CS 563 Advanced Topics in Computer Graphics The Wonderful World of Birds and Feathers by Brad Goodwin

Over-View  Introduction  Feather structure  Technique Using Bezier Curves  Technique Using L-system

Introduction  Mimic the complexity of nature  Two tasks  Model and render an individual feather (covered in all techniques)  Growing Feathers on birds (covered only by L-systems)

Feather Structure  Techniques use biological model to structure feather

Feather Types Four main types: A)Filoplume  Specialized B)Contour  Flying C)Plume  Barbs longer than rachis D)Semiplume  Contour/plume

First Technique  Only models a single feather  Bezier Curves  Parameters of Feather (biological)  Rendering

Bezier Curves Simple Math  Four points  Two endpoints  Two control points  Two equations  One for Y and one for X  All controlled by time t  If you need more than that check out: 

Defineing Bezier Curves  Bezier for both Vanes with 5 control points  Bezier for Rachis  Define a single barb with four points  P0, P3 on vane and rachis  Random (v1,v2)(v3,v4) determine P1, P2  Parameters that affect barbs

Parameters  N b Number of barbs for each vane (2000 in practice)

Parameters Continued  Fpv – Use to vary the length of the barbs  So 0 is no change and 1 is the largest change

Parameters Continued  F b -Used to control v1, v2, v3, v4 for each barb  So affects P1, P2 scaling the distance from P0 to P3

Boolean Parameters  S v -T or F symmetry of vanes  U f -T or F uniform barbs (same shape or dif)  U c -T or F modify length for each barb  If T then F pv isn’t modified for each barb  Is -(not boolean) allows the division of Rachis into two segments to create an irregular feather.

Rendering  Non-photorealistic  No texture map  Just a stick figure  Photorealistic  Texture map  Set up with map for both sides  Two parameters s,t  S is 0 to 1 along the barb  T is 0 at base and 1 at top

Samples

L-System Technique  Individual and growing  Mesostructure including barbules  Modeling and Rendering with BTF (Bidirectional Texture Function)

Remember  Techniques use biological model to structure feather

Shape interface control  Allows user a lot of control on the shape of the feather  1 -rachis; 2 -left and right barbs; 3 –texture; 4 -outline

Curve Illustration  O-Outline left and right  b-Barb left and right  r-Rachis All are controlled in the User interface

L-System Model  N – length of feather and density of Barbs  M L and M R – Define lengths of left and right Barbs  R(0) generates feather based on Rachis and barb curves  P1 generates first segment of rachis and two barbs  P2 generates left barb segment  P3 generates right barb segment

L-System Continued  This set up is modified to take into account variations in the feather  Example if a twig were to brush through the barbs  Little hooks called cilium that hold barbs  Good for fiying

Final L-System  Random force F e which if it exceeds threshold of F L or F R then the barb is rotated by some random degree theta

BTF  BTF – Bidirectional Texture Function  6 Dimensions  Viewing direction v  Lighting direction l at surface point (x, y)

BTF - Continued  Set on X axis so only 5D but renders just as well in most cases  Considers Opacity and difuse and Specular Relflections  Allows for detail up close  Can model Oil film interference and iridescence

An Entire Bird  Skeleton Structure

Growing Positions  Tried just at poly-mesh vertices and looked bad  Used turk’s algorithm to divide polygons

Growing Direction

Interpolate the rest

Adding Wing Feathers

Growth Collision

Recursive Collision Detection  Rotates until no collision and moves along the edge to a connected vertex

Questions???????

References  Chen Y et al, "Modeling and rendering of realistic feathers ", Proc. ACM SIGGRAPH 2002  Franco C.G. and Walter M., "Modeling and Rendering of Individual Feathers", Proc. SIBGRAPI 2002  Franco C.G. and Walter M., "Modeling the Structure of Feathers", Proc. SIBGRAPI 2001  or/bezier/ or/bezier/ 