Originally Posted by archiguy
You could build up a whole library of various booms!
after awhile, amortizing out the costs. Not my field, but I'd be interested to hear what an AVS member like vfxproducer who's in the business thinks about it.
Whenever possible, VFX crews will try to get productions to shoot specific fire and explosion elements for a scene that can be composited into the scene in post production. It happens all the time. It's especially important when the fire needs to feel like it is interacting with an environment - let's say for example a fireball that needs to feel like it is racing down a hallway towards camera. And every VFX facility in the world will save these elements and reuse them over and over again in other productions, building large libraries of stock practical fire, smoke, and explosion elements over the years. There are companies that have been doing what you suggest since the beginning of the VFX industry.
This works well for *small* fires and explosions. The problem is, these need to be filmed against a black or green screen so they can be separated over the background and cut out easily. And if your explosion needs to be really big (say like a building exploding), that's impossible. So sometimes you try to shoot a small miniature explosion and scale it up, but there's a point where that just doesn't feel right and doesn't work. My guess is that a lot of the fire and explosions that you *think* are CGI area actually real stock fire elements that have been composited into a scene poorly, or scaled up too much or layered together in a way that isn't quite natural.
Sometimes you can't get production to shoot elements or pay for you to shoot elements, or it's just impossible to shoot a specific element that you need. And that's when people will turn to CGI fire. It is almost always a last resort, because we know it doesn't look great, and it will almost always be more expensive than using real fire/explosion elements.
I'll give you an example. In the recent series finale of Burn Notice, there's a climactic scene with a series of explosions. I won't give much away so as to avoid a spoiler for a show in the wrong thread. But suffice to say that there are a series of cuts that alternate between practical on-set explosions, practical stock explosions composited into the scene in post production, and CGI explosions. The only place the CGI was used was when it had to look like the entire building was going up in flames. There was just no way to do that with real fire. If a company does a scene like that well, the audience won't be bothered by the difference between the real and fake fire and probably won't notice. If a company does a scene like that poorly (or the production won't pay for them to spend the time to do it well), then all the fake fire will pop out and be very obvious and distracting to the audience.
On a related note, my avatar for this forum is a single frame from a practical miniature underwater explosion that was filmed to use as a torpedo detonation in a submarine movie I worked on. The real fireball was about 2 foot across, but when composited into the scene it was about 1/3 the length of a November class Soviet submarine.