Yes, I remember that documentary. It was, I believe, the single most complex scene ever to be pushed through an optical printer. As you said, nowadays the compositing is all done digitally, so the old optical printers are now gathering dust and I don't think anyone will ever try to do anything like that again.

The grid was necessary because they needed to know which order the objects had to be placed in the optical printer through its multiple passes.

And, if you'll recall from the documentary, there was actually a glitch in the ordering, where you could see a distant tie fighter group overlaid atop the Falcon. They left it in because it was so tiny and hard to see, and it wasn't worth going through that insane amount of work on the optical printer again.

Funny thing is, up until I'd seen that documentary, I'd always suspected there was something not quite right about that scene, something I couldn't quite put my finger on. I got the impression that some of the smaller distant ships really had passed in front of the Falcon, but I couldn't confirm it just by watching the movie. Another problem is a nearby clip where a group of tie fighters seems to appear out of thin air instead of fading in from a distance. Again, I suspect a slip up at the optical printer stage with lots of complex elements.

But I have to say, that moment in the Rogue game is rather cool. You get the order to pull out, and you turn with all the other fighters and capital ships to see that blanket of stardestroyers waiting for you... too cool!