CS 395: Adv. Computer Graphics

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CS 395 Adv. Computer Graphics

• Overview
• Surface Detail Maps
• Watt Chapter 8 readings
• Due Date Reminders Feb 4 - project
  proposal, or Feb 4 - reference list for
  survey paper 
• Jack Tumblin
• jet_at_cs.northwestern.edu

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Basic Problem

• Big problem even simple objects are not...

Image Credit Pixar/Renderman 1984
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Basic Problem

• Big problem even simple objects are not...
• Practical ways to add accurate local details?

Image Credit Pixar/Renderman 1984
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Surface Detail Mapping

• Basic Idea
• Simple polygons for primary 3D shape info
• Texture image secondary 2D/3D details
  stored in 2D
• Quickly paint 2D details onto simple
  polygons
• Hardware support now commonplace
• Texture
• Mapping...

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Digital Image a 2D Grid of Numbers

• NO intrinsic meaninguse it for anything
• reflectance, transparency, illumination, normal
  direction, material, velocity...

v
v
u
u
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Texture Map Bilinear Interpolation
v
Image Credit Nate Robins
u
Basic ideas Inverse mapping for each image
pixel (x,y) find texture address (u,v) get
value (may skip texels!) Forward mapping for
each texel (u,v), find image address (x,y) put
value (may skip pixels!)
x
v
u
y
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Bilinear Interpolation 2 Problems...
Image Credit Nate Robins
Aliasing Errors (1 pixel maps to gt 1 texel) (can
be much worse!)
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Bilinear Interpolation 2 Problems...
Image Credit Nate Robins
Aliasing Errors (1 pixel maps to gt 1 texel) (can
be much worse!)
Blurring (1 pixel maps to lt 1 texel)
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MIP-map Filter (L.Williams, 1983)

• Inverse sampling find texture address from x,y
• Sample texture from a pyramid of images, each
  one ¼ size of the previous.
• Match transformed pixel spacing to texel spacing
• Good, but blurry nondirectional

Image Credit Nate Robins
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Bump Mapping (Blinn 1978)

• Texture maps diffuse reflectance no lighting
  or viewing dependence
• Flat appearance can we suggest shape?
• Bump map texel value ? surface normal
  changes
• Make bump map
• make a 2D height field in u,v space
• find, store ? surface normal vec. at each u,v
  texel
• Use bump map
• surface normal (phong shading, etc) polygon
  normal ? bump map normal
• variants use for lighting normal, embossing,
  EMBM, visibility self-shadowing(Nelson Max,
  1999)., etc.

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Bump Mapping

• Texels displace Surface Normals
• Many clever, simpler (cheaper) variants...

Image Credit NVidia Corp GeForce 3 Demo
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Bump Mapping (Blinn 1978)

• Works well everywhere except silhouettes
• No self-shadowing (but see Nelson Max99)
• Complex hardware reqd forced many early
  approximations (Embossing, EMBM, etc.)
• Displacement Maps True geometry mods
• Requires micropoly shaders(Renderman)
• VERY powerful high-end modeling tool (lighbulb
  filament and screw base)

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Displacement Maps, Shading Languages

• Famous early Pixar resultREYES?Renderman
• photo-based environment map
• dicing,micropolygons
• shader language
• displacement maps
• Stochasticantialiasing (with motion blur)

Mental Ray displacement shader 1998
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Environment Maps

• How can I use texture maps for shiny things that
  move in real time?

Answer make a texture map for the the incoming
light, not the surface itself
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Environment Maps

• World-in-a-box texel light src. at infinity
• Fast, simple, looks good on flying chrome logos
• Complex lighting environments in real time...
• Limits bad on flat surfaces, no
  interreflections, no self-shadowing

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Environment Map

• A Box of images
• Can use Photos or ray-tracing
• all 6 images use the same
  Center-of-Projection point
• one texel light from one direction

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Environment Maps

• Result
• Curved surfaces are best
• Flat surfacessame normal?same color

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OpenGL Spherical Environ. Maps

• More Efficient than cube maps
• texture map image of perfect sphere
  reflectorseen from infinity (orthographic)

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OpenGL Spherical Environ. Maps

• Example
• Mirror Ball-like image Env. Mapped result

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Light Maps

• Multi-texturing idea arbitrary texel-by-texel
  shading calcd from multiple texture maps

ReflectanceTexture
? Light Map (Illumination Texture)
Display texture
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Light Maps

• Multi-texturing idea pre-render special
  lighting effects

ReflectanceTexture
? Light Map (Illumination Texture)
Display texture
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Advanced Topics

• Continuum geometry vs texture, material
• microstructure of surface, self-shadowing, etc
  determines surface appearance
• Surface is scale-defined carpet?yarn?fiber?
• Surface Mappings for volumes
• Lengyel 2000 approximating fur, hair,...
• BTF synthesis, volumetric textures
• Generalized Surface Mapping
• Hypertextures, turbulence functions(Perlin85)
• Texture Synthesis on Surfaces (Turk2001,...)

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Turk Diffusion, Texture Synthesis
Greg Turk, Ga Tech

• Diffuse outwards from seed vectors to build a
  smooth (u,v) map
• Works on any shape (any topology)

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Lengyel Texture-based Fur, Hair

• Semi-transparent layers of hair images
• Fins for silhouette hairs
• Real-Time Display, movement

Jed Lengyel, MS Research SIGGRAPH2000
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Merging with Other Areas

• Image-Based Modeling and Rendering
• Real-time Visibility (billboardssee book)
• Texture Synthesis
• Procedural Modeling
• Even the image/geometry line is blurring
• if we put a vertex at each texel, is it still
  a texture map?
• if an image holds positions of a mesh?