if I cut the subs directly from one frame of a 35mm scan, could that be turned into a soft sub?
I’ve already done this for Jedi, using 1080p image files you sent me as a source, remember? 😉 For Star Wars, I had to hack together something on my own from another source, and it’s not quite as nice. Both of these soft subs are already created and in the current version of Project Threepio. The filenames are ROTJ-eng-alien-35mm.sup and SW-eng-alien-35mm.sup – feel free to try them out.
If you have new image files for Star Wars, they could be turned into soft subs in fairly short order. I’d certainly prefer to use your image files as a starting point for both films.
The problems with soft subs are minor, but on a theatrical fidelity scale, they might be big enough. Soft subs have no gate weave, moving grain/dirt, etc, and attempting to simulate this through an animated sequence of subtitle images is very badly supported by players. The BD-SUP subtitle format uses a strange timestamp-at-the-wrong-framerate+framecount time specification method, which means it’s possible that soft subs will not appear and disappear on exactly the right frame. But for most purposes, they would be indistinguishable from burnt-in subs to even fairly picky viewers.
To be honest, though, I still prefer burnt-in for these particular films.
EDIT: Oh, and if you’re making new image files for Star Wars, try not to lose the positioning info. i.e. the best way to get them is to get the subtitles floating at the right location in a transparent 1920x1080 image.