Finding area on a mesh

Is there an opengl call that return the area of a shape (say, rectangle) on a 3D surface? (ie, a mesh - anything wich is not a plane)

i don’t understand very well what do you want.
is it the sqare area ? (as for ex: rectangle is lenght*widht ?)
if it’s that:
there are no opengl calls to do that, that’s just mathematics.

if it’s not a plane, it would be more difficult. just add all the planes ‘areas’ to your mesh, and you’ll get it.

sorry if i were to wrong.