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

Tom H tom at scriptsupport.co.uk
Wed Aug 13 21:24:05 EDT 2003

> > 
> > 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.
> Indeed, not a trivial problem.  In all the map graphics that I use in
> the PROJ.4 manuals, only vector plotting was performed.  The clipping
> problem for vectors is much simpler than polygons---that's why I didn't
> fill the continental outlines.  ;-)
> BTW, libproj4 forward routine returns HUGE_VAL when it cannot project a
> point.  Program [l]proj returns an "*".  The HUGE_VAL method has been
> in PROJ.4 use for a *long* time so I suspect that the problem lay in
> the calling software.

If HUGE_VAL was returned against the 0,0 would the image have had shards out
to infinity rather than to 0,0?

Thanks for your help.


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