02.04.2016, 15:15, Even Rouault kirjoitti:
A few thoughts and questions :
- I guess what would be cool from the perspective of a C++ user is to be
really to write the above pseudo code with C++ operator overload when
appropriate ! A += B, A.min(B), min_A= A.min(), etc...
Operator overloading is of course something to have. It is great for
interactive use too.
I would most probably implement that in the Perl bindings.
- There might be some numpy reinvention here... Or at least, it could perhaps
serve as an inspiration. I guess Python users would in priority use numpy for
the most basic operations. Of course the GIS specific methods would still be of
value.
I know little of numpy and how it is used by the Python bindings. Does
it load the whole raster band into memory or can it make use of the
block read/write? If yes, then that is probably preferable for
Pythonists at least when it comes to standard methods.
- Saturation arithmetics could be interesting. numpy by default do wrapped
arithmetics, you have to cast to a larger type and clip down afterwards,
whereas OpenCV does it by default :
http://opencvpython.blogspot.fr/2012/06/difference-between-matrix-arithmetic-
in.html
- Are the methods handling 2 bands able to deal with different block sizes ?
Yes, the band size needs to be the same currently but the caching and
cell index functions allow working with two bands at the same time even
when the cell neighborhood is needed. I'm not sure about the optimality
of the code however, the cache is cleaned of not-needed blocks at each
block step.
- And with bands of different data types ?
Yes. It creates pretty huge switch tables however. They are mostly
hidden in macros but are a problem. I don't see how I can get the
datatype into the templates from the bands and I don't like the idea of
leaving that to the users.
- Why the need to reinvent a hashset mechanism ? Isn't std::set good enough ?
- Same for gma_array vs std::vector ?
Simply ignorance of C++ I think. I do see that for example C++11 and
boost have lots of useful things but did not (yet) go that way. Earlier
C++ standards seem to have many tools too. I learnt C++ >20 years ago...
I believe I also need some small classes like "mapper", which
reclassifies data from old values to new values and are not readily
available.
- various methods take a void* argument, whereas it seems that could be
rewritten as datatype* for better type safety
Noted. The problem is often that the callback to the method specific
function needs a prototype, which can be used for many methods. Surely
minimization of void pointers is a goal. Generic objects and
introspection might be a partial solution.
- there seems to be a block cache mechanism. What is it for ? How does that
work ?
In the block loop (looping through all blocks in a band) at each step
the cache is checked for block(s) that are needed (if focal distance is
> 0 then more than one block may be needed) and which are not needed.
Then blocks are read and written/discarded.
- there is some overlap/connection with the pixel functions of VRT ( see
"Using Derived Bands" of http://gdal.org/gdal_vrttut.html).
ok
Thanks for the comments!
Ari
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
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
