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Non-Photorealistic Rendering. Mike Wade April 22, 1999. What is Non-Photorealistic Rendering?. Produces images which are non-photorealistic “NPR”. Why NPR?. Sometimes we don’t need/want photographs or rendered images Photographs, renderings might be too “good”

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non photorealistic rendering

Non-Photorealistic Rendering

Mike Wade

April 22, 1999

what is non photorealistic rendering
What is Non-Photorealistic Rendering?
  • Produces images which are non-photorealistic
  • “NPR”
why npr
Why NPR?
  • Sometimes we don’t need/want photographs or rendered images
  • Photographs, renderings might be too “good”
  • Hand-drawn/painted animations are more energetic
  • Pen and Ink
  • Toon shading
  • Painterly Rendering
  • Convert synthetic or natural scene into impressionistic image
  • Created an interactive painting program
  • Painting: an ordered list of brush strokes
  • User could define:
    • location, color, size, direction, shape
two approaches to animation
Two Approaches to Animation
  • Generate a new sequence of stroke attributes for each frame
    • Coherent?
  • Create single array of brush strokes
    • Move scene behind that array
    • “Shower Door” effect
painterly rendering for animation

Painterly Rendering for Animation

Barbara J. Meier


  • Eliminate “shower door”
    • but not too much
particle system approach
“Particle System Approach”
  • Particle set represents geometry of a surface
  • Particles are rendered as 2d brush strokes
generating particle set
Generating Particle Set
  • Surface is known
  • Decompose into triangles that represent surface
  • Randomly distribute particles into triangles
particles rendered painterly
Particles Rendered Painterly
  • Based upon particle:
    • orientation
    • color
    • size
    • position
      • Transformed to screen space
      • Furthest from viewpoint rendered first
  • Keep track of where the particles move
  • Draw each frame as above
maintaining hand drawedness
Maintaining Hand-drawedness
  • Use randomness
    • perturb brush stroke attributes
    • user defined
    • use same randomness for each frame for each particle in animation
  • Automatically generate impressionistic animations from video
generating strokes
Generating Strokes
  • Strokes have:
    • position
    • length
    • radius
    • orientation
    • color
injecting randomness
Injecting randomness
  • Used to create hand-drawn look
    • All previous stroke attributes may be slightly perturbed
    • Also: order drawn
    • User-defined limits
stroke clipping
Stroke Clipping
  • Used to show edges

1) Convert to grayscale, blur

2) Find the edges

3) Grow stroke from center until reaches an edge

  • Fall-off so the stroke is not perfect
  • May use a “fixed” orientation for stroke
  • Or: Angle lines normal to gradient of intensity image
    • Be careful with small gradients
rendering video
Rendering Video
  • First frame: as described above
  • Optical flow is used to find subsequent stroke centers
optical flow
Optical Flow
  • Avoid sparseness in image
    • Add stroke centers
    • DeLaunay triangulation
    • Mix new strokes in with old
  • Avoid clumping
    • Remove strokes which are “too close”