제 6 주. 응용 -2: Graphics Evolving L-systems to Generate Virtual Creatures G.S. Hornby and J.B. Pollack, Computers & Graphics, vol. 25, pp. 1041~1048, 2001 학습목표 그래픽분야의 인공생명 응용에 대한 실제적인 구현 예를 이해 함으로써 총정리 Creature construction  L-system  GA

개요 4 Virtual creature 의 역할 –Special effects and background characters 4 Evolutionary algorithm –Reduce difficulty and time to specify morphologies and behaviors 4 Limitation of conventional approach –Scalability beyond creatures with 50 components –Unnatural looking 4 Proposed method –Lindenmayer systems as the encoding of EA to create virtual creatures –Create creatures with hundreds of parts and more natural looking

Introduction 4 Factors to limit the richness of virtual worlds –Computing power  design ability 4 Evolutionary algorithms –Promising in automating the process of producing creatures –Direct encoding  one-to-one mapping No re-use, asymmetries, non-regularities  unnatural looking 4 Solution  graph structure (like Karl Sims) –Generative encoding for creatures –Developmental method with a set of grammatical re-writing rules –Fractal-like self-similarities  organic look & better scalability –Lindenmayer systems 4 Evolving controllers for pre-specified morphologies –Stimulus-response rules, neural controllers, genetic programs  oscillator circuits : with frequency and relative phase offset  recurrent neural networks

Methods: Overview 4 Components to produce moving creatures –Evolutionary algorithms: to optimize creature design Stochastic search & optimization inspired by natural evolution Iteratively replacing poor members with individual generated by applying variation to good members of population –L-systems: encoding to represent creatures for optimization Set of grammatical rewriting rules to model biological development Iteratively applying the rules to create complex string from simpler ones Characters for L-system  set of construction commands –Creature constructor: to construct creatures from encoding Follow string of construction commands to build creature piece by piece Evaluate how well it moves  pass the score to EA

Creature Constructor 4 Morphology –Bars connected by either fixed or actuated joints –Construction commands: specify how and where to attach bars to the existing design  LOGO-style turtle 4 Command language –Push and pop operators: [, ] Current location, orientation, relative phase offset –Turn left/right/up/down/clockwise/counter-clockwise(n) –Forward(n), Backward(n) –Revolute-1(n), Revolute-2(n) –Twist-180(n) –{, } : {forward(1)} (3)  forward (1) forward (1) forward (1) 4 L-system specification  constructor  behavior evaluation (quasi- dynamic simulator)

Parametric 0L-systems 4 Purpose –Compactly describe more natural-looking structures –Overcome overly regular designs from L-systems  G = (V, , , P) –:,  : Predecessor : condition  successor –Restricted condition: comparisons as to whether a production parameter is greater than a constant value

Evolutionary Algorithm 4 Initialization –Blank template of a fixed number of production rules –Push and pop, block replication symbols –Initial population consists of a variety of different solutions whose fitness is above the preset threshold 4 Mutation –Replacing one command with a random command –Perturbing parameter of a command by adding/subtracting a small value to it –Changing parameter equation to a production –Adding/deleting a block of commands in a successor –Changing condition equation 4 Recombination

Evolved Creatures 4 Evaluation function –Distance moved by the creature’s center of mass –Penalty proportional to distance covered by points that dragged along the ground  stepping and rolling motions encouraged 4 Interesting results : Fig. 2 –Roll along sideways : (a), (b), (c), (g) –Undulating sea-serpent : (d) –Inch-worm : (e) –Flipping of the creature : (f) –Pushing a coil : (h)  larger creatures –Four legs in an awkward walk : (i)  larger creatures

Midterm Exam : Take Home 4 Design the virtual creatures with evolving L-systems 4 Due date : April 29 4 Collaborators : maximum 3