2MotivationPhoto‑realistic image rendering is particularly difficult to compute because of the complexity of the physical nature of light. However, the radiosity global illumination methods approximates the physical nature of light and provides the necessary foundation for extremely high quality rendered photo‑realistic images.Radiosity has become established as the global illumination method for rendering the highest quality, view independent images for virtual environments and captures subtle lighting effects such as colour bleeding. The method is able to correctly compute shadows due to area light sources, producing accurate penumbra and umbra.
3MotovationRadiosity is a powerful tool for rendering photo‑realistic scenes. Once the radiosity of a scene has been calculated, a ‘virtual reality’ walkthrough of the scene is immediately available. However, this comes at a costly price as calculating the radiosity of a scene is anything but trivial.
4Introduction Real-time walkthrough with global illumination Possible under limited conditionsRadiosity (diffuse surfaces only)Real-time interactionNot possible except for special case local illuminationWhy is the problem so hard?
5LightRemember visible light is electromagnetic radiation with wavelengths approximately in the range from 400nm to 700nm400nm700nm
6Light: Photons Light can be viewed as wave or particle phenomenon Particles are photonspackets of energy which travel in a straight line in vaccuum with velocity c (300,000m.p.s.)The problem of how light interacts with surfaces in a volume of space is an example of a transport problem.
7Light: Radiant Power denotes the radiant energy or flux in a volume V.The flux is the rate of energy flowing through a surface per unit time (watts).The energy is proportional to the particle flow, since each photon carries energy.The flux may be thought of as the flow of photons per unit time.
8Light: Flux Equilibrium Total flux in a volume in dynamic equilibriumParticles are flowingDistribution is constantConservation of energyTotal energy input into the volume = total energy that is output by or absorbed by matter within the volume.
9Light: Equation (p,) denotes flux at pV, in direction It is possible to write down an integral equation for (p,) based on:Emission+Inscattering = Streaming+Outscattering + AbsorptionComplete knowledge of (p,) provides a complete solution to the graphics rendering problem.Rendering is about solving for (p,).
10Simplifying Assumptions Wavelength independenceNo interaction between wavelengths (no fluorescence)Time invarianceSolution remains valid over time unless scene changes (no phosphorescence)Light transports in a vacuum (non-participating medium) –‘free space’ – interaction only occurs at the surfaces of objects
11RadianceRadiance (L) is the flux that leaves a surface, per unit projected area of the surface, per unit solid angle of direction.nd = L dA cos dLdA
12RadianceFor computer graphics the basic particle is not the photon and the energy it carries but the ray and its associated radiance.ndALdRadiance is constant along a ray.
13Radiance: Radiosity, Irradiance Radiosity - is the flux per unit area that radiates from a surface, denoted by B.d = B dAIrradiance is the flux per unit area that arrives at a surface, denoted by E.d = E dA
14Radiosity and Irradiance L(p,) is radiance at p in direction E(p,) is irradiance at p in direction E(p,) = (d/dA) = L(p,) cos d
16Recall Reflectance: BRDF Reflected Radiance = BRDFIrradianceFormally:L(p, r ) = f(p, i , r ) E(p, i )= f(p, i , r ) L(p, i ) cosi diIn practice BRDF’s hard to specifyRely on ideal typesPerfectly diffuse reflectionPerfectly specular reflectionGlossy reflectionBRDFs taken as additive mixture of these
17The Radiance EquationRadiance L(p, ) at a point p in direction is the sum ofEmitted radiance Le(p, )Total reflected radianceRadiance = Emitted Radiance + Total Reflected Radiance
18The Radiance Equation: Reflection Total reflected radiance in direction : f(p, i , ) L(p, i ) cosi diRadiance Equation:L(p, ) = Le(p, ) + f(p, i , ) L(p, i ) cosi di(Integration over the illumination hemisphere)
19The Radiance Equation L(p, ) p is considered to be on a surface, but can be anywhere, since radiance is constant along a ray, trace back until surface is reached at p’, thenL(p, i ) = L(p’, i )L(p, ) depends on all L(p*, i) which in turn are recursively defined.p*iL(p, )pThe radiance equation models global illumination.
20Traditional Solutions to the Radiance Equation The radiance equation embodies totality of all 2D projections (view).
21Irradiance Power per unit area incident on a surface. E = d /dA Unit: Watt / m2arrivingdA
22Radiant Exitance Power per unit area leaving surface Also known as radiosityB = d /dASame units as irradiancejust direction changes.leavingdA
23Basic Definitions Radiosity: (B) Energy per unit area per unit time. Emission: (E) Energy per unit area per unit time that the surface emits itself (e. g., light source).Reflectivity: (r) The fraction of light which is reflected from a surface. (0 <= r <=1)Form- Factor: (F) The fraction of the light leaving one surface which arrives to another. (0<=F<=1)
24The Basic Radiosity Equation We will compute the light emitted from a single differential surface area dAi.It consists of:1. Light emitted by dAi.2. Light reflected by dAi.depends on light emitted by other dAj, fraction of it reaches dAi.The fraction depends on the geometric relationship between dAi and dAj: the formfactor .
25Mesh Surfaces into Elements Reconstruct and Display Solution Classic Radiosity AlgorithmMesh Surfaces into ElementsCompute Form FactorsBetween ElementsSolve Linear Systemfor RadiositiesReconstruct and Display Solution
26The Descrete Radiosity Equation Total power leaving an element iand reflected light.weighted by geometric couplingj->iand reflectivityis sum of emitted light by element iReflected light depends on contribution from every other element j
27The Form Factor:the fraction of energy leaving one surface that reaches another surfaceIt is a purely geometric relationship, independent of viewpoint or surface attributesSurface jSurface i
28The Reciprocity Relationship If we had equal sized emitters and receivers, the fraction of energy emitted by one and received by the other would be identical to the fraction of energy going the other way.Thus, the formfactors from Ai to Aj and from Aj to Ai are related by the ratios of their areas:Thus:The radiosity equation is now:
29Patches and ElementsPatches are used for emitting light. Some patches are divided into elements, which are used to more accurately compute the received light after the patch solution have been computed.
30Next Step: Learn ways of computing form factors Needed to solve the Descrete Radiosity Equation:Form factors Fij are independent of radiosities (depend only on scene geometry)
31The overall form factor between i and j is found by integrating Between differential areas, the form factor equals:The overall form factor between i and j is found by integratingSurface jSurface i
32Form Factors in (More) Detail where Vij is the visibility (0 or 1)
33We have two integrals to compute: Surface jArea integralover surface iArea integralover surface jSurface i
34The Nusselt AnalogIntegration of the basic form factor equation is difficult even for simple surfaces!Nusselt developed a geometric analog which allows the simple and accurate calculation of the form factor between a surface and a point on a second surface.
35The Nusselt AnalogThe "Nusselt analog" involves placing a hemispherical projection body, with unit radius, at a point on a surface.The second surface is spherically projected onto the projection body, then cylindrically projected onto the base of the hemisphere.The form factor is, then, the area projected on the base of the hemisphere divided by the area of the base of the hemisphere.
36Numerical Integration: The Nusselt Analog This gives the form factor FdAiAjAjdAi
37Method 1: HemicubeApproximation of Nusselt’s analog between a point dAi and a polygon AjPolygonalArea (Aj)InfinitesimalArea (dAi)
38The Hemi-cubeWe compute the delta formfactor of each grid cells DF and store in a table.Project all patches onto the ‘ hemi- cube ’ screen, drawing a patch- id instead of color.Sum the delta form factors of all grid cells covered by the patch’s id.Delta form factor
40The Hemicube In ActionThis illustration demonstrates the calculation of form factors between a particular surface on the wall of a room and several surfaces of objects in the room.
41Projecting all other surfaces onto the hemicube Compute the form factors from a point on a surface to all other surfaces by:Projecting all other surfaces onto the hemicubeStoring, at each discrete area, the identifying index of the surface that is closest to the point.
42Discrete areas with the indices of the surfaces which are ultimately visible to the point. From there the form factors between the point and the surfaces are calculated.For greater accuracy, a large surface would typically be broken into a set of small surfaces before any form factor calculation is performed.
43Hemicube Method Scan convert all scene objects onto hemicube’s 5 faces Use Z buffer to determine visibility termSum up the delta form factors of the hemicube cells covered by scanned objectsGives form factors from hemicube’s base to all elements, i.e. FdAiAj for given i and all j
44Hemicube Algorithms Advantages + First practical method + Use existing rendering systems; Hardware+ Computes row of form factors in O(n)Disadvantages- Computes differential-finite form factorAliasing errors due to sampling- Proximity errors- Visibility errors- Expensive to compute a single form factor
45We have found the Radiosity Matrix Elements EiBi
46Radiosity MatrixThe "full matrix" radiosity solution calculates the form factors between each pair of surfaces in the environment, as a set of simultaneous linear equations.This matrix equation is solved for the "B" values, which can be used as the final intensity (or color) value of each surface.
47Radiosity MatrixThis method produces a complete solution, at the substantial cost offirst calculating form factors between each pair of surfacesand then the solution of the matrix equation.Each of these steps can be quite expensive if the number of surfaces is large: complex environments typically have above ten thousand surfaces, and environments with one million surfaces are not uncommon.This leads to substantial costs not only in computation time but in storage.
62Discontinuity Meshing Dani Lischinski, Filippo Tampieri and Donald P. Greenberg created this image for the 1992 paper Discontinuity Meshing for Accurate Radiosity.It depicts a scene that represents a pathological case for traditional radiosity images, many small shadow casting details.Notice, in particular, the shadows cast by the windows, and the slats in the chair.