[postgis-devel] Status of raster mapalgebra?

[Nicklas Avén]

Thanks Pierre

I will take a look at the plpgsql-functions.

I understand that there is a lot of work to do. It will be exciting when
things are in place :-)

/Nicklas



On Mon, 2011-01-31 at 15:20 -0500, Pierre Racine wrote:
> Hi Nicklas,
> 
> >What is the status of map algebra functionality?
> 
> There is a one raster version on the way (Jorge what is the status?) and a two raster version planned. You can see slow plpgsql prototypes of those two functions in raster/script/plpgsql
> 
> >Is there any code that could be tested?
> 
> Just the plpgsql versions. But they might be sufficient to make a demo or proof of concept.
> 
> >In beginning of Mars I will give a talk about PostGIS i Stockholm. I
> >will try to tell about the news in 2.0.
> 
> MapAlgebra will be a very progressive work:
> 
> 1) One raster version - Pixel by pixel
> 2) One raster version - Accepting neighbour pixels in expression
> 3) One raster version - Accepting neighbour pixels from the whole coverage (hence getting raster values using world coordinates over the whole table instead of just raster coordinates)
> 4) Two raster version
> 5) Two raster version working with non-aligned rasters (forcing resampling of one to be aligned on the other one)
> 
> 1) might do it in 2.0.
> 2) maybe.
> The rest probably/certainly not. It will depend on the date of release.
> 5) need GDAL based ST_Resampling first (this is still far away).
> 
> >Most of my raster experience is about finding clear cuttings by
> >comparing satellite imagery from different years. It would have been
> >very nice to be able to show that in PostGIS. The software that were
> >used for that purpose was something called Enforma that as far as I
> >understand just compared band for band what had happened since last
> >year. The different bands give different result depending on what
> >wave-lengths the photosynthesis uses compared to what is absorbed to
> >naked ground and so on.
> 
> For this I guess you would need the two raster version computing difference between two bands years after years.
> 
> >Will there be anything to try during February?
> 
> I suggest you use the plpgsql versions as a proof of concept and comment on their usage. They might however be broken right now because of some change in the API but they should be easy to repair.
> 
> >Am I right that map algebra will be the tool to use?
> 
> Yes. Something like:
> 
> SELECT ST_MapAlgebra(rast1, 1, rast1, 2, "rast2 - rast1") FROM yourrastertable WHERE ST_Intersect(rast1, rast2)
> 
> Later MapAlgebra should be encapsulated by a number of specific functions. In your case probably something like ST_Difference(raster1, band1, raster2, band2)
> 
> >I have no idea how much work there is to do so I don't know what I am
> >asking for.
> 
> The two raster version is very complicated (you must deal with nodata values, non-aligned rasters). I'm working on an optimized version (setting large zones or raster with memcpy when possible) which still a lot of work.
> 
> Pierre
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