[Gmsh] Question regarding the step of high-order node placing

[Schmid V.S.]

Dear Prof. Geuzaine,

I am interested in the way how high-order nodes are projected onto the real geometry in gmsh, before additional optimisation algorithms could be applied. In your paper The Generation of Valid Curvilinear Meshes from 2015 in IDIHOM: Industrialization of High-Order Methods-A Top-Down Approach I found the explanation that they are orthogonally projected from the straight sided edge to the real geometry. I was wondering if this projection is done orthogonal to the straight edge, or orthogonal to the real geometry.
Could you provide me a short answer to this question or a reference where this is step in  the algorithm is explained in more detail?

Thank you in advance.

Best regards,
Verena Schmid