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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