Stop-Motion Looks with XSI or other CGI packages

A key to tricking the eye into believing we’re looking at something handmade is in the relationships of proportion. Although a real puppet builder might aim for realism, there are usually elements that retain a hand-built nature. For instance, when constructing Abe Lincoln (see below), we used very simple geometry to make his hat look like it could be molded plastic. His head is very large in proportion to his body, and his hands are very tiny. His jacket has large and obvious thread detail, helping to reinforce the illusion of scale. In this short, Abe’s proportions help him read as a small puppet.

When attaching a character to a rig, we usually try to achieve a smooth, realistic deformation. To simulate the look of a puppet that’s been rigged with some type of armature, we want to break from that tendency. The rig in this step is based on the Softimage|XSI 4.2 default rig, which includes a curve-based spine. It gives us a good basis for our puppet armature. Although we have more controls than we will actually use, it still lets us set up quickly.

When applying a flexible envelope, use the minimum number of deformers per point. In other words, each point should be mostly influenced by no more than one deformer. This will cause a more rigid breaking of the geometry during animation. Some smoothing will still be necessary, but keep it to a minimum since we are trying to mimic an armature with clothing. XSI has an "Apply Smooth" function that makes this quick and easy. As you can see in this image, Abe’s arm is unacceptable for most purposes, but this distortion happily aids us in our goal of attaining a stop-motion look.

Stop-motion animators commonly use a technique that involves swapping out parts of a puppet. For instance, swapping mouth parts creates the illusion of lip-sync. Tim Burton’s Jack Skellington character from his film The Nightmare Before Christmas had over 40 replacement heads. These are referred to as "replacement parts," and we will apply this concept to our model as well. The procedure is the same as regular shape morphing, but rather than letting the computer assist interpolation, you can keyframe shape changes over a single frame. We all know that smooth blending at the push of a button is easy in XSI, but once again, you need to resist the temptation to achieve your goal. Limit the computer’s influence by using blend shapes and changing quickly from one shape to the next.

"Final gathering" is an algorithm that speeds up global illumination, lending realistic lighting to your scene. Most 3D programs today have incorporated some type of this form of rendering, which lets you achieve a level of realism with very little effort. Using a large "bounce card" object, you can cast a soft, diffuse light over your set. I won’t go into detailed instruction on how to use final gathering in this tutorial. But, in this case, a lavender-colored dome over the set is producing almost all of the lighting in the scene.

Probably the most important aspect of emulating stop motion is how you shape your animation curves. A stop-motion animator may try to achieve smooth motion when it is appropriate, but will hold poses and use larger position offsets when the action is less important. You can mimic this style by combining spline interpolation keys with hand keys and constant interpolation. In constant mode, each keyframe’s value is simply held until the next keyframe. In this example, Lincoln’s arm effector has a mix of constant interpolation and spline interpolation. As he rolls the snowball forward (see image, Step 1), his motion is smooth, as if the animator had done frame-by-frame adjustments.

Rob Cazin
Founder
So! Animation

Rob Cazin founded So! Animation in New York in 1992 as an open creative environment; a full-service design studio soon formed, built on a core of superior technology and artistic excellence. At So! Rob leads the Softimage|XSI animation team and provides original music and sound effects for independent films and broadcast branding. A recognized CGI pro, Rob has written in-depth articles for various trade publications and has won an Emmy. So! holds dozens of other industry awards.

This tutorial outlines the guiding principles and techniques that can give your 3D animation a traditional stop-motion look. Although this particular project was done in Softimage|XSI version 4.2, each step is generalized to apply to most 3D applications.

There are many other tricks that you can use to create a stop-motion feel. Try adding "jitter" to your "replacement parts" by rotating the part slightly at some of your shape keyframes. Or bend the ear of one of your heads, as if it were damaged in handling.

Another common trick is to apply film grain and/or damage using one of the many plug-ins that are available.

The final version of this Holiday ID short, which we did for the History Channel, can be viewed at www.so-animation.com.

Support Gear: Adobe After Effects and Photoshop; Avid|DS HD

So! Animation
www.so-animation.com
220 East 23rd St., Suite 1007
New York, NY 10010
ph. 212.696.1020
rob@so-animation.com