Here's what I can do, Bastien. I really think it's better to make these 'grilles' as objects in another program, and not use the displacement in TG. I've made a simple setup with either the single grass leaves or a displacement (enable the surface shader). And it's better, if using a mask like this A4, to make a white line on black instead of this.
If using the grass leaves, you might want to thin them and use a lot more of them.