Non-Photorealistic Rendering: Toon Shading David Luebke University of Virginia.
Surface shading: materials - University of California, San Diego · 2018. 1. 24. · Rendering...
Transcript of Surface shading: materials - University of California, San Diego · 2018. 1. 24. · Rendering...
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Surface shading: materials
Computer GraphicsCSE 167Lecture 5
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CSE 167: Computer Graphics
• Global illumination• Local illumination• Surface shading
– Materials– Lights
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Rendering Pipeline
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Modeling and viewingtransformation
Shading
Projection
Scan conversion,visibility
Scene data
Image
• Position object in 3D
• Map triangles to 2D
• Draw triangles
– Per pixel shading
• Determine colors of vertices
– Per vertex shading
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Appearance: lighting, surface reflectance, and shading
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Global illumination
• Photorealistic rendering using physics‐based simulation– Multiple bounces of light
• Direct (from light source) and indirect (from light reflected by other surfaces) illumination
• Computationally expensive (minutes per image)
• Used in movies, architectural design, etc.
CSE 167, Winter 2018 5Based on slides courtesy of Jurgen Schulze
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Global illumination
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Local illumination
• Not a physics‐based simulation• Uses simplified models that reproduce perceptually most important effects– One bounce of light
• Direct (from light source) illumination
– Indirect illumination with an “ambient” term
• Computationally very fast• Used in interactive applications
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Local illumination
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Local illumination
• Model reflection of light at surfaces– Assumption: no subsurface scattering
• Bidirectional reflectance distribution function (BRDF)– Given light direction, viewing direction, how much light is reflected towards the viewer
– For any pair of light/viewing directions!
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Local illumination
• Simplified model– Sum of 3 components– Covers a large class of real surfaces
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ambientdiffuse specular
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Local illumination
• Simplified model– Sum of 3 components– Covers a large class of real surfaces
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ambientdiffuse specular
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Diffuse reflection
• Ideal diffuse material reflects light equally in all directions
• View‐independent• Matte, not shiny materials
– Paper– Unfinished wood– Unpolished stone
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Diffuse reflection• Beam of parallel rays shining on a surface
– Area covered by beam varies with the angle between the beam and the normal– The larger the area, the less incident light per area– Incident light per unit area is proportional to the cosine of the angle between the
normal and the light rays
• Object darkens as normal turns away from light• Lambert’s cosine law (Johann Heinrich Lambert, 1760)• Diffuse surfaces are also called Lambertian surfaces
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nnn
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Diffuse reflection
• Given– Unit surface normal n– Unit light direction L– Material color (albedo) kd– Light color cl
• Reflected color cd is given by
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Do not allow angles less than 0 (light is behind back of surface)
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Diffuse reflection
• Notes– Material color (albedo) kd and light color cl are r,g,b vectors
– Need to compute r,g,b values of reflected color cdseparately
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Surface normals
• Important to bring out 3D appearance• Important for correct shading under lights
• The way shading is done also important– Flat: Each facet has single normal– Smooth: Each vertex has a single normal, interpolate normal vectors to determine normal at point on facet
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Flat
Smooth
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Diffuse reflection
• Provides visual cues– Surface curvature– Depth variation
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Lambertian (diffuse) sphere under different lighting directions
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Local illumination
• Simplified model– Sum of 3 components– Covers a large class of real surfaces
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ambientdiffuse specular
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Specular reflection
• Shiny surfaces– Polished metal– Glossy car finish– Plastics
• Specular highlight– Blurred reflection of the light source
– Position of highlight depends on viewing direction
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Specular highlight
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Specular reflection
• Ideal specular reflection is mirror reflection– Perfectly smooth surface– Incoming light ray is bounced in single direction– Angle of incidence equals angle of reflection
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Projection of vector on another vector
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Law of reflection
• Angle of incidence equals angle of reflection
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Specular reflection
• Many materials are not perfect mirrors– Glossy materials
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Glossy teapot
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Glossy materials• Assume surface composed of small mirrors with random
orientation (micro‐facets)• Smooth surfaces
– Micro‐facet normals close to surface normal– Sharp highlights
• Rough surfaces– Micro‐facet normals vary strongly– Blurry highlight
Polished
Smooth
Rough
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Glossy surfaces
• Expect most light to be reflected in mirror direction
• Because of micro‐facets, some light is reflected slightly off ideal reflection direction
• Reflection– Brightest when view vector is aligned with reflection
– Decreases as angle between view vector and reflection direction increases
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Phong shading model
• Developed by Bui Tuong Phong in 1973• Specular reflectance coefficient ks• Phong exponent p
– Greater pmeans smaller (sharper) highlight
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Phong shading model
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Local Illumination
• Simplified model– Sum of 3 components– Covers a large class of real surfaces
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ambientdiffuse specular
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Ambient light
• In real world, light is bounced all around scene• Could use global illumination techniques to simulate
• Simple approximation– Add constant ambient light at each point: kaca
• Ambient light color ca• Ambient reflection coefficient ka
• Areas with no direct illumination are not completely dark
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Complete Phong shading model
• Phong model supports multiple light sources• All light colors c and material coefficients k are 3‐component vectors for red, green, blue
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∙ ∙
Image by Brad Smith
Sum over all light sources