[GRASS5] [OFFTOPIC] Need some help...

[Roger Bivand]

On Tue, 20 Dec 2005, Daniel Calvelo wrote:

> Dmitry,
> 
> If you are willing to program the whole rendering algorithm, there are
> some optimizations you could use: you might want to use R-Trees for
> storage of the polygon features, and check, after the data is
> projected and before writing into the canvas, that the projected data
> does fill more than one pixel before rendering the whole feature.
> 
> Also, if your objective is only to display data, you could
> precalculate simplified versions of the polygon boundaries for the
> less detailed scales, and switch to the original data as you zoom into
> more detailed areas.
> 
> Google for R-Tree; a lot of research has been done on them, and
> several implementations are available from free software projects. You
> might want to ask in the freegis list
> (http://www.intevation.de/mailman/listinfo/freegis-list) where people
> are rather heplful.

A lightweight approach is to decimate the long-lat data first - I think 
the code at:

ftp://gmt.soest.hawaii.edu/pub/pwessel/gshhs

is pretty informative (or was until I tried it now - seems to be down 
temporarily - look for gshhs.h, gshhs.c, and especially gshhs_dp.c, 
which contains the line simplification code - looks as though they are 
copied in:

http://www.ngdc.noaa.gov/mgg/shorelines/data/gshhs/version1.3/gshhs_1.5_src.zip

).

In fact, my first cut on your task would have been to use GMT straight 
off, pushing the projected output Postscript into gs or some such to 
render to your window. Depending on the size of your window, the 
pre-thinned coarse or low GSHHS datasets might be good enough, but I don't 
know if you need more features than just shorelines.

> 
> Daniel.
> 
> On 12/20/05, Dmitry Suzdalev <dimsuz at mail.ru> wrote:
> > Hi, dear GRASS developers :).
> > Please, sorry for maybe offtopic, but I have one question.
> > I'm learning my way into world of map projections and my task is to display
> > all the world in a GUI window (QT-window, actually).
> > I've already managed to do it, using libproj4 :).
> > But... There is sooo much data to be displayed that it draws rather slow.
> >
> > And it's obvious, that it's not needed to draw EACH map data primitive,
> > because some of them are so small, that they're represented by one pixel,
> > when using large scales. And still, if I have 1000 primitives (polylines,
> > regions etc) that are so small, that they fit in one concrete pixel, they'll
> > all be drawn. In the same pixel. 1000 times. That's slow :).
> >
> > So the question is: is there any common algorithm to cope with this situation?
> > Which helps to somehow "filter out" the primitives which are represented by a
> > single pixel at the screen and draw that pixel only once?
> > Or should i investigate that algorithm by myself? :)
> >
> > Again, sorry if this is offtopic, I've googled for a few hours trying to find
> > some kind of web-forum for such kind of questions, but I didn't find it :).
> > Googling on "gis developers forum" pointed me to your mailing list, so I
> > decided to give it a try :).
> >
> > Maybe at least, you can point me to the right place to ask such a question? :)
> >
> > Thanks in advance!
> >
> > And sorry for my maybe not very good English ;).
> > Cheers, Dmitry.
> >
> > _______________________________________________
> > grass5 mailing list
> > grass5 at grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grass5
> >
> 
> 
> --
> -- Daniel Calvelo Aros
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no