Python: determine "valley" of quad-face

I’m a newbie to blender python scripting.
I wonder how it is possible do determine the “valley” of a quad-face.
Please see picture.
If you grab 2 opposite vertices of a face along z-axis, then there seems to be no rule if there becomes a “valley” between them or not.
Or is there a rule?
http://thumbnails6.imagebam.com/3911/7b9bdf39105375.gif

quad in blender are not necessarely rectangle flat !

but this is internal only

now there is a script somewhere that can give you this info
but don’t remember what the name is

in any case don’t worry too much about it

if you create you plane in one of the view it should always be flat !

Happy blendering

the “valley” is indeterminate… as you’ve noted the direction will change as you move different vertices around…

Graphics cards deal in triangles, a quad is not two triangles put together it’s a quad…this means it needs to be converted to triangles behind the scenes. how the geometry gets converted could vary with graphics drivers… the internal renderer may use a different tesselation scheme… (so rendered output may not match the viewport display…) the “valley” doesn’t really exist in the data.