[OSRS-PROJ] orthographic projection with proj4 and mapserver.

Judd Taylor judd at seas.marine.usf.edu
Thu Aug 14 10:21:04 EDT 2003

----- Original Message ----- 
From: "Craig Bruce" <csbruce at cubewerx.com>
To: "[OSRS-PROJ]" <osrs-proj at remotesensing.org>
Sent: Wednesday, August 13, 2003 6:32 PM
Subject: Re: [OSRS-PROJ] orthographic projection with proj4 and mapserver.

> "Gerald I. Evenden" <gerald.evenden at verizon.net> wrote:
> > On the surface, it appears that points beyond the visible rim
> > are not being properly sensed for their "invisiblity."  PROJ
> > should return an error for such points and the graphic program
> > should properly sense them.  With information provided, it is
> > difficult to know who to blame.
> Proper graphical clipping is a tough problem.  It looks like the specific
> problem in this case is with the polygons that are half visible and half
> invisible.  The invisible vertices appear to be all substituted with (0,0)
> coordinates, though I thought that the PROJ.4 library used -infinity as
> its 'invalid coordinate' value.  To plot such a polygon mathematically
> correctly, you'd need to cut it in two (or more) pieces along the line of
> the horizon of the projection, which would be a smooth curve in this case.
> Other short-cut cheats are also possible.

I could use some pointers in this arena as well, if anyone on this list has
any. Right now I'm using the General Polygon Clipping (GPC) library to clip
my polygons from the GSHHS coastline set (GPC website:
http://www.cs.man.ac.uk/aig/staff/alan/software/gpc.html). Unfortunately,
this takes forever to clip the larger polygons in that coastline set (~1.4
million points).

Anyone know how I can speed this up? Maybe a different library? Maybe a
different format for the polygons? I can't seem to find any good references
for this sort of thing anywhere online. Any advice would be appreciated!

    Judd Taylor

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