field indicating this is a GroundOverlay and not a
Placemark. This is enough to load and work with the bounds, etc. and the
URL can be passed to GDAL for raster loading (via /vsicurl/ if
necessary).

This will not allow a user to pass a KML directly to GDAL and get access
to the embedded images.

Thoughts?

------------------
Trevor R.H. Clarke
Ball Aerospace & Technologies Corp
Opticks, Free Your Imagery (http://opticks.org)
GPG key available on random.sks.keyserver.penguin.de





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