[postgis-devel] wktraster ST_ConvexHull
lr at pcorp.us
Tue Apr 6 04:28:58 PDT 2010
Thanks. I'm feeling déjà vu now. I'm sure Pierre explained this to me
before and pointed me at the spec and I forgot, I guess cause it feels kind
of unintuitive to me. I apologize for my denseness.
So the ST_ConvexHull is really only useful if you have an irregularly shaped
raster or a rotated raster which I guess I don't commonly run into.
The ST_AsPolygon I guess I also got wrong. I keep on thinking about it as
what is referred to as ST_Shape except my concept of ST_Shape would take all
bands (leaving out nodata) into consideration instead of a specific.
Would seem better to call ST_AsPolygon - ST_DumpPolygons or something like
that as it seems to be more in line with the PostGIS concept of ST_Dump,
ST_DumpRings, ST_DumpPoints... that return geometry_dump.
The distinction I see it would always return a dump of polygons rather than
an arbitrary dump of geometries
And the path value would be replaced with a pixel value.
Getting to names -- ST_AsWKTPolygon should not start with an ST_ if it is
not intended for end user use.
From: postgis-devel-bounces at postgis.refractions.net
[mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of Jorge
Sent: Tuesday, April 06, 2010 6:11 AM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] wktraster ST_ConvexHull
On Tue, Apr 6, 2010 at 3:14 AM, Paragon Corporation <lr at pcorp.us> wrote:
> I'm a little bit confused about what ST_ConvexHull is supposed to
> return. I thought it would return the convex hull of the polygon
> formed by the pixels that aren't the BandNoDataValue value.
> It seems to always return the same result as ST_Envelope regardless of
> what I set the ST_BandNoDataValue too.
The resulting polygon from implemented ST_ConvexHull function encloses every
pixel, even those containing NODATA values. You can read the specs here
Anyway, that point is not included in final specs yet. So, comments are
welcome. Pierre can provide more qualified opinion.
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