Lighting and Shading - University of Calgary in...
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1 Lighting and Shading • Adding colour • Combine Shading - recap r v r K L n l K L K I sr dr dr ar ar r a ) ( ) ( = r L b v r K L n l K L K I sb db db ab ab b a ) ( ) ( = g v r K L n l K L K I sg dg dg ag ag g a ) ( ) ( = g I g L b I b L 1 L b g r I I I r I
Transcript of Lighting and Shading - University of Calgary in...
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Lighting and Shading
• Adding colour
• Combine
Shading - recap
rvrKLnlKLKI srdrdrararrα)()( ⋅+⋅+=
rL
bvrKLnlKLKI sbdbdbababbα)()( ⋅+⋅+=
gvrKLnlKLKI sgdgdgagaggα)()( ⋅+⋅+=
gIgL
bIbL
1L
bgr III ++
rI
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• Adding colour
• Combine
Shading - recap
rvrKLnlKLKI srdrdrararrα)()( ⋅+⋅+=
rL
bvrKLnlKLKI sbdbdbababbα)()( ⋅+⋅+=
gvrKLnlKLKI sgdgdgagaggα)()( ⋅+⋅+=
gIgL
bIbL
1L
bgr III ++
rI
Gouraud Shading (intensity interpolation)
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Phong Shading (normal vector interpolation)
• Normal vector interpolation along each edge and each scan line
Shading comparisonsFlat• single colour to each
polygon
• fast • facetted look
Phong• vector normal
interpolation• more accurate• more realistic highlights• much less Mach banding
• much more computation
Gouraud• intensity interpolation• smoother effect
• some mach banding• specular highlights
handled poorly• more computation
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Human Perception and Light
• Difference Signaling• Contrast vs. value information• Light value information vs. object information.
• cell has normal rate• light in centre excites • light in surrounds inhibits
• Colour and Lightness Constancy
Consequences
• Hermann Grid Illusion
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Consequences• Scintillating Grid
Consequences• Simultaneous Brightness Contrast
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Consequences• Grating Induction Effect
Consequences• Mach Banding
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Consequences• Chevreul Illusion
Consequences• Crispening
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Consequences• Craik-Cornsweet Effect
Consequences• Craik-Cornsweet Effect
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Seurat: The Bathers• Craik-Cornsweet Effect
Shading Comparisons
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More on Phong shading
Problems with interpolation methods• Expensive: since illumination calculation is repeated for
every pixel. Also calculating and normalising new normal. (Schlick equation and table look up methods).
• Silhouette edge: Attempts have been made to modify normals to match a curve fitted to the edge. (see Van Overveld and Wyvill)
• Perspective distortion: Intensity interpolation is performed in screen space. The z has undergone a non-linear transformation.
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Problems with interpolation methods• Orientation Dependence: The results of an interpolation model are
not independent of the projected polygons orientation. Results may differ when the polygon is rotated affecting animations.
• Problems at Shared Vertices: This problem is analagous to the cracks problem in subdivided meshes. The undivided edge is interpolated differently to the two divided edges.
• Unrepresentative Vertex Normals: Pathalogical cases where interpolated normals give poor results. Examples are staircase example and mobius strip.
![Lighting and Shading - Jernej Barbicbarbic.usc.edu/cs420-s20/11-lighting/11-lighting-6up.pdfLighting and Shading Light Sources PhongIllumination Model Normal Vectors [Angel Ch. 5]](https://static.fdocuments.in/doc/165x107/5f5e0385f74eea5569795372/lighting-and-shading-jernej-lighting-and-shading-light-sources-phongillumination.jpg)
