NonPhotorealistic Rendering

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Non-Photorealistic Rendering

• MAJ Stevan J. French
• Naval Postgraduate School
• french_at_cs.nps.navy.mil

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Overview

• Definition
• Motivation
• NPR Programs
• Conclusions
• References
• Questions

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Definition

• NPR is the deliberate attempt to make complex
  graphics look like they are drawn by hand.
• NPR is also known as Expressive Rendering.
• This is because NPR can express a complicated
  object with much less detail.

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Motivation 1 of 2

• Computer Graphics are great for precision
  rendering
• But sometimes we dont want precision!

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Motivation 2 of 2

• To show a work in progress. A hasty appearance
  can signify that the idea is fresh.
• To make the displayed graphics appear more
  user-friendly.
• To simulate the type of natural media that
  artists use (pencil, charcoal, painting, etc).
• To show more graphics in less time (multiple
  objects, animation, etc.).

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NPR Programs

• SKETCH
• Orientable, Image-Based Pen Ink Illustrating
  (OIBPII)
• Real-Time NPR
• Teddy
• Art-based NPR
• View-Dependent Geometry (VDG)

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SKETCH 1 of 4

• Created by Robert Zeleznik, Director of User
  Interface Research _at_ Brown University.
• Sketch provides an interface for drawing 3D
  scenes with 2D tools.
• Designed to bridge the gap between hand sketches
  and computer-based modeling programs.

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SKETCH 2 of 4

• People normally initiate designs with paper a
  pencil instead of a computer for many reasons
• Ease of use.
• Precision is not required with a pencil!
• Low cost, etc.
• Sketch incorporates these features with those of
  CAD systems to approximate 3D polyhedral modeling.

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SKETCH 3 of 4

• The user simply sketches 2D primitives onto the
  computer instead of the paper.
• SKETCH then instantiates the corresponding 3D
  primitive into the 3D scene.
• SKETCH uses an automatic grouping mechanism to
  make it easier to transform aggregates of
  geometry.

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SKETCH 4 of 4

• The set of geometric primitives is more
  restricted than those in most CAD systems.
• The user must therefore approximate complex
  shapes with aggregates of simpler primitives.
• Using NPR, SKETCH allows users to see their
  concepts much quicker than traditional CAD
  programs.

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Sketch Example
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Sketch Example

• The final product is a 3D representation which
  can be looked at from all angles!

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OIB Pen Ink Illustrating 1 of 5

• OIBPII was created by Michael Salisbury and
  assoc. at the University of Washington.
• OIBPII was designed to abstract away detail,
  clarify shapes, and focus attention on specific
  points.
• OIPBII creates an illustration from a
  geometric-based referenced image.

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OIB Pen Ink Illustrating 2 of 5

• Each illustration gets three defined components
• A grey scale --- defines the tone at every
  point
• A direction field --- defines the desired
  orientation of texture
• A stroke example set --- fills the area with
  examples

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OIB Pen Ink Illustrating 3 of 5

• OIBPII drawing can easily be resized.
• The components and scale create an orientable
  texture that forms the surfaces in the scene.
• Several layers of imaging can be emplaced upon
  each other to give an overlapping appearance to
  the illustration.
• The final result is quicker rendering than
  traditional CAD.

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OIB Pen Ink Illustrating 4 of 5

• Advantages over geometry-based systems include
• Reduces task of geometric modeling.
• Dont have to specify surface reflectance
  properties.
• Can use any type of image input.
• More direct user control in manipulating tone,
  texture, or sketched line orientation.

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OIB Pen Ink Illustrating 5 of 5

• OIBPII also works well with images as
  complicated as the human face and/or furry
  creatures.

Prof. Sam Buttrey ?
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Real-Time NPR 1 of 4

• Created by Lee Markosian and associates and
  presented at SIGGRAPH 97.
• Motivation was the fact that NPR was primarily a
  batch-oriented process.
• Sought to convert NPR into real-time.

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Real-Time NPR 2 of 4

• Markosian was able to determine visible lines and
  surfaces by improving upon Appels hidden-line
  algorithm.
• This new method created
• an acceptable alternative to wire-frame
  rendering.
• An acceptable frame rate.

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Real-Time NPR 3 of 4

• Markosians point was that there are times when
  an artist does not want photograph quality
  graphics --- it is a question of intent.
• If a designer doesnt want complex graphics, he
  shouldnt have to pay for it (in processing
  time).

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Real-Time NPR 4 of 4

• The key concept is Economy of Line (EOL).
• EOL is the idea that a great deal of information
  can be conveyed by very few strokes.
• The algorithm is thus preferential towards
  silhouettes, key features, and shading.
• Shading strokes are used to help represent the
  depth of images.

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Teddy 1 of 5

• Was created by Takeo Igarashi and presented at
  SIGGAPH 99.
• Teddy was inspired by SKETCH, extending its
  concepts.

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Teddy 2 of 5

• Teddy is a sketching interface for quickly and
  easily designing freeform models such as stuffed
  animals and other rotund objects.

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Teddy 3 of 5

• The user draws several 2D freeform strokes
  interactively on the screen and the system
  automatically constructs plausible 3D polygon
  surfaces.
• The user doesnt have to manipulate control
  points or combine complicated editing operations.

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Teddy 4 of 5

• Like SKETCH, Teddy is designed for the rapid
  construction of approximation models (not for the
  careful editing of precise models).
• The emphasis is on encouraging creative
  exploration.

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Teddy 5 of 5

• Teddy can be downloaded off the web and
  immediately used to create objects.
• I will email everyone the web site later today.
• This version of teddy uses a mouse to manipulate
  the created objects.
• There is an online tutorial at the web site.

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Art-Based NPR 1 of 4

• This is another NPR project by Lee Markosian and
  his friends at Brown University.
• The idea is that techniques used by
  artists/illustrators can be applied to increase
  the expressive power of 3D computer graphics.

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Art-Based NPR 2 of 4

• In order to create fuzz, fur, or grass, an artist
  must create the impression of free-form shapes.
• This is difficult to do with conventional 3D
  modeling systems.
• However, it can be done efficiently by adding
  graftal textures.

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Art-Based NPR 3 of 4

• The overall affect is a less complex image with
  as approximately the same amount of perceived
  detail.
• There are two techniques for adding the graftal
  textures
• Alpha blending (continually phasing detail in or
  out).
• Fixed Placement (choosing what not to print).

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Art-Based NPR 4 of 4

• These techniques typically involve
• An overall reduction of visual detail.
• An enhancement of the most important details like
  facial features and object outlines.
• With careful stylization, just a few strokes can
  be made to evoke a sense of complex geometry like
  leafy bushes or flowing, glossy hair.

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Art-Based NPR Example
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VD Geometry 1 of 4

• VDG was created by Paul Rademacher at the
  University of North Carolina at Chapel Hill.
• VDG is designed to be utilized in cartoon
  animation, to deal with artistic distortions.
• Different models of the same character often
  conflict with each other.
• VDG ensures that the presented model continually
  changes IAW the current viewpoint.

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VD Geometry 2 of 4

• The below rabbits ears change directions as his
  head turns.
• This affect is next to impossible to capture in
  traditional CAD.
• This is where VDG steps in!

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VD Geometry 3 of 4

• The user creates traditional models of the most
  common viewpoints.
• VDG then creates deformed models for all
  additional key viewpoints.
• As the animated objects moves, the software
  continually updates its representation based upon
  the current view (taking the 3 closest models,
  and merging them together).

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V-D Geometry 4 of 4

• The result is that the VDG model captures more of
  the intended artistic distortions that
  traditional CAD.

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Conclusion

• NPR is a great technique for either adding needed
  detail, or reducing processor time.
• Now go play with Teddy!

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References

• SIGGRAPH 97 99.
• www.cs.washington.edu
• www.cs.brown.edu
• www.mtl.t.u-tokyo.ac.jp/takeo
• www.cs.unc.edu

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Questions ?