RADIOSITY

Question 1

How does radiosity treat light in comparison to ray tracing

Answer 1

Raytracing treats light as rays whilst radiosity treats it as energy being exchanged between areas of a scene

Question 2

What is radiosity good at modeling

Answer 2

Diffuse interreflection
subtle lighting
coloured bleeds
soft shadows
all happen naturally (“flat and diffuse effect”)
whilst transparency and specular reflection are much more tricky (opposite of raytracing)

Question 3

What is the cornell box

Answer 3

a physical box built at cornell univeristy with objects lighting and cameras set up and controlled
The aim was to use rendering techniques and reference back to the original box to measure how good the render is (can subtract one image from the other to find the errors (differences))

Question 4

What is colour bleeding

Answer 4

For example
If a white wall of a box faces a red wall and so the reflection of the red wall makes the white wall appear having a red tinge

Question 5

Light source in radiosity vs raytracing

Answer 5

In raytracing, the light source does not have to have a physical representation, rays just come from it (just a point)
In radiosity, it is modeled as part of the geometry of the scene (a polygon emitting energy)

Question 6

Radiosity for thermal calculations

Answer 6

Based on the energy equilibirum of a scene:

Thermodynamic radiosity aka Radiance = The flux leaving a surface at point X

Irradiance - the flux arriving at point X

Flux = a flow of particles
in this case the particles are photons

Question 7

Radiosity equation between surfaces

Answer 7

Have a surface Ax with emits energy to instances of the scene Ax’
We get this equation:
B(x)δA = E(x)δA + p(x)δA ∫B(x’) . F(x,x’)δA’
Where B(x) is the total energy leaving the small area δA
That total energy is made up of 2 components:

• The first is any energy directly emitted from that delta area: E(x)δA
• The second is any energy being indirectly emitted (reflectivity) from that delta area:
  • p(x)δA is the reflectivity of that point
  • S: runs over all surfaces S
  • F(x,x’) form factor of x to x’ which is 1 if x to x’ is completely visible to each other

Question 8

How to find a δA from the scene

Answer 8

We take the original mesh of the scene and split it down into much smaller polygons until we are happy to represents each polygon as just one colour

Question 9

Why does radiosity have no viewpoint

Answer 9

Because it is an entirely object space algorithm
we never need to care about a viewpoint
after we have rendered the scene we could bake the radiosity into a texture - we do not need to change it again

Question 10

The Form factor

Answer 10

A number between 0 and 1 which determinates the visibility of a patch to another
1 - completely visible to each other

Question 11

4 main factors that affect the visibility of one patch to another

Answer 11

Size of the patches
Bigger patch = more energy

Distance between patches in space
small increase in distance has a big affect

The orientation of one patch compared to another
If they do not face each other - no exchange

Whether or not something stands in the way to completely the block exchange

Question 12

The definitive radiosity equation

Answer 12

Bi = Ei + p Σn,j=1 FijBj

Question 13

What is progressive/shooting radiosity

Answer 13

The recursive way we can build up the energy exchange in the scene
Begin with the patch emitting the most energy (probably the light source) and follow the energy and repeat
Get a better image the more we repeat