Non-Photorealistic Rendering
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Transcript of 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”
• Hand-drawn/painted animations are more energetic
How?
• Pen and Ink
• Toon shading
• Painterly Rendering
Paint By Numbers:Abstract Image Representations
Paul Haeberli
SIGGRAPH 1990
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
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
Barbara J. Meier
SIGGRAPH 1996
Goal
• Eliminate “shower door”– but not too much
“Particle System Approach”
• Particle set represents geometry of a surface
• Particles are rendered as 2d brush strokes
Generating Particle Set
• Surface is known
• Decompose into triangles that represent surface
• Randomly distribute particles into triangles
Particles Rendered Painterly
• Based upon particle:– orientation– color– size– position
• Transformed to screen space
• Furthest from viewpoint rendered first
Animation
• Keep track of where the particles move
• Draw each frame as above
Maintaining Hand-drawedness
• Use randomness– perturb brush stroke attributes– user defined– use same randomness for each frame for each
particle in animation
A brief interlude of fun
Processing Images and Video for An Impressionistic Effect
Peter Litwinowicz
SIGGRAPH 1997
Goal
• Automatically generate impressionistic animations from videovideo
Generating Strokes
• Strokes have:– position– length– radius– orientation– color
Injecting randomness
• Used to create hand-drawn look– All previous stroke attributes may be slightly
perturbed– Also: order drawn– User-defined limits
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
Orientation
• May use a “fixed” orientation for stroke
• Or: Angle lines normal to gradient of intensity image– Be careful with small gradients
Rendering Video
• First frame: as described above
• Optical flow is used to find subsequent stroke centers
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”