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Local Illumination
The Phong model is a local illumination model • shaded color depends purely on local surface configuration
- – surface normal, viewing direction, light directiondiffuse, specular, and ambient reflectances Other surfaces have no effect • they don’t block light (no shadows)
- they don – consider’t illuminate the surface glMaterial(…,GL_EMISSION,…)
- – just causes that surface to glowdoesn’t effect other surfaces
But in reality surfaces interact a great deal
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Towards Global Illumination
Illumination of one object depends on others • other objects can block light — shadows
- • may reflect image of other objectsother objects may illuminate this object — reflection — emissive surfaces Ray tracing already models some of these interactions • shadows
- • specularspecular reflectionstransmission (refraction)
- and with distributed sampling we extended this to – soft shadows These are only^ –^ glossy reflections some of the possible phenomena
Direct Ray Tracing vs. Global Illumination
images from Henrik Wann^ Direct lighting only Jensen and Per Christensen.^ Global Illumination Efficient Simulation of Light Transport in Scenes With Participating Media Using Photon Maps. SIGGRAPH 98.
Color Bleeding
A hallmark of global illumination • floor is illuminated by walls
- • the floor itself is grayred & blue “bleed” onto floor Why is this? • light hitting walls is white
- • the wall is red (or blue)thus it reflects mostly red light
- this red light illuminates floor^ –^ absorbs other wavelengths
- the floor is gray – reflects wavelengths equally
- thus it reflects red light previous image with 90% color saturation increase
Indirect Illumination
Not all surfaces are directly lit by lights • the world is rife with indirect light
- often preferred for lighting a room Consider • light is pointed at ceiling torchiere-style lamps
- • opaque basket beneath lightis the floor in shadow? no! image from Cornell Program of Computer Graphics http://www.graphics.cornell.edu/online/research/
Two Views of Global Illumination
Every surface is a • at each surface, some light is arriving from all directions “light source” illuminating all the others
- • some of it is reflected back of the surfacedistributed over some set of outgoing directions
- solving this is tricky – assuming everything is diffuse makes it tractable — each surface depends on all the others Must trace all possible paths of light from sources to eye • backward ray tracing explores a very limited subset
- and it • how to shoot rays at a prism to guarantee that they hit a light?’s difficult to extend — consider caustics
- • forward ray tracing could work, but could be very expensivemost workable ray-based methods combine these two
- – bi-directional path tracing, photon maps, Metropolisand they are far beyond the scope of this course
Global Illumination Algorithm: Radiosity
View a surface element as illuminating all other surfaces • but we assume that all surfaces are perfectly diffuse
- • in other words, they reflect light equally in all directionsthis actually makes the problem tractable In radiosity • assuming we use physical material models , we simulate physically correct transport of light Thus the goal is true^ •^ there will actually be conservation of energy and such photorealism
- • before we were interested in whether images looked goodnow we can consider whether they are measurably good
Physical Image vs. Radiosity Simulation
Which image is the photograph and which is the simulation?
Solving the Radiosity Equation
This is really the main part of the radiosity method
How do we solve this (recursive) integral equation? • next time we’ll look at some simple methods
incoming power from patch outgoing power emitted power^ reflectance
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