[OSRS-PROJ] Re: Orthographic Projections and MapServer

[Frank Warmerdam]

Gerald I. Evenden wrote:
> In general, these comments are getting off track as I feel the basis
> of the thread was to solve the problems of "mapserve" and not advertise
> a graphics package.  A package, I might add, that package seems to be
> only available for Apple systems.

daan / Gerald,

While I raised this issue with regard to MapServer, I brought it to the
osrs-proj list because I thought there might be a more general issue
that many applications using PROJ might need to solve dealing with "limits"
of various projections, and that it might be useful to include aspects of the
solution within PROJ.  So, I certainly feel that the discussion is going very
well, and I have learned alot.

Also, I am very pleased that daan (and others) are contributing and giving
some context on the issue from other projections packages and application
environments.  I don't think daan was out of line at all in relating his
experiences with his package.  It was hardly spammy self promotion without
relationship to the discussion.

One thing about which I have been very pleased is the evolution of the
osrs-proj list with members who don't even use the PROJ package, but who
have an interest in projections, and geodesy.

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.

  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.

  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.

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.

Best regards,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent


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