[postgis-devel] Raster todo list

[Pierre Racine]

Hi Bborie,

> ST_Resample:
> 
> this would be adding the ability to change the scale (pixel size) and the origin
> (upper-left corner) of a raster using the GDAL library.  How about de-skewing a
> raster?

I already wrote specifications for that function. I did not put them in the wiki yet though. I can do it ASAP.

> ST_AsRaster:
> 
> convert a geometry to a raster using the GDAL library

I also have spec for that. Still have to put them in the wiki.

> ST_Accum:
> 
> be able to bring together a set of tiles to make one unified raster.  I really need
> to be able to do the following:
> 
> SELECT
> 	observation_date,
> 	ST_Accum(rast)
> FROM tmax_2011
> GROUP BY observation_date
> 
> This function will entail a bunch of issues, such as irregular scales, unclean
> overlap of tiles, skew and projection.
> 
> ST_Clip:
> 
> be able to clip/crop a raster using a geometry or raster as a mask
> 
> I haven't had much time yet to think of the specifics for each function but I'll get
> to each when I get there.

Isn't it more something like ST_Union that you need? ST_Accum create an array of geometry. ST_Union merge them together.

I think those two last functions should wrappers around an optimized version of two raster MapAlgebra. The non optimized version of Mapalgebra compute one pixel at a time.

There should be an option in the two rasters version of mapalgebra (optimized or not) to say what we want the resulting raster area to be: same as the FIRST raster, same as the SECOND, same as the INTERSECTION of both or same as the UNION of both.

INTERSECTION is very useful to implement ST_Clip  and UNION is very useful to implement ST_Union (or the merge of two rasters).

When you use UNION, very often there are areas that do not overlap. For those areas you want to be able to provide an alternative expression for the missing data. This is also the case when they overlap but one of the raster pixel is nodata.

Also when you use UNION, and where the two rasters expression do not apply. You can optimize the setting of these sub-areas quickly as block (like a memset), not pixel by pixel. You may want to set the whole sub-area to nodata, to a constant or to a copy of one of the two raster (this is the optimum case exploited by ST_Union).

When you use intersection and the expression resume to 'rast1' you have a clip. You also, in this case, want to just copy the pixel values from the first raster to the second. This can be optimized by a ST_SetValues function just copying pixel values from one to the other.

So in the end you have a very flexible Mapalgebra working on two rasters helping you to implement a bunch of other raster processes and optimized for the very useful case of doing Clips and Unions. In bonus you get some editing functions like ST_SetValues very usefull to implement who knows what...

That how I see all this.

Please have a look at script/plpgsql/st_mapalgebra.sql and script/plpgsql/st_mapalgebra_optimized.sql

Pierre