It is finally coming to an end: this is the forth part of our “Howto” tutorial going through the steps of creating the effect of a person freezing in an instant. We’ll start with the creation of the ice texture covering up our actor, move on to the transition matte to make the spread of ice grow and end up with the snow particles and the final comping. This is our final result:

In order to get ice moving along with the actor’s body-parts you’ll first have to get yourself a nice looking lockup to bring back into Nuke. Export a still frame from somewhere towards the end and bring it into Photoshop.
As you can see we started by taking ice textures from our stock footage and sxc.hu and placing them over his body. Make sure you keep as many steps as possible in separate layers for more control later. You might need to change the blendmode of some layers to Screen or Soft Light, but make sure you’ve got some shadow information in there as well – ice doesn’t just consist of highlights floating in the air. Using the brush we generated for our Summer-To-Winter tutorial (download here) we painted in some frost and snow particles (note the crisp flakes around his ear!). Remove the background to get the ice texture only. And you might want to make the delicate parts like the eyes extra, which is what we did:

To slightly distort the ice texture according to our actor’s depth we’ve created a black-and-white version of the talent to use as Displacement Map. Save the black and white image as a separate PSD file.

Filter > Distort > Displace will ask you for a scale value (both horizontally and vertically) – we go with the default value of 10% and both Stretch to Fit and Repeat Edge Pixels selected. If the given effect is too strong, simply go back and reduce the contrast of your displacement PSD by subtly pumping up the brightness.

Although we intended to make the shape of the ice fit as good as possibly we’ve tried to leave it as flat and undistorted as possible at that point – we are going to need it flat once we start distorting parts with a 4-Corner-Pin. Always think two steps ahead. A good example: what blendmode are you going to use to place the ice over him? If you are not going to use Screen or Add, you must not have a black background. So take the time to think everything through from the beginning.

To make the ice grow over your actor use carbon dioxide snow for practical effects and a transition matte for post effects. In the latter case figure out first if you want the ice texture to grow over him from bottom to top or rather from behind to front, and so on. We decided to let it move from bottom to top, starting with the arm first and then continuing on the rest of his body.

We won’t go into the details of how to create a noisy transition matte, but basic skills should suffice to pull that off – and no one says you necessarily have to create this matte in Nuke.
If you really are lost here we highly encourage you to watch the following tutorial from Andrew Kramer in which he is using the same technique: 88. Disintegration. You see our result on the right.

Our goal was to make tiny snow particles go off from where the ice currently builds up. So we now needed to sync the snowflakes with the previously created matte for the growing ice. In our settings below you can see we set our Layer Emitters Layer to a comp holding the before-mentioned animated matte, eventually creating the desired effect.

Our next step leads us into After Effects. We used Trapcode Particular to create the ice cold breath and snow particles. For the breath use a Point Emitter with a low lifespan and some crazy transparency. You can see our settings below, but feel free to blow everyone away with your ideas.

Let’s get to the rotoscoping part. Back in Mocha we’ve imported the original greenscreen footage and started drawing bezier shapes around every single extremity – the ears, the mouth, you get our point.

Adjust the tracking settings and let the automatic tracking run through the sequence. Adjusting the bezier points you should be able to perfectly rotoscope the separate parts. As we will eventually need CornerPin Nodes inside Nuke to make the different layers appear moving with his bodyparts, we go to the AdjustTrack tab in Mocha and hit the Align Surface button beneath your layer panel:

As you see we also adjusted our Reference Points (shown as crossed circles in red) to cover the entire shape that we were currently tracking, like the four corners of a three-dimensional plane. Once this is accomplished we can export our shapes as Nuke RotoPaint Nodes and the Reference Points as Nuke Corner Pin (*.nk) like described here in our first tutorial.
You should end up in Nuke with both a RotoPaint and a CornerPin2D node containing the data from the same extremity.

Now import the previously generated ice texture (yes, Nuke knows what to do with a *.psd), premultiply and reposition it, if needed.

We now have a matte on the left and the Corner Pin holding the ice texture on the right. Using a Copy node we can take the mattes alpha channel and copy it to our Corner Pin, leaving us with the texture only where we want it to be. Our matte is pretty sharp edged right now, so we’d like to blur it a bit without distorting the ice texture itself. As we mentioned before, we need to repeat these steps a few times, thats where a Nuke Gizmo comes in handy. Gizmos are Grouped Nodes that can have multiple In- and Outputs, calculations and even user defined settings. Select everything from the Copy Node down to the Premult and Press Cmd + . You should now have a Gizmo called Group 1, which you can rename, recolor and reuse as often as you want. And that’s exactly what we did for a whole lot of body-parts.

But before going through that whole process make sure to adjust your CornerPin2D Node’s From points. To do so plug your plate into the current body-part’s RotoPaint Node and send it to your Viewer so you can see the plate behind the points of the CornerPin. Remember this is only to make our life easier for this step, you want to undo this step afterwards in order not to mess up everything.
In your CornerPin2D settings switch to the From tab and reposition the points to their place on the plate. This for example is where the From points of the left eye should be placed on our ice plate:

To get a better overview and for the case we need e.g. the whole head masked out at a later point of time, we merged together the eyes, mouth, nose, ears, headbase and hair and repeated this step for every bigger group of elements (like the arm). Merging all our icy parts together we ended up with a Merge containing our final match-moving ice plate.

So now its time to reintroduce our previously generated transition matte. After importing the image sequence we need to copy its alpha channel into the ice plates. Plates, as plural, because we wanted the effect to start at his arm and move on to the rest of his body. To do so we first merge all body parts covering his arm into one Merge and the rest into a second one. We then copy the transition mattes alpha channel into both of these Merge Nodes and offset both with the help of a Time Offset Node. To make the effect appear on his arm first we simply push the Torso’s Time Offset back in time half a second.

Finally copy all the alpha channels and the iced body parts together, premultiply it and merge it over our talent footage right before pushing it into 3d space.

Do a little color correction and you’re done.

It took us a while to get here, but here’s our final Node Tree:

Now looking closer you’ll see that the ice still looks a bit flat and agitated, but consider we just started breaking him down into separate parts! One corner pin can only cover a single plane, which is why you start to see some slippery movement at the areas where we got lazy in Mocha. To cut a long story short: you’ll gain way more detail and accuracy the more subdivisions of the body parts you create.

It’s been quite a journey to get us to the final animation, and it all wouldn’t have been possible without Boris Zrost‘s fiesty yet imperturbable dedication as an actor, having spent over 450 hours inside a cooling chamber for reference. As you can see we are in love with the idea of freezing people, so if you have another approach to this: please do share!

Jake & Dan

...you might also be interested in:

• Freeze Effect Part 3/4:
  Integrating The Elements
• Freeze Effect Part 1/4:
  Shooting and Keying
• Freeze Effect Part 2/4:
  The Backdrop