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

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CS 563 Advanced Topics in Computer GraphicsThe
Wonderful World of Birds and Feathers

• by Brad Goodwin

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Over-View

• Introduction
• Feather structure
• Technique Using Bezier Curves
• Technique Using L-system

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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)

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Feather Structure

• Techniques use biological model to structure
  feather

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Feather Types

• Four main types
• Filoplume
• Specialized
• Contour
• Flying
• Plume
• Barbs longer than rachis
• Semiplume
• Contour/plume

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First Technique

• Only models a single feather
• Bezier Curves
• Parameters of Feather (biological)
• Rendering

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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/

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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

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Parameters

• Nb Number of barbs for each vane
• (2000 in practice)

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Parameters Continued

• Fpv Use to vary the length of the barbs
• So 0 is no change and 1 is the largest change

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Parameters Continued

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

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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 isnt modified for each barb
• Is -(not boolean) allows the division of Rachis
  into two segments to create an irregular feather.

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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

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Samples
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L-System Technique

• Individual and growing
• Mesostructure including barbules
• Modeling and Rendering with BTF
• (Bidirectional Texture Function)

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Remember

• Techniques use biological model to structure
  feather

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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

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Curve Illustration

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

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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

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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

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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

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BTF

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

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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

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An Entire Bird

• Skeleton Structure

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Growing Positions

• Tried just at poly-mesh vertices and looked bad
• Used turks algorithm to divide polygons

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Growing Direction
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Interpolate the rest
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Adding Wing Feathers
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Growth Collision
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Recursive Collision Detection

• Rotates until no collision and moves along the
  edge to a connected vertex

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Questions???????
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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/