What do you mean by true dimension and ISO-Constraints?
? Can you clarify what you are talking about?
Let me give you a overall picture then you might be able to understand what constraints are in Catia Sketcher.
In Catia , usual design process for part modeling is through Sketches only, You prepare the sketches first & then there is sketch based features that allows you to create 3D geometries.
Sketcher is a 2D workbench where you could create 2D geometries always on a single plane only.
Now you want to not only create 2D geometries but you also want them be at a particular length, angle radius ,diameter or offset etc. so with the help of constraints you can do that.
And what does this have to do with OpenMV?