[gdal-dev] Map algebra

[Ari Jolma]

Folks,

I played around a bit with the map algebra idea and came up with 
something that could work nicely as a plugin. The initial code is at 
https://github.com/ajolma/gdal/tree/trunk/gdal/map_algebra

The idea is based on processing raster bands block by block and calling 
a given function on each block. Using macros and templates the code 
should be rather nice and manageable even when supporting multiple data 
types for raster cells.

Further, the idea is to support three kinds of map algebra methods, 
mostly working on a raster band in-place.

1) Simple ones, which do not take any arguments beyond the raster band 
itself, examples are functions like log, sin, rand.

2) Those which take one non-spatial argument. The argument can be a 
scalar like a number, or more complex like a reclassifier.

3) Those which operate on two rasters. Examples are summation of rasters 
and computing flow directions on a DEM. The latter is a bit more complex 
since it is a focal method and requires a 3 x 3 matrix of blocks from 
the other operand raster.

Maybe a fourth kind of methods are those which compute something from a 
raster.

This would lead to four functions in the C API and raster band methods 
in the bindings.

Comments are welcome, the initial code base contains a small demo 
program. Eventually, if this works, I'll make a RFC from this.

Ari