On 06/15/2011 07:03 AM, Pierre Racine wrote:
I may be missing the boat here, but it seems to me that a version of
ST_Intersection() which returns a raster would eliminate the need for a
separate geometry parameter, and would be more in line with the spirit
of "seamless vector/raster" processing. It also would seem that your
separate geometry parameter would behave very much like a
raster-returning ST_Intersection(), except that it is only usable "vicariously"
via
the stats functions.
Right. It would even be of greater value for other functionalities even if, I
think,
it would be slower for the particular case of computing stats .
Sorry but I must come back on this.
This is true when the vector area is small in comparison with the raster area AND if the
raster coverage is tiled. In this case vectorization is possible and provide results very
fast. This is true also if the raster is not tiled but is small. This operation provide
very precise numbers because the intersection is done in vector mode and every pixel are
cut when necessary. What you get is not a mere count of the pixels intersecting "a
la Starspan" but true areas.
In the other case (if the vector area is big in comparison the raster area OR the raster
is big and not tiled) the all vector method is catastrophic as you have to vectorize many
time large parts of the raster and this become unpracticable. This is why we need a pixel
counter (ST_Histogram) accepting geometries. The provided result are somewhat coarser
(take all the pixel or nothing) and provide a "a la Starspan" result, but at
last, provide a result in reasonable time.
This is why we need ST_Histogram, ST_SummaryStat, etc., accepting a geometry as
parameter.
Pierre,
Is it safe to assume that what you're looking for is a ST_AsRaster to
convert the geometry into a raster so that we can do raster on raster
filtering?
I've been wondering as to which method (covert raster to vector or
convert vector to raster) is more efficient, hence my delay in adding a
geometry parameter to filter a raster.
Then again, I've been thinking that there are fundamental functions that
should be implemented first, including:
1. ST_AsRaster(geometry)
2. ST_Resample(raster)
3. ST_Intersection(raster, raster)
4. ST_Clip(raster, geometry)
-bborie
--
Bborie Park
Programmer
Center for Vectorborne Diseases
UC Davis
530-752-8380
bkp...@ucdavis.edu
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