by Lee Montgomery
One of the fundamental principles in traditional animation is “Squash & Stretch,” which is used to show the change in volume and weight in organic or flexible objects. Without squash and stretch, organic or flexible objects and characters will appear lifeless or rigid.
To achieve the effect, traditional cell animators must draw the change in shape of an object or character to suggest the shift in volume. Traditional cell animators new to Disney were originally given a couple of tests to measure their skill at representing squash and stretch on organic objects including a flour sack and bouncing ball.
Frank Thomas and Ollie Johnston explain how the principle applies in the real world in Chapter 3 of Disney’s book, The Illusion of Life:
‘’Anything composed of living flesh, no matter how bony, will show considerable movement within its shape in progressing through an action”
‘’Only the wax figure in the museum is rigid”
‘”Through the test we learned the mechanics of animating a scene while being introduced to Timing and Squash and Stretch’’
Squash and stretch is sometimes over-exaggerated in cartoon cell animation for added effect. Stylized cartoon characters often deform beyond their natural limits when squash and stretch is used in the animation.
In 3D Animation, we can use a number of different tools and techniques to re-create the squash and stretch principle on organic objects and characters. In Maya, some of the solutions we can use include:
* Skin and blend shape deformers to animate deformations and change in volume in 3D models.
* Dynamics, soft-body deformers, and the Maya Muscle system to accurately simulate squash and stretch as it would apply based on the forces and gravity in the real world.
Blend Shape Deformer – Squash & Stretch:
The Blend Shape Deformer in Maya allows you to blend from one model to another. The models need to be exact duplicates and the Deformer allows you to blend in the weight effect of one object on another. Blend Shapes are typically used for facial animation in Maya, but can also be used to correct skin deformation or simulate muscle effects. For facial animation, there is a lot of squash and stretch motion in the skin when the mouth, jaw, and eyes open and compress. This makes Blend Shapes ideal for Lip-Sync.
Blend shapes in 3D Animation could be considered analogous to traditional claymation used on stop-motion features (http://en.wikipedia.org/wiki/Clay_animation)
In claymation, duplicate models are made from flexible material such as plasticine or flexible clay, with sculpted duplicates being made for each expression. The sculpted duplicates are swapped in at different frames to create the effect.
In Maya, we have the benefit that the deformations between the models are smooth as the software interpolates the blending between the models in real-time.
Setting up effective Blend Shape targets is primarily a modeling task. For the setup, targets can be set up for localized areas on the face such as the eyelids or corners of the mouth and the blending will be evaluated in parallel alongside other model blending.
Once set up, the target shapes can be blended through the sliders in the Blend Shape window in Maya. The blend shape window in Maya looks similar to a mixing desk’s sliders in a recording studio and can be bewildering.
For animation, control objects can be set up to control the blending–this provides a more visual interface for the animator to control the animation with the setup being done through the Set Driven Key window in Maya to make connections between the control objects and blend shape influence.
Skin Deformation – Squash & Stretch
Using a simple joint rig setup in Maya, you can also get convincing squash and stretch through manually animating the scaling of joints that are bound through the Smooth Bind Skin Deformer. For extra control, you can look at using control objects with constraints or a more complex setup using expressions to automate the scaling.
If you were scaling the joints manually to create squash and stretch, the process is fairly straightforward and can be applied to Disney’s Flour Sack animation test in 3D in Maya.
For squash: Scale the object downwards (typically in Y-axis) to compress, then scale the other two axes (typically the X and Z axes) outwards to maintain the volume in the shape.
For stretch: The joints scaling would be opposite to create the effect. Scale the object upwards (typically in the Y axis) to compress then scale the other two axes (typically the x and z axes) inwards to maintain the volume in the shape.
Maya Muscle – Squash & Stretch
The Maya muscle system is a full muscle setup and simulation system included with the software. It allows you to set up muscle objects for characters that act as influence objects and deform the character mesh.
Basics of the setup include creating and connecting the muscle objects to the character skeleton. Once set up, joint motion such as the curl of the forearm to meet the upper arm create a natural squash in the muscle, which mimics the flex on the muscle.
Once the weight influence is painted for the muscles on the character mesh, you can get some really nice, convincing squash and stretch deformations. The nice thing about the setup is that, because it’s a full dynamics system, the muscles react to real-world forces to create the effect—so it’s automated. The system also includes additional control through the muscle attributes for advanced effects including jiggle and sliding on the muscles.
Lee Montgomery is currently a Senior Technical Support Specialist at Autodesk. He’s worked within the 3D animation and VFX industry for over 9 years. Prior to joining Autodesk, he was a Senior Animator and Artist on a number of AAA video game titles for well-known studios. His projects have included Grand Theft Auto and Manhunt for Rockstar North. Lee is currently at work on a Focal Press book on applying the rules of traditional animation in Maya.