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

[Craig Bruce]

"Gerald I. Evenden" <gerald.evenden at verizon.net> wrote:

> Whatever "on-the-fly" means?
>
> Presumably "mapserver" is some kind of graphic program.

"On-the-fly" presumably means that the source data is reprojected in real
time as requested when the map is drawn.  The "map server" is likely an
OpenGIS® Consortium Web Map Server (OGC WMS):

http://www.opengis.org/techno/specs/01-068r3.pdf

Our OGC-compatible map server and client have the same feature (long URL):

http://demo.cubewerx.com/demo/cubexplor/cubexplor.cgi?imageClick.x=297&imageClick.y=199&clickMode=zoom+in&layerToQuery=%28value+of+%22Layers%22+field%29&pixelSensitivity=3&featureCount=5&dataStores=Foundation&themes=Boundaries&layers=Barrier+Line+Features&styles=%28default%29&newScale=230M&newX=0&newY=0&newImageWidth=600&newImageHeight=400&newImageType=image%2Fpng&newQuality=BEST&zoomFactor=2&newSrs=42003+%28WGS+84+%2F+Auto+Orthographic%29&sldUrl=&layersToPlot=Coastlines%2CPolitical+Boundaries%2CGlobal+30+Second+Elevations%2CTwo+Minute+Shaded+Relief&currentY=-8.00433241552028e-11&currentX=0&currentScale=230M&currentImageWidth=600&currentImageHeight=400&currentSrs=AUTO%3A42003%2C9001%2C2.136416958402712%2C52.37936226431053&currentImageType=image%2Fpng&currentQuality=BEST

> 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.

--------------------------+------------------------+--------------------------
Dr. Craig S. Bruce        | Tel: 819-771-8303 x205 |             CubeWerx Inc.
Senior Software Developer |    Fax: 613-771-8388   |  Gatineau, Québec, Canada
csbruce at cubewerx.com      | http://www.csbruce.com |  http://www.cubewerx.com/
--------------------------+------------------------+--------------------------
          "Do not try to think outside the box.  That's impossible.
           Instead, realize the truth.  There is no box." -- unknown
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