[mapserver-users] confusion about postgis bounding box -> valid map extent

[Steve Lime]

>>> Alex Rice <alex_rice at arc.to> 08/23/02 01:46PM >>>
> I am using PostGIS and getting a bounding box of objects in 
> PostGIS with the functions Envelope() or Box3d(). The bounding 
> box will not be the same aspect ratio as my Map object.

> Here are some things I am confused about.

> - Is it correct that in Perl mapscript, the only way to set the 
> extent of the map object is directly accessing 
> $mapObj->{extent}->{minx} etc.? Pretty sure this is true, just 
> want to verify I'm not missing something.

This is true. PHP MapScript has a couple of utility methods that need
to be replicated in the Swig version.

> - What's the expected behavior if in Perl mapscript I set the 
> extent as above, call prepareQuery(), then draw, if the new 
> extent has a different aspect ratio than the output image size?

In the C code the extent will be adjusted for output image size.

> - In php_mapscript.c  there are functions for dealing with the 
> map extent based on a box inside the visible image, or a point + 
> scale, etc.  Does anyone have example Perl code, for instance, of 
> the map->zoomRectange() function in php_mapscript.c? I have seen 
> the &set_extent() method from mapquakes.pl and seen the examples 
> Lowell posted in Perl, but those are zoom & pan, not fitting an 
> extent to another rect. If there isn't sample perl code

I've got all those written in C and javascript. Let me know what you'd like
to see. Otherwise MapScript already has a method to fit a rectangle to
a new window size. It's part of the rectObj class (of which extent is one)
and is called fit. For example:

  $cellsize = $map->{extent}->fit($newwidth, $newheight);

> - In maptemplate.c, the function setExtent() has several cases 
> for the "source" of the change in extent. Do  any of those cases 
> apply to my situation? Maybe I can adapt the C code to a Perl 
> method.

Nope. The code for zooms etc. is in mapserv.c

> - Is there a way to get PostGIS to fit a bounding box to an 
> aspect ratio? It would be nice if I didn't have to do it in perl 
> at all. A function like this is what I'm imagining

> select extent2extent(5, 3, box3d(the_geom), centroid(the_geom));

> where 5:3 is the aspect ratio
> box3d is the extent of the object
> centroid is the center of the requested new extent

Already addressed in a generic way with the fit method.

> Thanks,

> Alex Rice, Software Developer

Steve