[OSRS-PROJ] Re: Orthographic Projections and MapServer

Gerald I. Evenden gerald.evenden at verizon.net
Fri Aug 22 20:27:13 EDT 2003

On Fri, 2003-08-22 at 14:43, Frank Warmerdam wrote:
> My take so far from the discussion is that:
>   o It is not unreasonable for a coordinate systems package (taking the
>     broader view of PROJ) to offer information about bounds of validity
>     for a projection, though I for one don't forsee this being engineered into
>     PROJ in the near future.

Here I take the same position that I do with datum operations and
maintain that the projection operations are as independent
parts as many other mathematical/programmatic aspect of a larger
>   o For the particular case of orthographic mapping, it should be practical to
>     do some sort of polygon clipping in geographic space discarding anything
>     more than 90 degrees away from the projection center.

I would say the general class of azimuthal perspectives have the same

>   o More generally, it should be possible to use repeated subdivision to fairly
>     closely identify where line segments cross the horizon, and though not
>     trivial, even to clip polygons using a similar mechanism for the general
>     case.

Code exists clipping lines on the limb.  The hassle here is handling
the problem of how to join the polygon-limb intersects and the
hidden region problem.

> I would like to implement the polygon clipping methods in MapServer, but it
> sounds hard enough that I am unlikely to do it on my own in the near future.
> If Gerald, or anyone else wants to take a crack at it in a general form I
> would be willing to take the time to adopt it in MapServer.

I have looked at portions of the MapServer code and I am fairly
comfortable with the style.  But, and a big but, I am not familiar

1. the overall flow,

2. the graphic engine it uses

3. and working in the networking environment.

If it can function on a stand alone system then I think I can work on
it and leave network testing for someone else.

I did run configure and determined that I need to plug a couple of
packages into my system and modify the code to link to libproj4
(this looks easy).  I think I have all the stuff on my source
distribution DVD.  But I will have to download a couple of items

I do note one problem, configure looks for GD 2.0.12 or higher
and I only have 2.0.9 on a 2003 distribution disk.  Hmmmm.
Have to google around and find the home for this package.

I wish this package output to PostScript so it would be useful
for Hi-res hard copy like pdf.

Looks like a few days just to install this thing and several
more learning how to use it.  Modifying it is another story.

If someone is willing to be bugged a few times a day for about
a week, I would give it a try.

> Best regards,
Gerald I. Evenden <gerald.evenden at verizon.net>

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