How do you filter out moving objects in a movie?

Question

Hello!
I'v searched the internet for solutions to this but I cann't find any.
Im prittie new and I need to make a house on fire while two guys (one is me)
will be walking away from it, towards the camera.
I want to filter out these two guys and only apply fire and flames to the house itself!
To do this I used Time Diffrence and tried to isolate the guy using Fast Blur and Luma Key
but Im prittie lost.. Nothing works and I really need guidence
Do you know a trick to use?
I cant do greenscreening as the green background would cost a fortune..
But isn't there anyway to filter out moving objects?

Answer 1

I'm not sure that you totally get the process. This kind of shot requires at least 3 elements or plates. These three plates become three layers in After Effects. The bottom layer would be the Background Plate which is a shot of the house without any actors in the shot. The next plate would be the Effect Plate or Fire Plate which could be any shot you can get or generate through plug-ins of flames. The top layer would be your Foreground Plate or your two actors.
Now that you know what you need you can plan your shoot. The background plate is easy. Just set the camera on a tripod and lock it off. NO zooming. No panning, No change of focus. The foreground plate is also easy. You don't have to green screen the entire set, all you need to do is to put a green screen behind your actors. I do this kind of thing all the time using a 6' X 6' (sometimes smaller) green screen (or blue or red or purple) made of fabric and stretched over a frame that I made from 1/2" EMT electrical conduit and 4 90º elbow connectors for less than $20 at Home Depot or Lowes. You get a couple of assistants to walk behind your actors holding the green screen behind them. If you need to include their feet they can be easily rotoscoped out later and you can garbage matte everything else. If it's impossible to green screen the shot then you are stuck with roto, but that's much easier in CS5 using the roto brush. If you use your show with the actors as the background plate roto is even easier because you can hide a multitude of sins by simply feathering the roto and letting the fire light wrap around the actors a bit.
The fire plate can be purchased footage, or you could shoot a real fire against a black background, or you could create the flames using AE or even a 3D app like Blender (free) or a bunch of other apps.
Making the shot look real requires a few tricks once you have your clean plates. Fire creates light so you've got to add lighting effects to the background plate to sell the fire. You've also got to wrap light around your actors. Look for tutorials on light wrap for this. You can really help sell the effect if you place some flickering light sources behind and to the side of your actors when you shoot the foreground (actors) plate.
Once you get a handle on these techniques it's fairly easy to make the transition to pulling of this kind of a shot with a hand held camera (now you have to learn motion tracking). On a project I recently completed we had to put a fire in a prop fireplace while actors moved around bringing milk and cookies for Santa. It was all roto and the fire was shot at night with nothing more than a black cloth (Duvetyne) behind the fireplace grate. The final composite was 10 layers and was so totally believable that no one questions the shot.
The layers were from top to bottom, reflection in grandmas glasses, light wrap, actors (roto of original background plate), fire flicker layer, color mask layer (more orange around fire, fire glow layer, fire layer in Add mode, second fire layer in screen mode, third fire layer Luma keyed, and finally the original plate which contained all the actors, carefully lit with flickering firelight simulated by dangling a stick with fabric cut into 1/2 inch wide strips about one foot long in front of a orange gelled light behind and to the right of  the actors.
If I get a chance I'll post a screenshot.
Here you go:
Light Wrap:
The Movie
Part of the original render before color grading.
Enjoy