I've try what you suggest me.
With a resolution of 4096 it's almost perfect and the render time do not change. What's change? the memory usage?
I also try removing the translation and now with a resoluzion of 4096 is really better, only some blades pass throw the mesh. I notice that is better if I increase the density and the treshold too. Is it true? Probably i've got a too high value in the translation roll out....I'm experimenting with the parameters
If i go above to 6000 px resolution it seam to go crazy and put the grass everywhere
I've tried also with a spline. And i think that for a so simple geomtrey is the best way to go. Almost no blade inside the woodtile
Thanks for the quick and precise reply.