[GRASS-dev] v.what and spatial index

[Moritz Lennert]

Glynn Clements wrote:
> Moritz Lennert wrote:
> 
>>>> How would this change from the first approach (if v.what could easily be 
>>>> made to accept multiple coordinates) ? This would again mean 'blindly' 
>>>> collecting points before seeing the results, or having to call v.what 
>>>> multiple times.
>>> v.what would read one coordinate pair, perform the lookup, write out
>>> the result(s), repeat until EOF.
>>>
>>> gis.m would start one v.what process; each time you click the mouse
>>> button, it would send the coordinates to the v.what process, read the
>>> result, then display it. It would need to restart v.what if the set of
>>> maps changed, but could use a single process to look up multiple
>>> points.
>> Here's my attempt at applying what you suggest. It's not very 
>> complicated, and seems to work fine here, but I'd appreciate if someone 
>> could check that I didn't do anything wrong before committing.
> 
> One issue which won't show up when running it directly from the shell,
> but will if you run it via pipes: you need to explicitly set stdin and
> stdout to line-buffered operation with e.g.:
> 
> 	setvbuf(stdin,  NULL, _IOLBF, 0);
> 	setvbuf(stdout, NULL, _IOLBF, 0);

I assume this can be done anywhere as long as its before the call to stdin ?

> 
>> Now we will have to change the query function in the gis manager (from 
>> line 1474 of gui/tcltk/gis.m/mapcanvas.tcl) to use v.what with the -s 
>> flag and to provide continuous flow of coordinates and an EOF at the 
>> end. Michael, can you do this ? Or could someone tell me in rough lines 
>> how this can be done ?
> 
> Ah. Tcl's "open" function doesn't directly support using pipes for
> both stdin and stdout; you can choose read (stdout is a pipe, stdin is
> inherited) or write (stdin is a pipe, stdout is inherited), but not
> both.
> 
> You will need to create a named pipe (with mkfifo) then explicitly
> redirect one of the ends to it, e.g.
> 
> 	set fifo $tmpdir/$tmpname
> 	exec mkfifo $fifo
> 	set rfh [open "|v.what map=$maplist <$fifo" r]
> 	fconfigure $rfh -buffering line
> 	set wfh [open $fifo w]
> 	fconfigure $wfh -buffering line

Ok, I understand the principle of this, but don't know how and where to 
apply it in mapcanvas.tcl. Don't have the time to dig into it right now. 
Michael ?

Another question is whether it is worth doing all this (don't know how 
much work it is to apply Glynn's solution) if we are going to change GUI 
in a foreseeable future...

Thanks, Glynn, for your help.

Moritz