Hy guys…I have another question…
how can I detach a peace of object after I have masked it (with and without using dynamesh) ?
Invert the mask, Hide Pt (Visibility>Hide Pt…hides unmasked polygons), Subtool Menu >Split Hidden (splits hidden geometry into its own subtool.) This is going to make for some fairly ugly , open geometry though, that you will probably want to use Geometry>Close holes, or otherwise remesh in order to make it useable.
With Dynamesh, Mask as desired, invert mask, Hide PT, assign polygroups (Polygroups>group visible), and then Dynamesh with the Groups function pressed, which will split the polygroups into separate chunks.
great …thank U 