A special edition of “Avatar” debuts today with three Blu-ray discs and just as many versions of the film: the original theatrical release (clocking in at 161 minutes), the special edition re-release (which pushed right up to the Imax format’s 170-minute max) and a new just-for-homevid cut that boasts a full 18 minutes of new material (plus an additional 45 minutes of deleted scenes for auds to consider).While it’s nice to see that added footage, the movie was perfectly wonderful without it and, at the end of the day, it’s still the same movie. Apart from the famously finicky George Lucas, Cameron is probably the director most commonly associated with going back and making his opuses even more magnum. After being forced to trim his original 1986 release of “Aliens” — including a scene about the fate of Ripley’s daughter on which Sigourney Weaver had based her entire performance — Cameron later went back and restored nearly an entire reel. “The Abyss” exists in two radically different versions roughly half an hour different in length. Digging into the nearly three-hour “collector’s extended cut” of “Avatar,” we discover a new opening segment set on Earth. “Aliens” riffed on a visual world conceived by Ridley Scott in his 1979 “Alien” while Cameron’s “Avatar” depiction of a crowded metropolis, with its neon-bright skies and rain-slicked streets, clearly recalls Scott’s “Blade Runner” (which itself underwent multiple reinventions after the original release). The additional “Avatar” scenes are instructive but not essential, providing a couple of nice character moments, including one in which feisty protag Jake Sully (Sam Worthington) starts a bar fight, despite his wheelchair-bound condition. Perhaps the most significant alteration was Cameron’s decision to restore the backstory of Grace (Sigourney Weaver): Now we visit the schoolhouse where her character once taught young Na’vi students and discover the tragic incident that severed her connection with the natives — a massacre resulting in the dealth of Neytiri’s sister, Sylwanin, the details of which are peppered throughout the extended cut in a series of poignant new scenes. The detail that “Avatar” never should have done without amounts to a single shot during the love scene between Jake and Neytiri, restored for the summer re-release and included in the two longer cuts, in which Cameron shows the two characters connecting ponytails. It’s not a prurient moment (after all, we’ve seen the Na’vi make the same bond to beasts and trees), but a useful detail in understanding the unique anatomy of these creatures. Without it, this pivotal scene had amounted to smooching and softcore groping. But now Cameron’s unique concept of how these creatures bond to one another and the world around them feels complete. In that spirit, the most valuable additions to this cut help expand the alien world of Pandora (with a bit more time spent admiring the planet’s stunning flora and fauna) and the unfamiliar culture of the Na’vi (as seen in a thrilling hunt sequence). Interested parties can dig even deeper into the design and concept of Cameron’s fascinating creatures in a series of short documentaries provided on the disc and uncover a few surprises among the deleted scenes, including a more-than-professional relationship between Norm (Joel Moore) and Trudy (Michelle Rodriguez) that seems ready-made for the fan-fiction community. Yet the movie is better off without this distraction and, unlike the incident involving Grace’s schoolhouse, it doesn’t enrich the central story in any way. As “Blue Valentine’s” Cianfrance told his cast after cutting their merry-go-round scene, “The movie wouldn’t be the movie had you not experienced that together” — we don’t need to see the characters barfing on the playground to recognize the bond between them.