I was on-set filming as VFX Supervisor when the first Assistant Director (1AD) had a simple question for me, "Do you want to film this shot with the smoke on, or add it in later? I know some VFX artists like to get the plates clean."
This is a question I quite often end up fielding in one way, shape, or form. It was a wide shot with a large bluescreen behind it and giant fans were blowing smoke through the scene, whipping actors hair and all the rest. From time to time I’ll ask for them to shoot it both ways, but that has almost never worked out in my favor, and today, with limited time for the main actor, that wasn’t even an option. I had to make a call. This is as exciting as VFX supervision gets, and the only wrong answer is, “um…..”
Let me address a few thoughts before I continue with how I answered this question.
In-camera smoke and added-in-post smoke elements are generally indistinguishable in the shot. It isn’t modeled by light, it’s just more or less washed out to smoke-color. The nuance of the texture and movement can be important, but you are far better served with an element that does what you want.
Elements are the way to go for windy/snowy/fast moving white buffeting shapes. They look the same and you can control them. Need more smoke: add more. Need less, lower the transparency or drop the gamma. Bigger smaller, upside-down. All easy to do. Smoke elements FTW.But, feeling confident about my tracking and keying skills, I replied, “Smoke is good. It’ll interact with the scene.” I pictured whorls of smoke trailing off objects, big misty shapes being smashed apart as they hit solid forms. Serious, dynamic stuff. The deep skills of my team would deal with the added difficulty in tracking and keying.
We filmed with the smoke in the shot; white puffs whipped through frame, just as they would have if I’d added them in post. They didn’t appear to be affected by the objects in the scene. There was no magic to be had. The 1AD was right, I should have just shot the plate clean.
Needless to say, at the 11th hour when a previously non-VFX shot was added to my plate I was cursing my decision to add smoke in-camera. I spent hours on what should have been a very easy track. Well into after-dinner hours, if my memory serves.
I should have known to not shoot smoke in-camera. Years before I was handed a job that needed blizzards, and the plates were snow and cloud free. We tracked them, added snow plates, and added smoke elements to further enhance the blizzardy nature. They looked great. Now that we know the rule; here are some examples of when to break that rule and shoot the smoke for real:
If it has to catch light in a specific way, like a shaft of light in a shot. The light’s not going to be visible unless you bounce it off some haze. That said, if there’s a lot of motion, you still may be better served with a post-solve.
If it has to track as it’s emitted, like a cigarette, for example. You can’t track a plate of cigarette smoke onto a moving cigarette, it will look fake because the smoke is being emitted from a moving emitter. You’re gonna have to shoot it for real or simulate it.
I’ve never run into this personally, but, if the smoke is big and thick enough to cast shadows. For example, if you were detonating something huge and smoky and the shadow casting was important to the shot, then you may need to shoot it for real. Good luck with the permits on that, though.
Learn from my mistakes: don’t shoot with smoke in the shot.
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