How to Key smoke, dust, and other particles

[casey warren]

What are some techniques that work for keying smoke, and particles. I've seen may times in behind the scenes stuff the FX guys keying elements to add in later. For example, dirt and dust particles to add into 3d footage to make it more real when the 3d elements come in contact with the live action footage.

 

[oneinfiniteloop]

You will usually shoot the dust/debris in negative space and use a blending mode to add it to your comp.

 

[casey warren]

I've also seen things like that blue screened too, are there any benefits and also how do they do it? wouldnt smoke be hard to key. (example: in the King Kong production diaries, they shoot tons of things like trees breaking, rocks falling, things kicking up dust, smoke, and flying dirt particles in front of blue sceens, then they later comp them into shots of live action/3d animation)

 

[kai]

You can shoot it against black and use blending modes, as oneinfiniteloop mentioned. Or, you could use unmult from the knoll factory to remove the black matting and comp it over your footage.