The 7 Most Common CGI Screw-Ups (Explained)

#3. Stiff Faces

Often cited as one of the creepiest movies ever made, sandwiched between Nosferatu and The Exorcist, Polar Express usually gets called out for its creepy eyes. While the eyes alone are sufficient to transport the viewer into the uncanny valley, another big source of unintentional horror is the stiff faces.

This is a big problem in a lot of near-photorealistic motion capture movies. They're essentially pasting a photographed human face onto the model, like a sticker wrap, and the motion of the body is lifelike, since it has all the minute swaying and tiny movements of a real actor, but facial movement is still as limited as Woody from Toy Story or Keanu Reeves in The Matrix.

More so on one than the other ...

This looks fine on Woody, since he is a cartoon toy, and his limited facial movement matches the cartoonish movement of his body and the cartoonish look of everything around him. On a near-photoreal model, however, it looks like the little girl has been embalmed and can only move her eyes and jaw.

In this scene from mocap and acting masterpiece Final Fantasy: The Spirits Within, characters move their eyebrows, nose, mouth and eyes like they are all separate pieces moving independently.

Sometimes animators are to blame, because in the end, you've got to move the parts right, but most of the time, stiff faces come from limited rigs. You see, after the model is made, but before the animator starts moving it, riggers come in and rig the model with all kinds of controls. They put in a skeleton, so that if you bend a joint, the arm bends, or they designate areas on the skin (like a cheek) that move when the animator turns a dial or types in a number. This is basically like putting the strings on a marionette and handing them to the puppeteer (the animator).

Fittingly, marionettes are also creepy as hell.

If you don't give the animator any controls for cheek muscles, for example, the animator couldn't move the cheeks even if they wanted to. Human faces have, like, a million muscles (I didn't take anatomy), and you have to give animators a shit ton of complex controls in order to make a realistic-looking human face move as realistically as it looks. They didn't have the experience or software to do it in Polar Express and Final Fantasy, but the team on Avatar managed to figure it out. So now nobody has an excuse.

These are all mocap movies, so a lot of the problem was also how to get the data from actors' faces automatically onto the CG faces, which is a pretty hard thing to do. Usually, no matter how good the process is, you still have an animator going in and touching it up, so a lot of it depends on how well a guy can look at a video of someone's face and click his mouse around on a computer to make the CG face match it.

#2. Characters Don't Look Like They're in the Same World

The dinosaur stampede in King Kong was infamous for "looking like bad green screen." At times it looked like Adrien Brody, Jack Black and numerous forgettable cast members were actually running in front of a dinosaur movie playing on a wall behind them.

You expect that kind of laughable B-movie effect from Jean-Claude Van Damme's Derailed:

But it's kind of weird to see it in a Weta (Lord of the Rings, Avatar) movie like King Kong. All I can figure is that they were rushing the crew -- too many shots in too little time. Technically, what's wrong is the compositing. Compositing is when you take all the stuff that has to go together -- backgrounds, CG characters and props, live actors shot on green screens -- and put them all together.

It's basically like doing Photoshop on a moving picture. You get shitty compositing for pretty much the same reasons you get shitty Photoshop. The characters aren't cut out well (green screen matte extraction was done badly), the lighting doesn't match or the focus doesn't match. You can see some lighting problems here:

Even when Jack Black runs under the dinosaur and should be completely in the shade, his hat stays bright. It's not like most people really notice that specifically, but when you watch the scene play out, your brain tells you something is wrong and you can't put your finger on it. There's also some shots where the actors are way too sharp compared to the background.

I mean, yes, obviously they should be sharper than the background, because the background is moving and has motion blur, whereas the camera is following the actors, so they should be more in focus. But if they'd gotten the relative focus of each object correct, it wouldn't look like ass when you saw it in motion, and it looks like ass when you see it in motion, which means there's too much blur on something or not enough blur on something else.

Maybe it's the edges of the actors that need to be blurred to blend in. I don't know, I'm not a compositor. If you know the answer to this, try going into comp, it doesn't pay too bad if you are good at it.

Fortunately, sometimes bad compositing can be so bad it's funny, as in this Gone in 60 Seconds clip (start at about 7:22).

Here's the crucial shot if you missed it:

Man, it's like someone cut that wall open with an X-Acto knife.

#1. Clothes Look Like They Are Pasted On

I tried to stay away from The Hulk, but these pants really have to be addressed.

These pants look like pants from a video game. A video game from 2003, when this movie came out. The fake-looking surface is a texturing/shader issue like I talked about before. It's pretty egregious, but that is not the worst thing. The worst thing is that they appear to be painted on him.

See, a character moves around when you move a skeleton, which is pretty much what it sounds like -- imagine a bunch of joints connected by wire, and then you sculpt soft clay around it. You bend the joints, and the sculpture moves. There's two ways of making the model move. One is that you say it's pretty much like clay that's attached to the skeleton. This is called "skinning." When it goes wrong, you get melty faces and Gumby elbows.

(A lot of tutorials cover the "collapsing elbow" problem.)

The other thing you can do is a simulation, which is something you do for hair and cloth and tassels and maybe dangling genitals or something. You tell it where it's attached, or pinned, to the skin or skeleton, and it sticks there, and everywhere else it uses a bunch of complex physics equations to tell it how to move. Like with a skirt, you tell it the waistband is "pinned" to the character, and the rest of it figures out what to do based on the pinning and air resistance and whatever it crashes into.

The Hulk's pants had a minimal amount of simulation, if any, so it looks like a second skin, which looks weird most of the time, but looks absolutely bizarre during certain movements, like this:

What the hell are those pants doing?




OK, I've got to go watch Crouching Tiger, Hidden Dragon again so I can stop being mad at Ang Lee. Now that you are a CG expert, I will leave you with this clip as a test to see if you can identify what parts went wrong. (Hint: Everything.)

For more from Christina, check out The 5 Miserable VFX Jobs That Make Movies Possible and 4 Reasons 3-D Movies Don't Have to Suck.

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