Pardon my ignorance, can you explain what is DI ?
1. Gas piston operated (can be long stroke, short stroke, annular, tappet).
Here gases from the barrel accumulate in a gas chamber and pushes a piston rearwards to start the cycling process after a round is fired. Like AKs, FAL, Garand, Dragunov.
2. Direct Impingement (DI) or true Direct Impingement.
Probably the simplest form of gas operation. A tube starts from the barrel and terminates on the face of the bolt carrier. Gas pressure accumulate in the tube and starts pushing the bolt carrier directly without need of any piston. As of now this type of operating mechanism is pretty much obsolete. A good example is MAS-49.
3. Modified Direct Impingement or hybrid gas piston.
Whenever you here someone say DI in today's context, it's most probably this one. Designed by Eugene Stoner, it uses a similar setup of gas tube like that of true DI to bring hot gases to the face of the bolt carrier but then uses an annular short stroke piston to actually start the cycling process. Here the bolt carrier itself acts as the piston head. All Stoner type rifles like AR-10 and AR-15 uses this.