[GRASS-dev] lidar tools update in grass7

[Markus Metz]

more updates in r40860, mostly documentation

Hamish wrote:
> Markus Metz wrote:
>   
>> The core of the lidar tools is the lidarlib, that is AFAICT
>> robust and working. Bugs are most likely in modules, so I'll try
>> to add comments there. I have reorganized the code for v.surf.bspline
>> in the hope that it is now easier to read.
>>     
>
> Slightly off-topic, but while we are thinking about this code ...
>
> A while back I did some crude profiling of the v.lidar tools and found
> that it was spending lots and lots of time in the Tcholetsky decomposition
> loop (3-for loops deep).
>   
That's better now because the matrix is a bit smaller.
> It seemed like a good & simple test case for using OpenMP to multi-thread
> it, but I got stuck with it segfaulting. AFAIR the problem was that OpenMP
> wanted you to malloc the entire thing before starting, and this could
> get big.
>
> if it interests you, see the attached patch and
>   https://trac.osgeo.org/grass/ticket/657
>   http://lists.osgeo.org/pipermail/grass-dev/2009-June/044709.html
>   http://lists.osgeo.org/pipermail/grass-dev/2009-June/044705.html
>   http://grass.osgeo.org/wiki/OpenMP
>   
Hmm, if I understand the code right, only the innermost for loop can be 
executed in parallel because the first two for-loops need results of 
previous runs (not possible to run i + 1 before i, same for j). But I 
don't know that parallel stuff, I would in any case first optimize the 
code without parallelization, only then add multithreading.
>
>   
>> I forgot to mention, there is a problem with sqlite, it is much slower
>> than dbf, no idea if this is a problem of my system or of the grass 
>> sqlite driver or of the way auxiliary tables are accessed by the lidar
>> tools.
>>     
>
> that sounds vaguely familiar, but as a general thing not necessarily
> to do with v.lidar. probably something about it in the archives?
>   
I discovered two little tricks: first, create the table, close database 
and driver, open database and driver again, work with table, second, 
creating an index on the table if possible helps too. Tested with sqlite 
and dbf.

Markus M