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: http://www.moshplant.com/direct-or/bezier/

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

Nb 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

Fb -Used to control v1, v2, v3, v4 for each barb So affects P1, P2 scaling the distance from P0 to P3

Boolean Parameters

Sv -T or F symmetry of vanes

Uf -T or F uniform barbs (same shape or dif)

Uc -T or F modify length for each barb If T then Fpv 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

ML and MR – 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 Fe which if it exceeds threshold of FL or FR 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

http://www.moshplant.com/direct-or/bezier/

http://www.geocities.com/cocoo_chen/