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Non-Photorealistic Rendering

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

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  1. Non-Photorealistic Rendering Mike Wade April 22, 1999

  2. What is Non-Photorealistic Rendering? • Produces images which are non-photorealistic • “NPR”

  3. 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

  4. How? • Pen and Ink • Toon shading • Painterly Rendering

  5. Paint By Numbers:Abstract Image Representations Paul Haeberli SIGGRAPH 1990

  6. Goal • 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

  7. 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

  8. Painterly Rendering for Animation Barbara J. Meier SIGGRAPH 1996

  9. Goal • Eliminate “shower door” • but not too much

  10. “Particle System Approach” • Particle set represents geometry of a surface • Particles are rendered as 2d brush strokes

  11. Generating Particle Set • Surface is known • Decompose into triangles that represent surface • Randomly distribute particles into triangles

  12. Particles Rendered Painterly • Based upon particle: • orientation • color • size • position • Transformed to screen space • Furthest from viewpoint rendered first

  13. Animation • Keep track of where the particles move • Draw each frame as above

  14. Maintaining Hand-drawedness • Use randomness • perturb brush stroke attributes • user defined • use same randomness for each frame for each particle in animation

  15. A brief interlude of fun

  16. Processing Images and Video for An Impressionistic Effect Peter Litwinowicz SIGGRAPH 1997

  17. Goal • Automatically generate impressionistic animations from video

  18. Generating Strokes • Strokes have: • position • length • radius • orientation • color

  19. Injecting randomness • Used to create hand-drawn look • All previous stroke attributes may be slightly perturbed • Also: order drawn • User-defined limits

  20. 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

  21. Orientation • May use a “fixed” orientation for stroke • Or: Angle lines normal to gradient of intensity image • Be careful with small gradients

  22. Rendering Video • First frame: as described above • Optical flow is used to find subsequent stroke centers

  23. 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”

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