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To do this in After Effects alone is going to require a lot of work. To make the hair look realistic you'll have to do some careful compositing and it will require some good Photoshop skills to create the strands of hair and make them look real. Most of the time the attacking hair wold be created in a 3D app and composited into the scene.
let me assume for a moment that you have a 3 second shot with two actors. Actor 1 is the female with the attacking hair. Actor 2 is the actor being attacked. You have blocked the shot so that Actor 1 comes from behind Actor 2 and the hair attacks. This would be a typical workflow using just what is included in After Effects.
- To make it easier to add the hair and do step 2 you need to Motion Stabilize Actor 1 so the head of Actor 1 stays pretty much in the same place in the frame. You'll probably want to add Scale and Rotation to the tracking. If Actor 1 is hidden by Actor 2 you would Motion Stabilize Actor 2. You want one of these actors to be as stationary on the screen as possible. To learn how to do that type stabilize motion in the Search Help field at the top right corner of AE and check out the community resources. Do Not confuse Motion Stabilize with Warp Stabilize.
- Create hair extensions in Photoshop using a reference frame exported from the first frame of the Motion Stabilized footage. You'll want a separate hair extension layer for each strand of hair you want to attack the victim. I would keep them as straight as possible. If the hair is wavy then a little waviness is OK, you just need long enough extensions to allow you to animate their position and shape in AE. Import the Photoshop File a composition retaining layer sizes.
- Add the hair extension layers you made in Photoshop on top of the stabilized footage by opening the PSD comp and copying and pasting all of the hair extension layers in the main comp. If you used the first frame of the motion stabilized footage the position should match. If it doesn't then reposition the layers so they line up with Actor 1's head on the first frame and look like they might be part of her hair. You will need several layers. Use the puppet pin tool to animate the hair so it's moving around like you want it to move. You will have to do this for each strand of hair that you want to attack Actor 2. You cannot apply any other animation to the layers you are animating with puppet pin. You are just concentrating on the movement of the hair. To learn how to use puppet pin type that in the Search Help field in AE.
- When you have the hair moving in attack mode select all of the hair layers and pre-compose them. Name the Pre-comp Attacking Hair. Once they are precomposed you can animate position and rotation to match the movement of Actor 1's head. When you get the hair to look right pre-compose the pre-comp again and name the comp Hair Master.
- Duplicate the stabilized footage layer, move it above the Hair Master pre-comp and then use rotoscoping or the roto brush to separate actor 1 from the background so you can put the animated hair behind Actor 2. You only have to separate the part of Actor 2 that is going to have hair behind the actor from the background. It is very important that you plan carefully because rotoscoping, even using rotobrush takes a long time. To learn how to rotoscope use the Search Help; field to find resources for Rotoscope or Rotobrush.
- Duplicate the Hair Master pre-comp in the timeline and drag this layer above actor 1 and then trim the in point of the top copy of the Hair Master layer to the point where you want the first strands of hair to start wrapping around Actor 2. You may need to use some animated masks to get the timing right. You may also need to have more than one attacking hair pre-comp so you can wrap the hair around the actor in a convincing way.
- Once you have all of the hair moving and wrapping around actor 2 the way you want it to move Add a null to the top of the comp. Rename the bottom layer - the original footage that you Motion Stabilized - and call it "stabilized."
- Apply this Animation preset to your null layer: Dropbox - destabilize Rotation Scale.ffx. The preset will apply the following expressions to the position scale and rotation of the null.
and scale (this one is a little more complex)
x = value;
y = value;
tx = thisComp.layer("stabilized").transform.scale;
ty = thisComp.layer("stabilized").transform.scale;
nx = x/tx*x;
ny = y/ty*y;
- Parent all layers to the null and you'll remove the Stabilize Motion and square up the shot
- Make any final adjustment to color and grading to sell the shot.
- If everything works well then render a production master of the shot using a suitable production format.
There you go. I'm guessing the total time for production of this shot if it is actually that simple and is photographed in a way that will allow you to use Rotobrush to separate Actor 2 from the background will be about 4 to 6 hours. As the complexity and the running time of your shots goes up the time required to complete the shots increases exponentially. To complete say 6 complex shots for a sequence that lasted 15 seconds with 4 or 5 actors in each shot attacking 4 or 5 victims would probably take one AE artist several weeks and maybe months to complete. A simple shot like the one I described would take an experienced artist about a half hour to Motion Stabilize and to roto. Animating the hair would probably take another half hour. If the motion is simple it may take another half hour to get the Hair Master to match up with Actor 1's head movements. Maybe an hour in Photoshop to create the Hair extensions. Color grading and final editing maybe another half hour. So once you know what your doing you can probably count on a pretty a half day to create a 3 second shot. Each shot in the sequence should be a separate comp. You should give your shots "handles," a few extra frames at the head and tail so you can fine tune the edit of the composites in Premiere Pro. Speaking of editing, you should rough cut your attacking sequence in Premiere Pro before you start doing anything else so you know what the in and out point of each shot will be. This is NOT the kind of project that is suitable for Dynamic Link. You will want to render each of your shots to a suitable production format for editing in Premiere Pro. DO NOT render mp4 or other compressed files to be used as in the rest of the production.
I hope this helps you figure out the steps. I hope the footage has not already been shot and you can carefully plan each shot to make compositing easy. If this were to be done with a 3D app and motion tracking to capture the movements of your actors it would require even more careful pre-production planning and would probably require some green screen work to save time. This kind of stuff isn't easy and you can't do it by pressing a button. It requires very careful production planning, blocking of the actors and careful camera work to keep from turning into a mess that takes months or years to sort out.