Hi Pilou,

> For the moment i believe you can't draw 3D free lines in 3D space,
> even you draw in 3D window, you will be on the ground!

Right, I mean your computer screen is a 2D thing, so a mouse cursor location on the screen does not define a single point in 3D space, it defines an infinite ray that shoots out through the viewing window into the scene.

When you are drawing something like a line, you need to specify a single 3D point, not an infinite ray, so that ray needs to be intersected with something in order to produce a point. The plane that is intersected with is the construction plane - by default it is flat on the ground in the 3D view but you can change it to set it at any particular orientation you want.

It's also possible to draw directly on surfaces - that will intersect the view screen ray with the surface instead of the drawing plane.

Also you can use object snap to snap the endpoint of a line on to any particular existing point in the model like on to ends of existing pieces, etc...

One good method for placing a point at some 3D location in space is to drag a construction line up from the ground plane (make sure "Straight snap" is enabled so that you will get the z axis snap line) - that will give you a vertical line that you can track along to place the point you want.

Also it is possible to set up a script so that you can orient the cplane to the current viewing direction of the 3D view, that's available here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2599.4

- Michael