November 18, 2003Doug JamesCarnegie Mellon Universityhttp://www.cs.cmu.edu/~djames/15-462/Fall03Pen-and-Ink IllustrationsPainterly RenderingCartoon ShadingTechnical IllustrationsPen-and-Ink IllustrationsPainterly RenderingCartoon ShadingTechnical IllustrationsNon-Photorealistic RenderingNon-Photorealistic Rendering15-462 Computer Graphics ILecture 2212/2/2003 15-462 Graphics I 2Goals of Computer GraphicsGoals of Computer Graphics• Traditional: Photorealism• Sometimes, we want more– Cartoons– Artistic expression in paint, pen-and-ink– Technical illustrations– Scientific visualization12/2/2003 15-462 Graphics I 3Non-Photorealistic RenderingNon-Photorealistic Rendering“A means of creating imagery that does not aspire to realism” - Stuart GreenDavid GaineyCassidy Curtis 199812/2/2003 15-462 Graphics I 4Some 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.12/2/2003 15-462 Graphics I 5Emergence of NPREmergence of NPR2D Paint (Pixel Oriented)Bitmap paint systems2D Paint (Brush Oriented)User intervention2D/2.5D Paint Post-ProcessingAutomatically generated fromaugmented images3D RenderersAutomatically generated based on 3D data3D Photorealistic RenderersTraditional ComputerGraphics12/2/2003 15-462 Graphics I 6OutlineOutline• Pen-and-Ink Illustrations• Painterly Rendering• Cartoon Shading• Technical Illustrations12/2/2003 15-462 Graphics I 7Pen-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 structure12/2/2003 15-462 Graphics I 8Pen-and-Ink ExamplesPen-and-Ink ExamplesWinkenbach and Salesin 199412/2/2003 15-462 Graphics I 9Rendering Polygonal SurfacesRendering Polygonal Surfaces3D Model LightingVisible PolygonsStroke ClippingOutlineDrawingCameraProceduralStroke TextureHow much 3D information do we preserve?12/2/2003 15-462 Graphics I 10Strokes 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 textures12/2/2003 15-462 Graphics I 11Stroke Texture ExamplesStroke Texture ExamplesWinkenbach and Salesin 199412/2/2003 15-462 Graphics I 12Prioritized 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 textures12/2/2003 15-462 Graphics I 13Stroke Texture OperationsStroke Texture OperationsScalingChanging Viewing Direction(Anisotropic)12/2/2003 15-462 Graphics I 14IndicationIndication• Selective addition of detail• Difficult to automate• User places detail segments interactively12/2/2003 15-462 Graphics I 15Indication ExampleIndication ExampleBold strokes indicate detail segmentsWith indicationWithout indication12/2/2003 15-462 Graphics I 16OutlinesOutlines• Boundary or interior outlines• Accented outlines for shadowing and relief• Dependence on viewing direction• Suggest shadow direction12/2/2003 15-462 Graphics I 17Rendering Parametric SurfacesRendering Parametric Surfaces• Stroke orientation and density– Place strokes along isoparameter lines– Choose density for desired tone– tone = width / spacinguv12/2/2003 15-462 Graphics I 18Stroke WidthStroke Width• Adjust stroke width retain uniform toneWinkenbach and Salesin 199612/2/2003 15-462 Graphics I 19Parametric Surface ExampleParametric Surface ExampleConstant-density hatchingSmooth shading with single lightLonger smoother strokes for glassEnvironment mappingUpdate reflection coefficientStandard rendering techniques are still important!12/2/2003 15-462 Graphics I 20Parametric Surface ExampleParametric Surface ExampleWinkenbach and Salesin 199612/2/2003 15-462 Graphics I 21Orientable TexturesOrientable Textures• Inputs– Grayscale image to specify desired tone– Direction field– Stroke character• Output– Stroke shadedimageSalisbury et al. 199712/2/2003 15-462 Graphics I 22Orientable Stroke Texture ExampleOrientable Stroke Texture ExampleSalisbury et al. 199712/2/2003 15-462 Graphics I 23Illustrating Smooth Surfaces[Hertzmann & Zorin, SIGGRAPH 2000]Illustrating Smooth Surfaces[Hertzmann & Zorin, SIGGRAPH 2000]12/2/2003 15-462 Graphics I 24Animating Traditional Pencil DrawingsAnimating Traditional Pencil DrawingsFrom SIGGRAPH 2003 course notes on NPR; Daniel Teece, Walt Disney Feature Animation12/2/2003 15-462 Graphics I 25Animating Traditional Pencil DrawingsAnimating Traditional Pencil DrawingsFrom SIGGRAPH 2003 course notes on NPR; Daniel Teece, Walt Disney Feature Animation12/2/2003 15-462 Graphics I 26OutlineOutline• Pen-and-Ink Illustrations• Painterly Rendering• Cartoon Shading• Technical Illustrations12/2/2003 15-462 Graphics I 27Painterly 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 controversy12/2/2003 15-462 Graphics I 28Physical Simulation ExamplePhysical Simulation ExampleCurtis et al. 1997, Computer Generated Watercolor12/2/2003 15-462 Graphics I 29Computer-Generated WatercolorComputer-Generated Watercolor• Complex physical phenomena for artistic effect• Build simple approximations• Paper generation as random height field• Simulated effects12/2/2003 15-462 Graphics I 30Fluid SimulationFluid Simulation• Use water velocity, viscosity, drag, pressure, pigment concentration, paper gradient• Paper saturation and capacity• Discretize and use cellular automata12/2/2003 15-462 Graphics I 31Interactive PaintingInteractive
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