Matchmoving Tips & TricksDate: 09-01-11 2:01PM | Category: Tutorials - 3D
Matchmoving...just the term alone make me shudder as I think about the countless hours I've spent trying to perfect and master this effects technique.
In simple terms, it's the process of replicating the motion of a real world camera or subject in a CG scene. Special software tracks & analyzes motion throughout the shot, and creates a CG element that moves identically in 3D space. So filmmakers can shoot with a moving camera in an empty field, and digitally insert any number of CG layers.
Here's a scene from Jurassic Park, which is a great example of this technique.
The process of matchmoving is fairly automated using today’s software...which is actually what makes it so frustrating and difficult. The artist will setup a scene and input as much helpful data as possible, but after that it’s up to the software to create a good track. If it doesn't create a good match on the first attempt, it's really difficult to manually correct these issues. However, there are a few key tips that can get you on the right track ;).
There's alot that goes into the process, and by definition, it will involve jumping back and forth between several different software packages before you have your finished shot. The problem is that there's a ton of things that can go wrong between those tools and you can end up with an accurate track that appears to be broken. So the first (and possibly the most important) tip is to always trust your matchmoving software. If the tracks are accurate and sticking in the preview, then it's a good track, there’s just some detail that’s being misinterpreted in your other tools.
Short answer; alot. It’s amazing how much information is being transferred from one tool to another, and every single one needs to be identical or the track will break. Here’s the main issues you’ll need to be aware of on every shot: frame rates, scale, coordinate system orientation (Y-up vs. Z-up), resolution, aspect ratio, and of course the common 1 frame offset. This is a specific issue that occurs between multiple animation tools, which revolves around whether the start frame of your animation is 0 or 1. It’s an easy problem to solve (just offset your CG animation 1 frame in your editing software), but it’s difficult to figure out unless you’ve seen it before.
Frame rates can cause a ton of issues in your track, and must be set independently in each piece of software. Even the simplest of tracking shots will involve 3 different tools: your matchmoving software (boujou, PFTrack, SynthEyes), your 3D software, and your editing tool. So be aware of the framerate of your shot, and make sure it’s one of the first things you setup in each one.
Scale is always an issue that needs some assistance from the artist. Ultimately the tracking software is just picking out points of high contrast in a shot, and tracking how those pixels move throughout the frame. It will take a guess at scale, but it’s just that, a guess. In my experience the scale usually comes in on the small side, so you usually have to scale the entire scene up. In After Effects, I usually scale my tracking data up 240.
The coordinate system orientation is similar to the 1 frame offset. It’s an important piece of data that different between tools. Basically, 3D space in computer is broken up into 3 axis; X, Y, and Z. Some software thinks Y is straight up and down, and some software thinks it’s Z. This is usually a trial and error one, but is easier to identify. If you bring in your tracking data, and everything looks like it’s been flipped 90 degrees, than find the coordinate system settings in and make sure they’re the same.
The last thing to touch on is a generic notion about where and how to insert your CG element into the scene (in the 3D space mentioned above). Remember, the tracking points are the known positions in 3D space that are going to move along with your footage with the greatest accuracy. Always align your CG elements with the tracking points and you’ll know they’re going to stick. Once you understand this concept, you can begin to setup a shot with this in mind. For example, let’s say you’re filming a simple shot with a CG object sitting on your kitchen table. Put a dime or a penny right where you want the object to sit, and you know the software will identify this area in the footage and place a tracking point there. Then, just align your CG model with the tracking point, and it’ll stick like glue.