I've got this into a rather nice working shape. The code needs to be
reorganized (it is now, horror, all in header files) but that's another
thing.
I've so far built this around a basic DEM catchment analysis workflow.
It includes many kinds of steps so it is a pretty good benchmark. In
pseudo code the workflow is something like
dem = open_from_file
dem->fill_depressions
fd = dem->compute_flow_directions
fd->route_flat_cells
ua = fd->compute_upstream_area
c = new_raster // compute the catchment into this
c->set_border_cells // we're interested in the outlet cells
c *= ua
c *= c > 10000 // we're interested in the big catchments
outlets = c->get_cells // outlets is an array of cells, and a cell is an
object containing (x,y,value)
c = 0
c->mark_catchment(fd, outlets[0], 1) // assuming the outlet cell is now
in outlets[0]
ua2 = new_raster // for visualization log of upstream area is nice
ua2 += ua
ua2->log
The C code is of course not this nice but it is not very far from this.
All the basic four method types I mention below are implemented and
adding new methods is not very difficult. The code makes heavy use of
templates and macros in an attempt for versatility (GDAL rasters can
have many datatypes) and easiness to write methods in a few lines of
code. The whole set of code is now ~2500 lines but it creates rather
large executables due to templates. The code is C++ but the API is C
like, I've so far done no changes to existing GDAL code because of this.
As written below, the code works on two levels of x,y loops: the blocks
within a band, which is generic and cells within a block, which is
separate for each method. The methods work on line by line (no recursion
etc.) and thus for example the fill_depressions is not optimal but
instead very simple. Also, very large rasters can be processed.
I think this may be somewhat similar thing in relation to GDAL as the
network support, so it could be a thing to add to the library if wished.
I've added RFC 62 "raster algebra" to spark discussion.
Ari
20.03.2016, 10:21, Ari Jolma kirjoitti:
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
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