Takatsugu Kuriyama and Takuro Wada (2009-2010)
(Special thanks: Takashi Sagisaka)
In many computer games and digital movies, physical simulation is applied to simulate bullet trajectory, collision, water flow and so on. As computers become faster, attractive but heavy computational simulations become practical. Soft body simulation is a good example. It simulates elasticity of soft bodies so that new CG expressions, such as deformed soft creatures and waving clothes, become available. SimJelly is a work where we can play with and intuitively understand the soft body simulation. SimJelly, deformed like a real jelly.
SimJelly is based on spring-damper model to simulate elasticity. In spring-damper model, a body is divided into tetrahedrons. Each edge of a tetrahedron is represented as a stick that extends and shrinks in a linear fashion, but it is never distorted. Each edge has a spring and damper so that the it gets original length by itself after it is deformed. Spring generates force that is propotional to displacement in opposite direction so that the stick tries to extend or shrink to the original length but oscillates because it never stops in the original length. Damper generates force that is proportional to exteding/shrinking speed in opposite direction so that the stick tries to stop extenting or shrinking. With both spring and damper, the stick can get the original length elastically.
In SimJelly, a cube is divided into 40 tetrahedrons. Each edge of a tetrahedron is replaced by overlapped thin and thick metal pipes so that the stick extends and shrinks in a linear fashion. Thin and thick pipes are connected with a metal spring so that it playes the spring role. The pipes are capped by plastic caps with a tiny hole. Air flows through this tiny hole of the caps and generates registant force when the stick extends or shrinks, so it plays the damper role. Then, SimJelly is a complete replacement of the spring-damper computational model.
SimJelly simulates soft body behavior in the real world. The crutial advantage of this work is that we can interact with by our hands and intuitively understand this CG technique. If you push down or pluck the vertices of SimJelly, it is deformed like a real Jelly. If you let SimJelly down, it bounces like a real jelly. You can play with SimJelly as you want. No limit.