Here’s what I would do:
1) output your video as a DivX AVI or something similar. You’ll be using it for reference in pI3 only, so it doesn’t need to be great quality.
2) If it plays back fast enough in pI3 (note that you may never get real-time playback at HD res in pI3 as it’s not really optimized for that), use it, add your emitters, then when it’s ready move to step 5.
3) If it’s not fast enough, scale it down in your editor — half-size or whatever. Then use that in pI3, setting your stage size in pI3 to the smaller size of your video. Setup all of your emitters, then when they’re perfect…
4) Use the “scale project” function in pI3 to scale your emitters and every back up to HD size.
5) Turn off your background video (click the “bg” icon in the layers window) then render just your particles with alpha. Use a format that can save alpha (AVI using HuffYUV or Lagarith codecs, or PNG image sequence).
6) Load the rendered AVI or PNG sequence into your editor, layering it above your original video.
Hope this helps.
Alan.
………………..
Alan Lorence
Product Manager
GenArts, Inc.
http://www.genarts.com
http://www.wondertouch.com