Expert: Andre Hickman - 10/13/2007

Question
I'd like to apply a fog effect to a film clip using Adobe After Effects CS3 Professional version. I applied a fractal noise effect to a solid but can't blend the fog to the footage. How can I apply fog to a video clip?

Answer
Hey Jim,

You are headed in the right direction, but it is going to take a bit more complexity to make it look believable.  I will try to give you a quick run-down of what needs to be done in essence...

Fog looks more dense at far distances, and less dense as it gets closer to you.  With your film clip, chances are you do not have any Z-buffer data built in, which would basically tell how far an object is from the camera, so you are going to have to rotoscope (create masks that change with time) for different depth planes.  

Start from the back.  Create a fractal noise solid, and make the fractal size relative to the distance away.  assume that is your furthest plane. Now first the fog is going to stop at the ground at that plane, so you need to crop or mask where you think that plane would hit the ground at that distance (imagine a piece of paper or a wall standing up from the ground in the distance) Now you have to imagine what parts in the film are physically in front of that plane (wall). You will have to 'cut a hole in it around those objects so that they aren't covered by a layer of fog that is supposed to be be behind them.  Of course if the objects are moving, that is where rotoscoping the masks comes in...changing the shape of the mask frame by frame. FEATHERING THE MASK will help hide imperfections.  After you have it the way you want it, you want to set the transfer mode to "screen" and adjust the opacity of the layer.

Next do the same technique, imagining the 'wall' is closer to the camera.  Obviously the fractal noise needs to be larger,  and what is in front of this new wall of fog will be different than the last wall.

You will probably want 3-4 walls, and know that the transparency becomes more and more, until the closest wall covers the entire film, and is very transparent, and probably will not require a mask.

Tweak and re-tweak your settings to get the desired look!

I hope this helps! ask more if you need more instructions...It's hard to explain this stuff without pictures, so I try to be as illustrative as possible with words...

Cheers!
Andre'

(P.S. JUST REMEMBER TO SET THE TRANSFER MODE ON THE LAYER TO 'SCREEN' AND THEN PLAY WITH THE OPACITY %)