Advanced rendering techniques4/2/02

Rendering for animationThe big difference: real time vs. off-lineReal time: sacrifice quality for performanceHardware support necessary Use polygons and scanline rendering Use simple lighting models

Phong+diffuse+ambient New hardware might change this

Applications: interactive systems Games Walkthroughs

Off-line renderingPrimary 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

Ray tracingThe idea: follow light propagation through the sceneAlgorithm: 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

Recursive ray tracingIf 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

Stochastic supersamplingRay through the pixel center – aliasing artifactsIncrease number of rays per pixel, average results supersamplingBetter if point is chosen randomly Stochastic sampling

Turn regular artifacts into noise

Advantages / disadvantages

Mirror reflections / refractions are easyArbitrary surface reflectance properties BRDFs

Diffuse interreflections are difficultCan be very slow Need extra acceleration datastructures

Grids, octrees, etc. With these, speed is ok on modern machines

Scanline performance: number of objectsRaytracing performance: image resolution

RadiosityAssumption: 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 pointsCoefficients depends on mutual arrangement

RadiosityWrite equations of light transferCompute patch-to-patch transfer coefficients Form factorsSolve this system Get patch color at one pointInterpolate to get color everywhere on the patch

RadiosityLots 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

Advantages / disadvantages

Very nice images of diffuse environmentsA rather complex algorithm Form factor computation is slowDoes not handle mirrors Some form of raytracing is neededCurrently somewhat decreasing in popularity

REYES systemChampion in longevity Created in mid-80s by what now Pixar Basis for RenderMan – standard

rendering tool for movie industry1993 Academy award (“Oscar”)Major ideas: Splitting and dicing of primitives Surface shaders

Splitting and dicingDetermine if a primitive is on the screenCompute primitive size on the screen Use bounding boxesSplit if the size is “too large”Dicing – conversion to a “grid” Tesselation into mycropolygons

Size is about 1 pixel Their vertices are shaded

Shader conceptPrimitives have shaders attached to itShader – program which determines relevant parametersNot only surface color (surface shaders) Displacement shaders Light shaders Volume shaders Imager shaders (BMRT only)

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

Enhanced REYESMemory usage problem Visible point lists are hugeUse buckets – small pixel regions Sort primitives into buckets Process one bucket at a timeOcclusion culling Sort primitives by depth in each bucket Process close objects first

RenderMan / BMRT4/4/02

RenderMan rendering interface

RenderMan also specifies a rendering interfaceIndependent of implementation REYES system in Pixar’s RenderMan Raytracing in BMRT Mostly transparent for the user

Analog: OpenGL is an interface Hardware support – driver hides the details Software implementation (Mesa)

RenderMan interfaceScene description file .rib (RenderMan interface bytestream)Compiled shaders .slc – used by RenderMan directlyShading language High level C-like language .sl – run a compiler to convert into .slc

Using RenderManIt is run from a command lineRun “setenv” first to set up pathsrgl – 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

Using RenderManrendrib is the renderer rendrib ribname.ribCreates output according to rib specs Use –d to get display output directly -d 16 to get multiresolution

approximationRaytracing complex scenes can be slow Debug shaders on simple geometry

.rib file anatomyGlobal options

Frame block

Next frame blockAnother world block

World block Attributes, lights, primitives

Changed options

Image optionsCamera options

Parameter declarationsCan declare parameters with Declare “name” “declaration” declaration is an analog of type

class type actuallyType = float, color, vertex, vector, normal, point, string, matrixThis is globalIn-line decraration – only in particular command “class type name”

Attribute blocksEverything between AttributeBegin and AttributeEndInherits attribute state of the parentManipulates it, assigns to geometric primitivesAttribute state: color/shaders attached Transformation matricesTransformBegin / TransformEnd push/pop transform matrices

TransformationsApplies to local coord systemRotate angle vx vy vzScale sx sy szSkew angle vx vy vz ax ay azConcatTransform matrixIdentityTransform matrix

Special coord systemsCamera space Origin at the camera, Z+ in front, Y+ is up Left-handed !!! Created with

Projection type parameterlist

Everything else is relative to itBefore WorldBegin – form world-to-camera matrixEach object/shader created according to current transform matrix Coord system is stored as “object” / “shader” space

GeometryQuadrics: Sphere, cylinder, cone, paraboloid, hyperboloid,

disk, torusPolygons and meshes: Polygon, GeneralPolygon, PointsPolygon,

PointsGeneralPolygonParametric patches and NURBS: Basis, Patch, PatchMesh, NuPatchOther: trim curves, subdivision meshes, CSG

Primitive variablesAttached to geometric primitivesCan 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)

ShadersIn .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 variablesSets some global variables Final surface color Ci and opacity Oi Can also modify position P and normal N

Displacement shader

A simple shaderSurface metal (float Ka = 1, Ks = 1; float roughness = .1;)

{ normal Nf = faceforward (normalize(N),I);

vector V = -normalize(I); Ci = Cs * (Ka*ambient() + Ks*specular(Nf,V,roughness));

Oi = Os; Ci *= Oi;}

Simple shader usageIn .rib file the usage will be:

AttributeBeginTranslate 0 0 0Color 1 .3 .05Surface "metal" "roughness" [0.3] "Ks" [1.5]

ReadArchive "vase.rib"AttributeEnd

Simple shader notesGlobal 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 .ribShader language functions: Uses default ambient() and specular(…) to

do actual computation There is also diffuse(…)

Normalize(), faceforward()

Lights and illuminationCan access light information in illumination loops

color diffuse (normal Nn){ extern point P; color C=0; illuminance(P,Nn,PI/2){ C+=Cl*(Nn . Normalize(L)) } return C; }

Loops over all visible lights from P which are within PI/2 from Nn

BMRTBMRT implements RenderMan interfaceBut it is a raytracer Extra features availablecolor trace(point from; vector dir) returns incoming light from dirAlso Fulltrace, rayhittest, visibility, etc. RayTrace(…) – stochastic supersamplingEasy to do reflections

Simple shader using raytracingcolor 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;

…continuedif (twosided == 0 && N.I > 0)kr = 0;

vector IN = normalize(I), V = -IN; vector R = reflect (IN, Nf); return basecolor * (Ka*ambient() + Kd*diffuse(Nf) + Ks*specular(Nf,V,roughness) +

SampleEnvironment (P, R, kr, blur, ENVPARAMS));

}

Notes on raytracing shader

Mostly as beforeSampleEnvironment calls RayTrace Also includes environment mapping See reflections.hENVPARAMS is a bunch of stuff controlling ray tracing / env. mapping Number of samples, env.map name, etc.

Concluding notesReal power of RenderMan is in its flexibilityWant 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…