slow vector display in grass7 (was [GRASS-dev] vector large file support)

[Markus Metz]

Glynn Clements wrote:
>>>
>>> One relatively recent change that may cause a slow-down is that the
>>> display library no longer tries to automatically simplify vectors
>>> (reduce consecutive short line segments with a single segment). This
>>> could have a significant impact when viewing detailed vectors at low
>>> zoom.
>>>       
>> OK. It does have a significant impact when viewing detailed vectors at 
>> low zoom (relatively detailed: 25MB coor, 31MB topo, not that large but 
>> complicated topology). I can't see a speed difference with small 
>> vectors. That just got me by surprise because I was used to a 7.0 
>> display that's faster, better, nicer than the 6.x display. Usually, the 
>> screen resolution can't represent that detail (same for raster map 
>> display), but I guess it is more correct to try to display all detail.
>>     
>
> Part of the problem is that a driver can generate either raster output
> (PPM, PNG, etc) or vector output (PDF, PS, SVG), and the higher levels
> don't know which is the case. For vector output, there is no obvious
> threshold below which additional detail becomes unnecessary.
>
> The display library has a function, D_set_reduction(), which will
> merge adjacent line segments which are below the specified size, but
> nothing uses it yet.
>   
This high amount of detail is completely lost on me, because when I 
fully display a high-detail vector (zoom to map), the screen pixel width 
of the boundaries is often larger than the screen pixel dimensions of 
the area these boundaries belong to. With default settings, lines black 
and areas grey, all I see is black in those parts of the vector with 
many small areas, no detail recognisable. I use standard monitors each 
with 1280 x 1024 resolution (dual screen setup) and gui display 
maximized on one screen.
IOW, I could do without sub-screen-pixel detail :-)
But I don't have a solution.