Non-Photorealistic Renderingdjames/15-462/Fall03/notes/22-npr.pdfNon-Photorealistic Rendering 15-462...
Transcript of Non-Photorealistic Renderingdjames/15-462/Fall03/notes/22-npr.pdfNon-Photorealistic Rendering 15-462...
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November 18, 2003Doug JamesCarnegie Mellon University
http://www.cs.cmu.edu/~djames/15-462/Fall03
Pen-and-Ink IllustrationsPainterly RenderingCartoon ShadingTechnical Illustrations
Pen-and-Ink IllustrationsPainterly RenderingCartoon ShadingTechnical Illustrations
Non-Photorealistic RenderingNon-Photorealistic Rendering
15-462 Computer Graphics I
Lecture 22
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12/2/2003 15-462 Graphics I 2
Goals of Computer GraphicsGoals of Computer Graphics
• Traditional: Photorealism
• Sometimes, we want more– Cartoons– Artistic expression in paint, pen-and-ink– Technical illustrations– Scientific visualization
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12/2/2003 15-462 Graphics I 3
Non-Photorealistic RenderingNon-Photorealistic Rendering
“A means of creating imagery that does not aspire to realism” - Stuart Green
David GaineyCassidy Curtis 1998
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12/2/2003 15-462 Graphics I 4
Some NPR CategoriesSome NPR Categories
• Pen-and-Ink illustration– Techniques: cross-hatching, outlines, line art,etc.
• Painterly rendering– Styles: impressionist, expressionist, pointilist, etc.
• Cartoons– Effects: cartoon shading, distortion, etc.
• Technical illustrations– Characteristics: Matte shading, edge lines, etc.
• Scientific visualization– Methods: splatting, hedgehogs, etc.
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12/2/2003 15-462 Graphics I 5
Emergence of NPREmergence of NPR
2D Paint (Pixel Oriented)Bitmap paint systems
2D Paint (Brush Oriented)User intervention
2D/2.5D Paint Post-ProcessingAutomatically generated from
augmented images
3D RenderersAutomatically generated
based on 3D data
3D Photorealistic RenderersTraditional Computer
Graphics
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12/2/2003 15-462 Graphics I 6
OutlineOutline
• Pen-and-Ink Illustrations
• Painterly Rendering• Cartoon Shading• Technical Illustrations
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12/2/2003 15-462 Graphics I 7
Pen-and-Ink IllustrationsPen-and-Ink Illustrations
• Strokes– Curved lines of varying thickness and density
• Texture– Character conveyed by collection of strokes
• Tone– Perceived gray level across image or segment
• Outline– Boundary lines that disambiguate structure
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12/2/2003 15-462 Graphics I 8
Pen-and-Ink ExamplesPen-and-Ink Examples
Winkenbach and Salesin 1994
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12/2/2003 15-462 Graphics I 9
Rendering Polygonal SurfacesRendering Polygonal Surfaces
3D Model Lighting
Visible Polygons
Stroke Clipping
OutlineDrawing
Camera
ProceduralStroke Texture
How much 3D information do we preserve?
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12/2/2003 15-462 Graphics I 10
Strokes and Stroke TexturesStrokes and Stroke Textures
• Stroke generated by moving along straight path
• Stroke perturbed by– Waviness function (straightness)– Pressure function (thickness)
• Collected in stroke textures– Tone dependent– Resolution dependent– Orientation dependent
• How automatic are stroke textures
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12/2/2003 15-462 Graphics I 11
Stroke Texture ExamplesStroke Texture Examples
Winkenbach and Salesin 1994
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12/2/2003 15-462 Graphics I 12
Prioritized Stroke TexturesPrioritized Stroke Textures
• Technique for limiting human intervention
• Collection of strokes with associated priority• When rendering
– First draw highest priority only– If too light, draw next highest priority, etc.– Stop if proper tone is achieved
• Procedural stroke textures
• Support scaling• Also applies to non-procedural stroke textures
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12/2/2003 15-462 Graphics I 13
Stroke Texture OperationsStroke Texture OperationsScaling
Changing Viewing Direction(Anisotropic)
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12/2/2003 15-462 Graphics I 14
IndicationIndication
• Selective addition of detail
• Difficult to automate• User places detail segments interactively
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12/2/2003 15-462 Graphics I 15
Indication ExampleIndication ExampleBold strokes indicate detail segments
With indication Without indication
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12/2/2003 15-462 Graphics I 16
OutlinesOutlines
• Boundary or interior outlines
• Accented outlines for shadowing and relief• Dependence on viewing direction• Suggest shadow direction
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12/2/2003 15-462 Graphics I 17
Rendering Parametric SurfacesRendering Parametric Surfaces
• Stroke orientation and density– Place strokes along isoparameter lines– Choose density for desired tone– tone = width / spacing
u
v
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12/2/2003 15-462 Graphics I 18
Stroke WidthStroke Width
• Adjust stroke width retain uniform tone
Winkenbach and Salesin 1996
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12/2/2003 15-462 Graphics I 19
Parametric Surface ExampleParametric Surface Example
Constant-density hatching
Smooth shading with single light
Longer smoother strokes for glass
Environment mapping
Update reflection coefficient
Standard rendering techniques are still important!
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12/2/2003 15-462 Graphics I 20
Parametric Surface ExampleParametric Surface Example
Winkenbach and Salesin 1996
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12/2/2003 15-462 Graphics I 21
Orientable TexturesOrientable Textures
• Inputs– Grayscale image to specify desired tone– Direction field– Stroke character
• Output– Stroke shaded
image
Salisbury et al. 1997
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12/2/2003 15-462 Graphics I 22
Orientable Stroke Texture ExampleOrientable Stroke Texture Example
Salisbury et al. 1997
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12/2/2003 15-462 Graphics I 23
Illustrating Smooth Surfaces[Hertzmann & Zorin, SIGGRAPH 2000]Illustrating Smooth Surfaces[Hertzmann & Zorin, SIGGRAPH 2000]
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12/2/2003 15-462 Graphics I 24
Animating Traditional Pencil DrawingsAnimating Traditional Pencil Drawings
From SIGGRAPH 2003 course notes on NPR; Daniel Teece, Walt Disney Feature Animation
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12/2/2003 15-462 Graphics I 25
Animating Traditional Pencil DrawingsAnimating Traditional Pencil Drawings
From SIGGRAPH 2003 course notes on NPR; Daniel Teece, Walt Disney Feature Animation
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12/2/2003 15-462 Graphics I 26
OutlineOutline
• Pen-and-Ink Illustrations
• Painterly Rendering• Cartoon Shading• Technical Illustrations
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12/2/2003 15-462 Graphics I 27
Painterly RenderingPainterly Rendering
• Physical simulation– User applies brushstrokes– Computer simulates media
• Automatic painting– User provides input image or 3D model– User specifies painting parameters– Computer generates all strokes
• Subject to controversy
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12/2/2003 15-462 Graphics I 28
Physical Simulation ExamplePhysical Simulation Example
Curtis et al. 1997, Computer Generated Watercolor
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12/2/2003 15-462 Graphics I 29
Computer-Generated WatercolorComputer-Generated Watercolor
• Complex physical phenomena for artistic effect
• Build simple approximations• Paper generation as random height field
• Simulated effects
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12/2/2003 15-462 Graphics I 30
Fluid SimulationFluid Simulation
• Use water velocity, viscosity, drag, pressure, pigment concentration, paper gradient
• Paper saturation and capacity
• Discretize and use cellular automata
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12/2/2003 15-462 Graphics I 31
Interactive PaintingInteractive Painting
��������
Simulationin progress
Finished painting
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12/2/2003 15-462 Graphics I 32
Automatic Painting ExampleAutomatic Painting Example
Hertzmann 1997
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12/2/2003 15-462 Graphics I 33
Automatic Painting from ImagesAutomatic Painting from Images
• Start from color image: no 3D information
• Paint in resolution-based layers– Blur to current resolution– Select brush based on current resolution– Find area of largest error compared to real image– Place stroke– Increase resolution and repeat
• Layers are painted coarse-to-fine
• Styles controled by parameters
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12/2/2003 15-462 Graphics I 34
Layered PaintingLayered Painting
Blurring
Adding detail with smaller strokes
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12/2/2003 15-462 Graphics I 35
Brush StrokesBrush Strokes
• Start at point of maximal error– Calculate difference between original image and
image painted so far
• Direction perpendicular to gradient– Stroke tends to follow equally shaded area
• Stopping criteria– Difference between brush color and original image
color exceeds threshold– Maximal stroke length reached
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12/2/2003 15-462 Graphics I 36
Longer Brush StrokesLonger Brush Strokes
• For longer, curved brush strokes– Repeat straight line algorithm– Stop, again on length or difference threshold
• Use anti-aliased cubic B-spline
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12/2/2003 15-462 Graphics I 37
Painting StylesPainting Styles
• Style determined by parameters– Approximation threshold– Brush sizes– Curvature filter– Blur factor– Minimum and maximum stroke lengths– Opacity– Grid size– Color jitter
• Encapsulate parameter settings as style
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12/2/2003 15-462 Graphics I 38
Some StylesSome Styles
• “Impressionist”– No random color, 4 � stroke length � 16
– Brush sizes 8, 4, 2; approximation threshold 100
• “Expressionist”– Random factor 0.5, 10 � stroke length � 16
– Brush sizes 8, 4, 2; approximation threshold 50
• “Pointilist”– Random factor ~0.75, 0 � stroke length � 0
– Brush sizes 4, 2; approximation threshold 100
• Not convincing to artists
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12/2/2003 15-462 Graphics I 39
Style ExamplesStyle Examples
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12/2/2003 15-462 Graphics I 40
OutlineOutline
• Pen-and-Ink Illustrations
• Painterly Rendering• Cartoon Shading• Technical Illustrations
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12/2/2003 15-462 Graphics I 41
Cartoon ShadingCartoon Shading
• Shading model in 2D cartoon– Use material color and shadow color– Present lighting cues, shape, and context
• Stylistic• Used in many animated movies• Developing real-time techniques for games
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12/2/2003 15-462 Graphics I 42
Cartoon Shading as Texture MapCartoon Shading as Texture Map
• Apply shading as 1D texture map
u=N·L
Carl Marshall 2000
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12/2/2003 15-462 Graphics I 43
Shading VariationsShading Variations
Flat shading Shadow Shadow + highlight
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12/2/2003 15-462 Graphics I 44
OutlineOutline
• Pen-and-Ink Illustrations
• Painterly Rendering• Cartoon Shading• Technical Illustrations
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12/2/2003 15-462 Graphics I 45
Technical IllustrationsTechnical Illustrations
• Level of abstraction– Accent important 3D properties– Dimish or eliminate extraneous details
• Do not represent reality
Photo
Ruppel 1995
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12/2/2003 15-462 Graphics I 46
Conventions in Technical IllustrationsConventions in Technical Illustrations
• Black edge lines
• Cool to warm shading colors• Single light source; shadows rarely used
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12/2/2003 15-462 Graphics I 47
Technical Illustration ExampleTechnical Illustration Example
Phong shadingMetal shading(anisotropic) Edge lines
Tone shading(cool to warm shift)
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Scientific VisualizationScientific Visualization
– Effective visualization of large, multidimensional datasets
Turk & Banks, “Image-Guided Streamline Placement,” SIGGRAPH 96
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• How to evaluate/define?• Smart graphics
– design from user’s perspective– with data?– HCI, AI, Perceptual studies
• Artistic graphics– beyond imitating– a way to create art work– how to assess?
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Art-Based Rendering of Fur, Grass and Trees[Kowalski et al., SIGGRAPH 99]Art-Based Rendering of Fur, Grass and Trees[Kowalski et al., SIGGRAPH 99]
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Art-Based Rendering of Fur, Grass and Trees[Kowalski et al., SIGGRAPH 99]Art-Based Rendering of Fur, Grass and Trees[Kowalski et al., SIGGRAPH 99]
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Doug DeCarlo, Anthony Santella. Stylization and Abstraction of Photographs In SIGGRAPH 2002.
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Image Analogies [Hertzmann et al., SIGGRAPH 2001]Image Analogies [Hertzmann et al., SIGGRAPH 2001]
?
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Image Analogies [Hertzmann et al., SIGGRAPH 2001]Image Analogies [Hertzmann et al., SIGGRAPH 2001]
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Image Analogies [Hertzmann et al., SIGGRAPH 2001]Image Analogies [Hertzmann et al., SIGGRAPH 2001]
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