ArtBased Rendering of Fur, Grass, and Trees

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Art-Based Rendering of Fur, Grass, and Trees
CS780 Seminar

• Michael A. Kowalski Lee Markosian J.D.
  Northrup Lubomir Bourdev Ronen Barzel
  Loring S. Holden Jhon F. Hughes

Presentation by Kim Jae-seop
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Outline

• Introduction
• Related Work
• Software Framwork
• Graftal Textures
• Results
• Future Work
• Conclusions

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Introduction

• Effective communication by an artist
• Free-form shapes
• A few well-chosen strokes

The Lorax
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Introduction

• Mimic the artists
• Three goals
• Style of rendering
• Modeling burden
• Interframe coherence
• Procedural stroke-based textures

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Related Work

• Particle systems
• Reeves 1983
• Reeves and Blau 1985
• Graftals
• Smith 1984

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Related Work

• Stroke placing algorithm
• Salisbury at al. 1997

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Related Work

• Particle-based brush strokes
• Meier 1996

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Software Framework

• OpenGL
• Patches, textures
• Reference images
• Color reference image
• Texture placement
• ID reference image
• Texture position
• Visibility check

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Graftal Textures

• Fur, leaves, grass,
• Procedural placement
• At silhouettes
• Stick to the surface for interframe coherence

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Placing Graftals with DIA

• Difference image algorithm Salisbury at al.
  1997
• To match the stroke density with the desired
  gray tone

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Placing Graftals with DIA

• Modified DIA
• To control the graftal texture density

Updated
New graftal
Maximum desire (Next graftal position)
Previous difference image
Current rendering
Difference image
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Placing Graftals with DIA

• Interframe coherence
• Make the graftals stick to object surface
• Obtain 3D position by ID reference image and ray
  test
• Attempt to place graftals on the same 3D
  position
• Reject invalid ones
• Place new graftals by DIA

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Subtracting the blurred image

• Whats the blurred image of graftals?
• Gaussian dot
• Volume ? graftals screen space area

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Subtracting the blurred image

• Zooming away
• Realistic stroke would be too small on distant
  object

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Subtracting the blurred image

• Compensatory scaling
• Convert object-space length L to screen space
  length s
• Multiply weight r
• What is r?
• d a graftals desired screen space length
• v0 corresponding volume
• rd/s ? length d in every frame
• r1 ? realistic
• rw(d/s)(1-w)
• Scaled volume v
• v v0 rs2 d2
• vv0(rs/d)2

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Subtracting the blurred image

• The volume to be subtracted
• Find 2D Gaussian function such that
• Subtract g(x-x0) from the difference image

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Subtracting the blurred image

• Is the graftal really desired?

Full detail
Reduced detail
Reject
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Details of fur graftals
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Details of fur graftals

• Orientation
• Face to camera

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Results
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Results
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Results
Movie clip
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Future Work

• Handling flickering and poping
• Fading and alpha blending
• Graftals on the back face
• Organizing levels of detail with static graftals

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Conclusions

• Expressiveness
• Interactive frame rate (several frame/s)
• Temporal coherence
• There are still many problems.