Travelling Mattes In Wax
Author: Chuckdoon (chuckdoon_@_yahoo_._com)

In the following article, it is my intent to show you how to adapt conventional filmbased traveling matte and motion-control techniques for use in the 3D world using some wonderful freeware applications provided by some very generous folks in cyberspace. I know that I am in good company amongst the readers when I call myself a, “film geek,” having spent much time and energy watching special-effects intensive movies and intently thinking, “How did they do that?” Having said that, I’d like to talk about the conventional traveling matte technique that has been used in film for several decades, in one form or another. This will also help you to see who I, “stole,” the idea from! A number of different techniques have been used, but I will be discussing what is commonly known as the, “bluescreen technique,” followed by some explanation of the motion-control technology used to create complex traveling matte scenes with many levels of action. Afterwards, we’ll take these time-tested techniques into the digital world and make them work for us 3D “geeks.”

The special effects in the first Star Wars movie were quite an amazing technological feat at the time. The film industry had been using traveling mattes for a long time to produce scenes of foreground action in front of backgrounds that would have otherwise been difficult, unsafe or downright impossible to do on location. But at the behest of director George Lucas, John Dykstra, the genius behind the special effects powerhouse Industrial Light & Magic, took the traveling matte process to a whole new level of sophistication.

The conventional film-based process starts by filming an actor or a model (the foreground subjects) in front of an evenly lit pure blue or green screen, which will be removed later in post-production. Blue or green is used because they are least likely to be color components of the foreground subjects (usually an actor) and therefore won’t leave, “holes,” in them when that color is removed. Care also must be taken not to have any of the blue or green color reflect onto the subject, otherwise it will leave a visible line, or “halo,” around the subject in the final scene. In the processing laboratory, the foreground footage is passed through a series of colored filters and printed onto black and white film, ultimately creating a black background with a traveling, “hole,” and a “traveling matte,” or a moving black opaque mask in the shape of the subject with a clear background where the blue/green used to be.

The opaque traveling matte and the desired background footage (or “plate”) are then run through what the film industry refers to as an optical printer and onto a negative. So now we have a negative of the background with another traveling, “hole.” Now we take the original traveling hole with the opaque background and run it thought the optical printer with the bluescreen foreground footage onto a negative. Now we have two negatives: the foreground negative with a clear background and the background negative with a traveling hole in front of it. These two are run through the optical printer to get the final product of the moving foreground subject in front of the background footage.

As you can imagine, this is not a cheap process, especially with 35mm film! (Now you can see why a “low-budget film,” can run into the tens of millions of dollars, even in the 1970’s!)

Now imagine George Lucas coming along and saying to John Dykstra, “I have this scene that calls for an entire squadron of tie-fighters to fly in formation through space.” Well, being an inventive kind of guy, Mr. Dykstra came up with a computer-controlled camera, which would record and repeat its own movements (this was state-of-the-art technology at the time and was NOT an inexpensive project!) This computer-controlled camera (starting at different points) was used to make multiple passes on the same model in front of a bluescreen and then each pass was layered on top of one another using the previously explained traveling matte process, thereby giving the illusion of a squadron of space vessels flying in formation. Keep in mind that the model, generally speaking, is kept completely stationary and all of the motion is done with the camera: the truck, pan, tilt, crane, etc. This type of filmmaking requires a lot of choreography!

These days, a lot of TV shows and movies not only have their special effects done digitally in post-production, but more and more are actually being shot using digital formats (George Lucas strikes again!) Additionally, many of the subjects and some of the, “live-action,” characters you see on film and TV are merely animated 3D models. Sam Raimi’s Spiderman and Spiderman 2 (again, the special effects were done by John Dykstra!) had extensive CGI character footage, in addition to blue/green screen composite scenes.

Here comes the part we’ve all been waiting for, how WE can a make a traveling matte! You can use any 3D program that you want as long as it can render alpha channels for the animation. An alpha channel is simply the digital version of a matte (traveling for movies or stationary for stills); it is a black mask in the shape of the foreground subject, moving frame by frame exactly as the subject does. In my case, I am using Anim8or, a program by Steve Glanville, to model, animate and render my foreground subject and for my background, as well. I will be discussing the techniques from this perspective, because that is what I know. I hope that you’ll be able to adapt these techniques for use in your particular situation. I am using Wax, courtesy of Satish Kumar, as my, “optical printer,” for layering each camera pass on top of the background. I will list links to these applications under the resources section at the end of this article.

  1. First, I render out a background, “plate,” to use the old school parlance, of a starscape. It can be stationary, but in my case, I had the camera pulling back through the starscape to provide some interesting movement. It doesn’t matter where you get this background from, I just happened to make mine using Terranim8or (yet another wonderful freeware app, created by Leslie Korosi!) and animated and rendered it in Anim8or. Just for flexibility purposes, I rendered the animation uncompressed. I also happen to like using 30FPS, but you can use 15, 24 or whatever speed you require. It can be compressed later in the final stage using whatever codec you want. I now set the background animation aside in a nice safe place on my hard drive, perhaps in a project folder logically named, “LaserShip Squadron.” You can do the background plate after you’ve rendered all of the foreground passes, too. Make note of how many frames this scene is.
  2. Now, we get to the choreography part. I imagined a squadron of ten laserships coming at the camera and shooting lasershots (the lasershots are subject matter for a whole other article.) From overhead, here they are in figure A:

    Since I am a lazy modeler, I made mine rather simply, just for this screen test. John Dykstra saved some time and money by using the same model for each pass. Again, in the interests of laziness, I use one model, too! You can model yours any way that you want to and you can have a scene with as many of them as you want. Notice the numbers beside the models, which are used to keep track of which passes I’ve made and for the purpose of labeling each foreground file accordingly. Of course, we will be layering numerically backwards, from the background forward. Of all the camera passes, my first one will be the furthest back and the furthest to the right from the subject (#10). This will place the object the furthest away from the camera and to the leftmost position of the frame (See Figures B AND C.)

    Again, make sure that you render out the foreground animation (called a “color-channel”) with an accompanying alpha channel (See Figures D and E)!

    For flexibility purposes, I prefer to render uncompressed at 640x480. Now that you have Lasership #10 “photographed,” move the camera slightly to the left and using the same movements, render another animation, calling it, “9.” For each pass, the starting point for the camera will be different. There’s no reason you can’t give the laserships (or whatever you are animating) more interesting movements such as pitch, roll and yaw, merely by altering the camera movement. For a choreography reference, George Lucas used old WWII films of dogfight sequences; not a bad idea! Repeat section 2 above as many times as you want vessels in your squadron. Make sure the animation is the same speed and amount of frames as your background movie.
  3. Now that you’ve shot all of the passes that you want, lets do some compositing! Let’s open Mr. Kumar’s awesome application, Wax. Remember when I said to make note of how may frames your scene is? This is where it is crucial. Under the Project menu, select Settings (See Figure F.) Set the output frames to the length of your scene (See Figure G.) Make sure you do this BEFORE you add any media files! In my case, I have a 10 second scene at 30FPS, so it’s from 0 to 299. Change the frame rate as applicable here, as well as the frame width and height, if desired. If you want to compress (I don’t recommend it, unless you are strapped for hard drive space; my uncompressed AVIs are 200 MB each!) at this point, you would do so by clicking the options button in the video section of the render console. I believe that you’ll also want to select, “Save Alpha Channel Transparency,” here (See Figure H.) Incidentally, I am choosing the output mode as, “video only,” at this point. Don’t forget to change the filename and location so you can find and identify it. With 20+ layers, you’ll thank yourself later! I used, “StarscapePullback10a,” so I could remember that it is the background plate with a traveling matte of lasership #10 on top. In my project folder, I created folders for each pass. I store, “10 and 10a” in a folder called, “10.”

    OK, now that we have the rendered output settings established, lets Add Media Files. It doesn’t matter what order you add them to the list, but it matters what order they are placed in the timeline. I add the starscape background plate, along with the alpha channel for the lasership (which I labeled 10a) that will be the furthest in the background in the final shot. Now, we drag the alpha channel (10a) into the timeline then drag the starscape movie just below it (See Figure I.) The display widow will show only the top layer, the alpha channel, at this point. Now, going into the Track Properties box for the alpha channel, there is a drop down menu with a number of options, from which you’ll want to choose, “Multiply.” (See Figure J)

    The display now shows the starscape and, even though you probably can’t see it, the alpha mask on top of it. Hit the render key and you’ll have your first traveling matte! To make things less cluttered, I now delete the media files to make way for the next layer. This will be the “color-channel,” animated pass of lasership #10, which we will place in the, “hole,” created by the traveling matte. I go back into the Render Console under the Project menu and change the filename to “StarscapePullback10.” Now we add the media files called, “10,”) or whatever you called the lasership/space vessel file) along with the AVI we just rendered called, “StarscapePullback10a.” The former gets dragged into the timeline first, because we want it on the top. Then we drag the, “StarscapePullback10a,” just below it. In the Track Properties box for “10”, select, “Add.” (See Figure K) You can play it now in the display window to get an idea of how it looks, but it may look a bit jumpy in here; not to worry. Now hit the render button, and you’ll have your first ship animation completed. Play it back in your favorite player just to see if you like it (See Figure L.)

    For the next lasership in the squadron, #9, we layer the alpha channel for #9 (9a) on top of “StarscapePullback10,” using “Multiply” again and then layer the the color-channel (9) on top of that using “Add.” Repeat this layering process for each lasership pass. Again, make sure the layers are done from the back most to the front most.

There’s no reason not to go wild with creativity. Heck, use rotten tomatoes instead of laserships! Have them spitting seeds instead of lasershots! Animate as many as you want, make them do a dance, like the Air Force’s Blue Angels!

If you have any questions or comments (even something like, “You don’t know what you’re doing!” would be fair game) please feel free to email me at the address given at the top of the page.

If there is an easier way to layer multiple traveling mattes, then please let me know. And if I have unintentionally made anything unclear, please tell me so I can tweak this article accordingly.

Thanks For Reading,


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