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NPR Today. “Art-Based Rendering of Fur, Grass and Trees”, Michael A. Kowalski et. al., SIGGRAPH 99 “A Non-Photorealistic Lighting Model for Automatic Technical Illustration”, Amy Gooch, Bruce Gooch, Peter Shirley and Elaine Cohen, SIGGRAPH ’98. Art-Based Rendering of Fur, Grass and Trees.

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npr today
NPR Today
  • “Art-Based Rendering of Fur, Grass and Trees”, Michael A. Kowalski et. al., SIGGRAPH 99
  • “A Non-Photorealistic Lighting Model for Automatic Technical Illustration”, Amy Gooch, Bruce Gooch, Peter Shirley and Elaine Cohen, SIGGRAPH ’98

© 2005 University of Wisconsin

art based rendering of fur grass and trees

Art-Based Rendering of Fur, Grass and Trees

Michael A. Kowalski et. al.

Presented by Scott Finley

© 2005 University of Wisconsin

introduction
Introduction
  • Highly detailed objects containing fur, grass etc. are expensive to render.
  • This paper attempts to use a well known artistic technique to indicate complexity with simple shapes.

© 2005 University of Wisconsin

goals
Goals
  • Give the designer control over the style.
  • Simplify modeling by making rendering strategy an aspect of modeling.
  • Provide interframe coherence for the styles developed.

© 2005 University of Wisconsin

prior work
Prior Work
  • Reeves used particle systems to create complex geometry from simple shapes.
  • Alvy Ray Smith used particles and “graftals” to create his “Cartoon Tree”.
    • Badler and Glassner generalized the idea of graftals.
  • This paper uses a modified version of difference images proposed by Salisbury et al.

© 2005 University of Wisconsin

more prior work
More Prior Work
  • Meier’s particle-based brush strokes showed non-geometric complexity and fixed particle spacing on objects.

© 2005 University of Wisconsin

software framework
Software Framework
  • Models are broken into “patches”. Each is rendered by a procedural texture.
  • Two types of “reference images” are used:
    • Color reference image
    • ID reference image
      • Provides patches with list of pixels
      • Can be used to find visibility of known point

© 2005 University of Wisconsin

graftal textures
Graftal Textures
  • Place fur, leaves, grass etc. on geometric models.
  • Need to be drawn in a controlled way in screen space.
  • Need to stick to models for inter-frame coherence.

© 2005 University of Wisconsin

before
Before

© 2005 University of Wisconsin

after
After

© 2005 University of Wisconsin

difference image algorithm
Difference Image Algorithm
  • Each patch draws its region in the color reference image.
    • Darker areas indicate more “desire” for graftals to be placed.
    • In the examples here we want graftals along the silhouettes.
      • Render with point light at camera
      • Can be done explicitly by designer. (Bear’s feet)

© 2005 University of Wisconsin

difference image cont
Difference Image Cont.
  • Graftals are placed according to the desire in the color reference image.
    • This allows screen space density to be controlled.
  • Bin all the pixels according to the desire level and start placing graftals on the pixels with the highest desire.

© 2005 University of Wisconsin

creating inter frame coherence
Creating Inter-frame Coherence
  • Need to be sure that graftals persist across frames to avoid extreme noise etc.
    • In first frame place graftals according to DIA.
    • In further frames attempt place graftals from previous frame.
    • Place new graftals where needed according to the DIA.

© 2005 University of Wisconsin

subtracting blurred image
Subtracting Blurred Image
  • When a graftal is placed it subtracts a blurred “image” of itself from the reference image.
    • Graftals are treated as a point for this. The “image” is a Gaussian dot.
  • Size of the dot is proportional to the screen space area of the graftal.

© 2005 University of Wisconsin

graftal sizing
Graftal Sizing
  • Graftals can be set to scale according to perspective, have a constant size, or somewhere between.
  • Graftal size can be reduced if it tries to draw itself but there isn’t enough desire.

© 2005 University of Wisconsin

drawing graftals
Drawing Graftals
  • Fur graftals can be drawn at drawn at different details with triangle strips.
  • Drawing happens in surface normal plane.
    • Detail depends on angle to viewer

© 2005 University of Wisconsin

future work
Future Work
  • Reduce flicker/popping as graftals enter and leave.
    • Use alpha blending
    • Put graftals on the back of objects
    • Use several layers of statically placed graftals

© 2005 University of Wisconsin

new styles
New Styles
  • Dual layered fur
    • Suggests complex lighting

© 2005 University of Wisconsin

a non photorealistic lighting model for automatic technical illustration

A Non-PhotorealisticLighting Model for Automatic Technical Illustration

Amy Gooch, Bruce Gooch, Peter Shirley, Elaine CohenSIGGRAPH ’98

(presented by)Tom BrunetUniversity of Wisconsin-MadisonCS779

background
Background
  • Various NPR Techniques
    • Cassidy J. Curtis, Sean E. Anderson, Kurt W. Fleischer, and David H. Salesin. Computer-Generated Watercolor. In SIGGRAPH 97 Conference Proceedings, August 1997.
  • Technical-like
    • Takafumi Saito and Tokiichiro Takahashi. Comprehensible Rendering of 3D Shapes. In SIGGRAPH 90 Conference Proceedings, August 1990.
    • Doree Duncan Seligmann and Steven Feiner. Automated Generation of Intent-Based 3D Illustrations. In SIGGRAPH 91 Conference Proceedings, July 1991.
    • Debra Dooley and Michael F. Cohen. Automatic Illustration of 3D Geometric Models: Surfaces. IEEE Computer Graphics and Applications, 13(2):307-314, 1990.

© 2005 University of Wisconsin

contributions
Contributions
  • Reduction of dynamic range neededto portray shape
  • NPR method for appearance of metal

© 2005 University of Wisconsin

diffuse shading
Diffuse Shading

© 2005 University of Wisconsin

highlights and edges
Highlights and Edges

© 2005 University of Wisconsin

diffuse w edges highlights
Diffuse w/ Edges/Highlights

© 2005 University of Wisconsin

alter shading model
Alter Shading Model
  • Want to keep lighting from above
  • Extend shading across entire sphere:
  • Finally, mix a cool-warm hue shift with a luminance shift

© 2005 University of Wisconsin

near constant luminance
Near Constant Luminance

© 2005 University of Wisconsin

color luminance shift
Color & Luminance Shift

© 2005 University of Wisconsin

maintains color name
Maintains ‘Color Name’

© 2005 University of Wisconsin

metal appearance
Metal Appearance
  • Milling creates anisotropic reflection
  • Pick 20 strips of random intensity [0, .5]
  • Linearly interpolate

© 2005 University of Wisconsin

metallic anisotropic reflection
Metallic, Anisotropic Reflection

© 2005 University of Wisconsin

approximate in opengl
Approximate in OpenGL
  • Two opposing directional lights:
  • (kwarm - kcool)/2
  • (kcool - kwarm)/2
  • Ambient:(kcool + kwarm)/2

© 2005 University of Wisconsin

other results questions
Other Results/Questions

© 2005 University of Wisconsin

computer generated watercolor siggraph 1997

Computer Generated Watercolor(Siggraph 1997)

Cassidy J. Curtis

Sean E. Anderson

Joshua E. Seims

Kurt W. Fleischer

David H. Salesin

© 2005 University of Wisconsin

watercolor effects
Watercolor Effects
  • Drybrush
  • Edge Darkening
  • Backruns
  • Granulation
  • Flow Effects
  • Glazing

© 2005 University of Wisconsin

previous work
Previous Work
  • David Small. Simulating watercolor by modeling diffusion, pigment, and paper fibers. February 1991.
  • Qinglian Guo and T. L. Kunii. Modeling the diffuse painting of sumie. In T. L. Kunii, editor, IFIP Modeling in Comnputer Graphics. 1991.
  • Julie Dorsey and Pat Hanrahan. Modeling and rendering of metallic patinas. 1996.

© 2005 University of Wisconsin

improvements
Improvements
  • More complex paper model
  • Better compositing (KM)
  • Three layer simulation
    • Shallow-water layer
    • Pigment disposition layer
    • Capillary layer
  • Painting represented as layers

of wash (dried watercolor)

© 2005 University of Wisconsin

algorithm overview
Algorithm Overview

For each time step:

  • MoveWater
    • UpdateVelocities
    • RelaxDivergence
    • FlowOutward
  • MovePigment
  • TransferPigment
  • SimulateCapillaryFlow

© 2005 University of Wisconsin

algorithm
Algorithm

UpdateVelocities

  • Height gradient used to modify velocities
  • Simulate shallow water flow using Euler Method and standard flow equations
  • Velocity of pixels outside wet area mask are set to zero

RelaxDivergence

  • Distribute fluid to neighboring cells

© 2005 University of Wisconsin

algorithm1
Algorithm

FlowOutward

  • Remove water from each cell
  • p = p – n * (1 – M’) * M

MovePigment

  • Pigment distributed to neighboring cells

© 2005 University of Wisconsin

algorithm2
Algorithm

TransferPigment

  • Pigment is deposited or lifted
    • Density of pigmentation
    • Staining power
    • Granulation

SimulateCapillaryFlow

  • Transfer water from shallow water layer to capillary layer
  • Water is diffused to neighbors in the capillary layer
  • Wet area mask updated

© 2005 University of Wisconsin

rendering
Rendering
  • Layers combined using Kubelka-Munk method
  • Interactive pigment creation system
  • Supports various paint types
    • Opaque Paints
    • Transparent Paints
    • Interference Paints

© 2005 University of Wisconsin

rendering limitations
Rendering Limitations

Kubelka-Munk doesn't account for:

  • Media of different refractive indices
  • Uniformly oriented pigment particles
  • Illumination other than diffuse
  • Fluorescent paints
  • Chemical or electrical interaction between different pigments

Looks pretty good anyway…

© 2005 University of Wisconsin

applications
Applications
  • “Interactive” painter
  • Semi-automatic “watercolorization”
  • NPR rendering (“watercolorization” in post)

© 2005 University of Wisconsin

results
Results

© 2005 University of Wisconsin

results1
Results

© 2005 University of Wisconsin

future work1
Future Work
  • More effects
    • Spattering
    • Hairy brushes
    • Interaction with pen-and-ink
  • Fully automatic “watercolorization”
    • No manual masking
    • Find optimal palette
  • Generalization
    • Backruns and flow effects are really the same
  • Limit “shower door” effect in "watercolorized" animation.

© 2005 University of Wisconsin

questions
Questions?

© 2005 University of Wisconsin

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