[mapserver-users] MapScript and ZoomRectangle (and rectObj problems)

[Jim Strevinas]

> Date: Sat, 2 May 2009 00:35:46 +0200
> From: Baas at speerit.nl
> To: mapserver-users at lists.osgeo.org
> Subject: [mapserver-users] MapScript and ZoomRectangle (and rectObj problems)
> 
> Hello List,
> 
> Three years ago, I built a web interface for our primary application
> using MapServer. Now we're in the process of building that same
> application in VB.Net (but with more features) using MapScript.
> 
> Although everything comes along nicely (I love the fact that everything
> returns objects, etc) there is one huge problem that I can't seem to
> solve: creating and using the rectObj class. I read a lot about it and
> found many bug cases, older messages, etc, but I still can't make heads
> or tails of it. I'm sorry about asking it, since it must be the
> millionth time this question has been asked.
> 
> I *CAN* get ZoomRect with a rectObj to work when using image coords,
> even though that requires my own code that flips miny and maxy to solve
> that lovely error about miny => maxy or something. When I try to use it
> with map coords however, I can't get it to work. 
> 
> In my init code I create a new rect object:
>     Map = New mapObj(strMapPath)
>     FullExtent = New rectObj(0, 0, 0, 0, 0)
>     With Map.extent
>       FullExtent.maxx = .maxx
>       FullExtent.minx = .minx
>       FullExtent.maxy = .maxy
>       FullExtent.miny = .miny
>     End With
> 
> When I feed this to ZoomRectangle using 
> Map.zoomRectangle(FullExtent, MapWidth, MapHeight, Map.extent, Nothing)
> I get that famous "mapscript::mapObj::zoomRectangle(): General error
> message. image rectangle maxy >= miny". 
> 
> When I flip miny and maxy I end up in the middle of the Sahara (while
> the original coords should show the Netherlands).
> 
> Things I wonder about:
> 1) How can I get zoomRectangle to work with world/map coords instead of
> image coords?
> 2) Why does the constructor have an "imageunits as integer"? IMHO a
> simple rect object has nothing todo with the coordsys of the/a map
> (since I can more than one).
> 3) When will the much discussed bugfix be applied that automaticly flips
> miny/maxy?
> 
> Any help would be greatly appreciated,
> 
> With kind regards,
> Jelmer Baas
> _______________________________________________
> mapserver-users mailing list
> mapserver-users at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/mapserver-users

Hi,
  When I want to use ZoomRectangle with real coordinates, i always transform
the real coordinates to image coords based on the current rendering of the map.
I talk about current rendering because one must compute the pixels by geographical 
degrees of the image. A simple function to do such conversion could be:

# Width and height of the map image (from mapObj.width)
# X,Y are the real coord pair
# real_extent from mapObj.extent
def map2img (width, height, x, y, real_extent):
     ppd_x=0
     ppd_y=0
     new_x=0
     new_y=0
     ppd_x = width / ( real_extent.maxx - real_extent.minx )  # pixel per degrees
     ppd_y = height / ( real_extent.maxy - real_extent.miny )
     new_x=ppd_x * ( x - real_extent.minx )
     new_y=height - ppd_y * ( y - real_extent.miny )
     return (new_x , new_y)

Thus, the two pairs of real coordinates can be converted using two call to this
function (there exists a distortion though in large scales). 

 Beware that if your map is using a different "output" projection than the 
REAL zoom coordinates are (say, zoom coords are latlong while map is LAEA)
you should first use a method to reproject the coords to the map's projection equivelant
(i do sth like this using the OGR/GDAL's osr module)

Finally, prevent minx>maxx error by requiring that the users define the proper points for
the input rectangle say (UL and LR) or (UR and LL)

Hope this helps a bit.


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