[Mapserver-users] Cartographic question

[Ballini Yvain]

Hi List,

I would like to have a piece of information about the definition and the
use of the parameter extent in mapserver.
For another application I'm using a cartographic formula which
calculates the extent of a map with the following inputs:
- Coordinates (in latlong) of the center of the map (Xc, Yc).
- the zoom which is defined as the longitudinal scale (the scale along
the parallel Xc)
- the size of the map in pixel (Xp, Yp).
My question is the further one:
I would like to know if mapserver redefines the extent parameter after
that it has been passed in the url.
How mapserver is managing the scale paramater which should be different
along the X- and the Y-direction? 
When I'm asking to mapserver to draw a map with an extent = Xm, Ym, XM,
YM, (430*380), it turns me a map which is ok, and if I try with an
extent = Xm, Ym-alpha,XM, YM+alpha (430*380), mapserver turns a map
which has changed the extent in Y (which is ok) and also in X: that
seems it has redefined the extent parameter !
I may be wrong with the use of my formula (because I'm considering that
the scale in X is the same than the sclae in Y, which is only an
approximation) and that's why I would like to know if there is a basic
way to get the extent of a zone with the 3 inputs which I've mentioned
before.

Thank you very much for any answer.
Don't hesitate to ask me any additional information.

Yvain Ballini.