Advanced rendering techniques

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Advanced rendering techniques

• 4/2/02

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Rendering for animation

• The big difference real time vs. off-line
• Real time sacrifice quality for performance
• Hardware support necessary
• Use polygons and scanline rendering
• Use simple lighting models
• Phongdiffuseambient
• New hardware might change this
• Applications interactive systems
• Games
• Walkthroughs

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Off-line rendering

• Primary goal get the right appearance
• Less concern about time
• Done in software great flexibility
• Not only triangles
• Not only Phong
• Can use different rendering techniques
• Raytracing
• Radiosity
• REYES

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Ray tracing

• The idea follow light propagation through the
  scene
• Algorithm
• Shoot a ray through eye position and pixel center
• Determine the first surface it intersects with
• Compute surface color
• Shoot new rays to light sources (shadow rays)
• If blocked, no contribution
• Account for surface reflection, light and viewing
  direction

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Recursive ray tracing

• If surface is a mirror
• Shoot new ray in mirror direction
• Repeat the process
• If surface is diffuse
• Terminate
• Alternative shoot a ray in random direction
• Called pathtracing very slow
• Always terminate once contribution is small
• Rays carry light energy

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Stochastic supersampling

• Ray through the pixel center aliasing artifacts
• Increase number of rays per pixel, average
  results
• supersampling
• Better if point is chosen randomly
• Stochastic sampling
• Turn regular artifacts into noise

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Advantages / disadvantages

• Mirror reflections / refractions are easy
• Arbitrary surface reflectance properties
• BRDFs
• Diffuse interreflections are difficult
• Can be very slow
• Need extra acceleration datastructures
• Grids, octrees, etc.
• With these, speed is ok on modern machines
• Scanline performance number of objects
• Raytracing performance image resolution

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Radiosity

• Assumption all surfaces are Lambertian
• Uniformly diffuse
• Split all surfaces into patches
• Chose a point on each patch
• Light reflected from a patch at a point linear
  combination of light from other points
• Coefficients depends on mutual arrangement

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Radiosity

• Write equations of light transfer
• Compute patch-to-patch transfer coefficients
• Form factors
• Solve this system
• Get patch color at one point
• Interpolate to get color everywhere on the patch

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Radiosity

• Lots of different algorithms to
• Split surfaces into patches
• Respecting shadow boundaries, etc.
• Compute form factors
• Solve radiosity system of equations
• Efficient methods for special sparse systems
• Take into account only significant energy
  exchanges
• Some form is implemented in Blender

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Advantages / disadvantages

• Very nice images of diffuse environments
• A rather complex algorithm
• Form factor computation is slow
• Does not handle mirrors
• Some form of raytracing is needed
• Currently somewhat decreasing in popularity

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REYES system

• Champion in longevity
• Created in mid-80s by what now Pixar
• Basis for RenderMan standard rendering tool for
  movie industry
• 1993 Academy award (Oscar)
• Major ideas
• Splitting and dicing of primitives
• Surface shaders

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Splitting and dicing

• Determine if a primitive is on the screen
• Compute primitive size on the screen
• Use bounding boxes
• Split if the size is too large
• Dicing conversion to a grid
• Tesselation into mycropolygons
• Size is about 1 pixel
• Their vertices are shaded

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Shader concept

• Primitives have shaders attached to it
• Shader program which determines relevant
  parameters
• Not only surface color (surface shaders)
• Displacement shaders
• Light shaders
• Volume shaders
• Imager shaders (BMRT only)

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Visible surface determination

• Determine which pixels are affected by micropol.
• Each pixel has list of sample positions
• Stochastic point samples
• Test which are covered by a micropol.
• Each sample has associated visible point list
• Includes depth and transparency
• Once done, determine pixel color

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Enhanced REYES

• Memory usage problem
• Visible point lists are huge
• Use buckets small pixel regions
• Sort primitives into buckets
• Process one bucket at a time
• Occlusion culling
• Sort primitives by depth in each bucket
• Process close objects first

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RenderMan / BMRT

• 4/4/02

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RenderMan rendering interface

• RenderMan also specifies a rendering interface
• Independent of implementation
• REYES system in Pixars RenderMan
• Raytracing in BMRT
• Mostly transparent for the user
• Analog OpenGL is an interface
• Hardware support driver hides the details
• Software implementation (Mesa)

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RenderMan interface

• Scene description file
• .rib (RenderMan interface bytestream)
• Compiled shaders
• .slc used by RenderMan directly
• Shading language
• High level C-like language
• .sl run a compiler to convert into .slc

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Using RenderMan

• It is run from a command line
• Run setenv first to set up paths
• rgl fast OpenGL previewer
• Good for geometry/lights/camera positioning
• Usage rgl ribname.rib
• slc - shader language compiler
• slc shadername.sl
• Produces shadername.slc
• Need to do this for all shaders used

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Using RenderMan

• rendrib is the renderer
• rendrib ribname.rib
• Creates output according to rib specs
• Use d to get display output directly
• -d 16 to get multiresolution approximation
• Raytracing complex scenes can be slow
• Debug shaders on simple geometry

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.rib file anatomy
Global options
Frame block
Image options Camera options
World block Attributes, lights, primitives
Changed options
Another world block
Next frame block
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Parameter declarations

• Can declare parameters with
• Declare name declaration
• declaration is an analog of type
• class type actually
• Type float, color, vertex, vector, normal,
  point, string, matrix
• This is global
• In-line decraration only in particular command
• class type name

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Attribute blocks

• Everything between AttributeBegin and
  AttributeEnd
• Inherits attribute state of the parent
• Manipulates it, assigns to geometric primitives
• Attribute state
• color/shaders attached
• Transformation matrices
• TransformBegin / TransformEnd push/pop transform
  matrices

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Transformations

• Applies to local coord system
• Rotate angle vx vy vz
• Scale sx sy sz
• Skew angle vx vy vz ax ay az
• ConcatTransform matrix
• Identity
• Transform matrix

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Special coord systems

• Camera space
• Origin at the camera, Z in front, Y is up
• Left-handed !!!
• Created with
• Projection type parameterlist
• Everything else is relative to it
• Before WorldBegin form world-to-camera matrix
• Each object/shader created according to current
  transform matrix
• Coord system is stored as object / shader
  space

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Geometry

• Quadrics
• Sphere, cylinder, cone, paraboloid, hyperboloid,
  disk, torus
• Polygons and meshes
• Polygon, GeneralPolygon, PointsPolygon,
  PointsGeneralPolygon
• Parametric patches and NURBS
• Basis, Patch, PatchMesh, NuPatch
• Other trim curves, subdivision meshes, CSG

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Primitive variables

• Attached to geometric primitives
• Can be referred to directly
• P, Pw, N, Cs, Os, st
• These are
• Position in 3D (P), and in hc (Pw)
• Normal (N)
• Surface color (Cs) and opacity (Os)
• Texture coords (st)

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Shaders

• In .rib file, created by
• Surface shadername parameterlist
• Displacement shadername parameterlist
• Parameters are passed to the shader program
• Written in special shading language
• Has access to some global variables
• Sets some global variables
• Final surface color Ci and opacity Oi
• Can also modify position P and normal N
• Displacement shader

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A simple shader

• Surface metal (float Ka 1, Ks 1 float
  roughness .1)
• normal Nf faceforward (normalize(N),I)
• vector V -normalize(I)
• Ci Cs (Kaambient() Ksspecular(Nf,V,rou
  ghness))
• Oi Os Ci Oi

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Simple shader usage

• In .rib file the usage will be
• AttributeBegin
• Translate 0 0 0
• Color 1 .3 .05
• Surface "metal" "roughness" 0.3 "Ks" 1.5
• ReadArchive "vase.rib"
• AttributeEnd

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Simple shader notes

• Global variables N, I, Oi, Os, Ci, Cs
• Sets final surface color Ci
• Cs is from .rib file
• Parameters Ka, Ks, roughness are from shader
  parameterlist in .rib
• Shader language functions
• Uses default ambient() and specular() to do
  actual computation
• There is also diffuse()
• Normalize(), faceforward()

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Lights and illumination

• Can access light information in illumination
  loops
• color diffuse (normal Nn)
• extern point P
• color C0
• illuminance(P,Nn,PI/2)
• CCl(Nn . Normalize(L))
• return C
• Loops over all visible lights from P which are
  within PI/2 from Nn

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BMRT

• BMRT implements RenderMan interface
• But it is a raytracer
• Extra features available
• color trace(point from vector dir) returns
  incoming light from dir
• Also Fulltrace, rayhittest, visibility, etc.
• RayTrace() stochastic supersampling
• Easy to do reflections

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Simple shader using raytracing

• color MaterialShinyMetal (normal Nf color
  basecolor float Ka, Kd,
  Ks, roughness, Kr, blur uniform float twosided
  DECLARE_ENVPARAMS)
• extern point P
• extern vector I
• extern normal N
• float kr Kr

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continued

• if (twosided 0 N.I gt 0)
• kr 0
• vector IN normalize(I), V -IN
• vector R reflect (IN, Nf)
• return basecolor (Kaambient()
  Kddiffuse(Nf)
  Ksspecular(Nf,V,roughness)
• SampleEnvironment (P, R, kr, blur, ENVPARAMS))

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Notes on raytracing shader

• Mostly as before
• SampleEnvironment calls RayTrace
• Also includes environment mapping
• See reflections.h
• ENVPARAMS is a bunch of stuff controlling ray
  tracing / env. mapping
• Number of samples, env.map name, etc.

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Concluding notes

• Real power of RenderMan is in its flexibility
• Want complex appearance just write a shader
  function
• Hundreds of parameters for complex shaders
• Will see more on procedural techniques later in
  the course
• Including possibilities for some interesting
  shaders
• Assignment 5 asks you to play with shaders
• And write a few of your own