Thanks, Kevin, that's exactly what I want. I thought I'd scoured the
documentation but somehow I missed ST_Simplify.
- Dan
Kevin Neufeld wrote:
Have you tried putting your resultant multipolygons through
ST_Simplify or ST_SimplifyPreserveTopology? Both use the
Douglas-Peuker algorithm to weed out unwanted vertices.
Cheers,
Kevin
Dan Johnson wrote:
I have an application that keeps a database of video footprints, one
quadrilateral per frame of video. I consolidate each 5 seconds of
video footprints into a single multipolygon in order to speed up
queries for video of a specified area. That's typically a union of
150 (30fps) polygons with 4-vertexes. I build this multipolygon with
ST_GeomUnion, and it's acceptably fast. So far, so good.
I've been quite surprised by the variance in the number of vertices
in the resulting multipolygon - sometimes it's as high as 1200.
Usually it's lower than the equvalent set of frame footprints, but
not always. This hasn't been a problem for database queries, but now
it's time to build a user interface and of course the UI has problems
displaying that many vertices. (Actually, the UI is getting the
polygons via a SOAP message, and our web server's SOAP infrastructure
often just dies when the result is so long.) Eyeballing these
consolidated polygons, it looks like keeping only 20-30 vertices
would provide an excellent approximation. (It's quite common for the
camera to stare at a single point for a long time, in which case all
the frames are in about the same location.) This seems like the sort
of thing PostGIS would be able to do but I haven't been able to
figure it out from the docs. Does anyone know if this is possible?
(I've been told the footprints have to consider every frame, so
building the union with one frame per second or similar is out.)
Thanks,
Dan
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