Lighting

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Lighting Visual Effects

• CSE 191A Seminar on Video Game Programming
• Lecture 8 Lighting Visual Effects
• UCSD, Spring, 2003
• Instructor Steve Rotenberg

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Lighting
3
Normals

• Normals are usually specified per vertex (rather
  than per polygon)
• Normals are usually set up entirely through an
  interactive modeling program and are just read in
  as is into the real time renderer
• A smooth vertex normal is usually computed as
  the average of the normals of the triangles using
  that vertex (offline)
• Consider that a cube has 8 vertices 6 normals,
  but requires 24 unique vertex/normal pairs
• Each vertex/normal pair should be uniquely lit

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Light-Surface Interaction

• L Light
• N Normal
• R Reflection
• H Halfway
• V Viewer
• T Transmission

N
L
R
H
V
T
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Diffuse Lighting

• An ideal diffuse surface reflects light uniformly
  in all directions
• Cr,g,b
• CfinalClightCdiffuse(NL)

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Light Types

• Ambient uniform light from all directions
• Directional light from a single direction
  (usually approximates a distant light source such
  as the sun)
• Point light emitting from a point source (like a
  light bulb). Point lights should obey the inverse
  square law II0/distance2
• Spot light emitting from a point source, but
  aiming in a particular direction cone

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Specular Lighting

• An ideal specular surface reflects incident light
  rays in one direction (a perfect mirror)
• A less-ideal specular surface may scatter rays in
  a cone around the mirror direction
• Specular surfaces can be approximated with the
  (old fashioned) Blinn (or Phong) models
• H(LV) (halfway vector) (normalize)
• CfinalClightCspecular(NH)shine
• Computing specular lighting at the vertices
  doesnt always look very good because the
  lighting can vary a large amount over a small
  distance

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Environment Mapping

• Environment mapping is a technique that uses a
  texture map to fake the appearance of a shiny
  surface.
• The environment map itself is a 360 degree view
  of the world viewed from the objects position.
• There are a variety of actually mapping
  techniques (polar, spherical, cube, dual
  paraboloid)
• The view vector is reflected off of the normal
  and then converted to a texture coordinate
• Polar environment map
• Vector reflection R-V2N(VN)
• Tx(atan2f(R.x,R.z)PI)/(2PI)
• Ty(R.y1)/2 -or- (asin(R.y)PI/2)/PI
• Sphere map
• NNMview
• Tx(N.x1)/2
• Ty(N.y1)/2

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Environment Mapping

• Environment maps can be rendered on the fly or
  can be precomputed
• Environment maps can be blurred to simulate
  glossy reflections

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BRDFs Global Illumination

• Bidirectional Reflectance Distribution Function
• BRDF ?(?i, fi,?r,fr,?)
• Real materials reflect light in complex ways
• Real light bounces around in complex ways
• There is some modern research on implementing
  accurate BRDFs and global illumination in real
  time, but these techniques are still a little out
  of reach for mainstream gaming

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Practical Real Time Lighting

• Precompute any static lighting when possible
• Turn point lights into directional lights (if
  possible)
• Ignore darker lights (not necessarily distant
  ones)
• Use environment mapping for specular lights
  (rather than per-vertex)
• Use projected textures for spot lights other
  custom projection shapes (rather than per-vertex)

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Shadows

• Drop shadows
• Polygonal projection
• Texture projection
• Stencil

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Precomputed Lighting

• Ideal diffuse light is view independent and so it
  can easily be precomputed and stored
• Possible effects include
• Diffuse lighting
• Complex light types (point, spot, area)
• Shadows (and soft shadows)
• Diffuse inter-reflection
• Techniques for precomputing global illumination
• Photon mapping
• Monte-Carlo path tracing
• Radiosity
• Dynamic light can be layered on top of
  precomputed light

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Alpha Blending
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Alpha

• Alpha is a generic name for an extra parameter
  that can treated as a fourth color component
  (i.e., rgba red, green, blue, alpha)
• Often, alpha is used to represent opacity in a
  01 range (opacity 1-transparency)
• Alpha can be specified per vertex and can also be
  specified per texel in a texture map
• The alpha blending function can be controlled
  through graphics API calls
• Different hardware systems tend to have radically
  different alpha blending capabilities

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Transparency

• Usually, for transparency to work, you must
  render polygons sorted from distant to near
• When a partially transparent pixel is rendered,
  the incoming color (source color) is blended with
  the existing pixel color (destination color)
• CfinalasrcCsrc(1-asrc)Cdest

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Additive Blending

• Incoming color is simply added to the existing
  color in the framebuffer
• Useful for lighting effects such as glows, lens
  flares, lighting bolts, plasma beams, etc.
• CfinalasrcCsrcCdest
• or even
• CfinalCsrcCdest

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Color Modulation

• Useful for colored lighting (either precomputed
  or dynamic projected lights)
• Cfinal CsrcCdest
• or sometimes
• Cfinal asrcCdest

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Source Destination Factors

• One traditional method of specifying alpha
  blending is the use of src and dest factors
• CfinalFsrcCsrcFdestCdest
• Where Fsrc and Fdest can be
• 0, 1, asrc, adest, (1- asrc), (1- adest), Csrc,
  Cdest, or others
• This leads to lots of possible blending
  functions. Only a small number of them are
  generally useful.

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Multipass Rendering

• In multipass rendering, a polygon is rendered
  several times (passes) to combine various effects
• Useful for several lighting material type
  effects.
• Example
• 1. Precomputed diffuse light (overwrite)
• 2. Projected shadow textured light (add)
• 3. Diffuse material texture (multiply)
• 4. Specular map (overwrite into alpha)
• 5. Environment map (CfinaladestCsrcCdest

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Multistage Rendering

• Same idea as multipass rendering, except the
  individual passes and blending is done internally
  and the final color is only written into the
  framebuffer once.
• Because combination isnt necessarily linear, you
  can potentially do more complex effects
• Number of stages may be limited (4 on XBox)
• You can still do multipass rendering with
  multistage rendering to get more passes

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Vertex Pixel Shaders

• Shaders are microprograms that can run per-vertex
  or per-pixel
• Different hardware supports radically different
  capabilities
• As graphics chips become more complex, pixel and
  vertex programs become more general purpose
• Stream architecture

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Effects
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Particles

• Useful for tons of visual effects
• Fire
• Smoke
• Water
• Dirt
• Debris, explosions
• Trash, leaves blowing around
• Usually, particles have only a position and no
  orientation info
• Particles are usually rendered as sprites/quads
  with alpha effects

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Particles

• class Particle
• Vector3 Position
• Vector3 Velocity
• Vector3 Force
• Vector4 Color
• float Mass
• float Radius
• int TexFrame

class ParticleSystem int ActiveParticles int
MaxParticles Particle Particles float
CreationRate Particle Mean Particle
Variance
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Fog

• Fog (or depth cueing) is an important visual
  feature that provides perception of depth
• Different hardware supports different fog
  features
• Linear, exponential, exp2
• Depth based vs. distance based

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Billboards

• Complex geometric objects can be approximated
  with simple cards or billboards (trees are a
  common example)

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Texture Movies

• Useful for fire, water surface, clouds, misc.
• Cheap and powerful effect, but may require a lot
  of texture memory
• Streaming texture movies

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Lens Effects

• Glows, blooms, stars
• Halos
• Lens flare
• Internal reflections, scattering

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Off-Screen Rendering

• Shadow maps
• Environment maps (cube map)
• Imposters
• Full-screen effects
• 2D distortion, ripple, heat wave, shockwave
• Color night vision, visual adaptation
• Predator effect, etc.
• Supersampling
• Focus (depth of field)
• Motion blur
• Issues
• Video memory
• State changes
• Pixel fill

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Conclusion
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Preview of Next Week

• Networking with guest speaker Mark Rotenberg,
  Technical Director for Midnight Club 2