Actually, it's not hard to film on Location with a "lava planet"
You make set. A real set. Not a blue set. Something the actors can feel, and touch. Go to New Zealand. Find some cool rocky location. Build a cool little set....or film it in a soundstage.....hell, you can make fake lava spraying everywhere, and have it feel real....
You have to understand, imagination is key, but when actors end up being more focused on "You cant step here, theres a window there..." rather then "You're whole family was killed, you have nothing to live for....ACT!" it becomes a problem.
Plinkett made the best point about the fake sets. The length of the blue set could be 30 feet. Now, a scene where you NEED to run and move to get to a location, it's very hard to film that because you have a short space to run...which is probably why half of ROTS's tense moments consist of walking.
Now, back to the ships. CGI ships may look good at THAT moment in time, but within a few years, they'll look horribly dated, and fake.
Take Pitch Black, I thought the ships back then looked great.....now...? Ugh.
The point Plinkett tried to make, which I agree with, and have agreed with for sometime (before he made it) is, we've seen CGI. We all know it, it's not a huge deal any more. But IF they did that whole scene with models and CGI....you'd bet that'd look a lot more impressive, and be probably one of the coolest shots of all time.
Would it be hard? Oh yes. But in the end, it WOULD be worth it. That would be so impressive to see. Hell, the final battle at the end of Jedi still impresses me. That's a fantastic space battle, that holds up today.
ROTS does not....all the effects look cheap, and dated.
Also, if you film something on Blue Screen, and add a real minature background, that still doesnt count as a set. That's just compositing. Not that impressive. I can do compositing on my computer.
What impresses me is doing something no one has ever done before, and making it work.....like Gollum. (You could say Jar Jar...but he didnt work.
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And on the last thing about Avatar...James Cameron DID do something new....he didnt just do what Lucas did, he actually got a proformence from the actor, and used that exact data for the CGI characters. And not only that, he made them look real. And not only that, he created a 3D that was incredibly immersive, and impressive.
Not like ROTS.