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

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
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
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/director/bezier/
http://www.geocities.com/cocoo_chen/