Week 10 - Ray Tracing, Path Tracing and Radiosity

Question 1

What is forward tracing?

Answer 1

Trace starting point is the light source

Trace paths of photons from the light source

Question 2

What is backward tracing?

Answer 2

Tracing starting point is the eye point

Shoot out rays from eye position and trace backward

Question 3

What type of tracing is ray tracing (proposed by Whitted in 1979) based on?

Answer 3

Backward tracing

Question 4

What is a light ray also known as in ray tracing?

Answer 4

Shadow ray

Question 5

What is a light ray for?

Answer 5

Direct diffuse reflection as a ray from the point to the light source

Question 6

In light paths notation, what do these letters stand for?
L
E
D
S

Answer 6

L - light source
E - eye
D - diffuse reflection
S - specular reflection / refraction

Question 7

What is the general regular expression for the variety of different light paths in ray tracing/path tracing using the LEDS notation?

Answer 7

L(D|S)*E

Question 8

What is path tracing (Kajiya 1986)?

Answer 8

An extension of ray tracing. Uses multiple rays per fragment pixel instead of single ray. At each object, make a “smart” decision how to continue tracing

Question 9

What are the similarities between ray tracing and path tracing?

Answer 9

Both use backward tracing

Both are view-dependent algorithms

Question 10

What does BRDF stand for?

Answer 10

Bidirectional Reflectance Distribution Functions

Question 11

What is the general regular expression for the variety of different light paths in radiosity using the LEDS notation?

Answer 11

LD*E

Question 12

How does the radiosity theory work?

Answer 12

No recursion required, focuses on object interactions

Splits scene into patches

Formulates problem as conservation of energy

Requires RGB wavelengths

Question 13

What is the patching process?

Answer 13

Divide graphics scene into patches for computing radiosity

For small polygons, use the polygon map

For large polygons, they need to be subdivided

Since the emitted light will not be a constant across a large polygon we will need to see the subdivisions

The form factors couple every pair of patches, determining the proportion of radiated energy from one that strikes the other

Question 14

What is radiosity?

Answer 14

Energy per unit area leaving a patch per unit time

Question 15

What is the hemicube method?

Answer 15

Direct computation of the approximate form factor equation will be expensive to compute, so an approximate computation method (the hemicube) was developed

Using a bounding hemisphere it can be shown that all patches that project onto the same area of the hemisphere have the same form factor

Question 16

What are 5 computation errors with radiosity and why are they errors?

Answer 16

Alias errors - computation of the form factors will involve alias errors

Form factor reciprocity - form factors have a reciprocal relationship thus form factors for only half the patches need be computed

Interpolation strategies - visual artefacts can occur with interpolation strategies but may not be significant for small patches

Meshing - meshing is the process of dividing the scene into patches, so artefacts are scene dependent, and errors can be caused by discontinuities in the radiosity function

Adaptive meshing - the idea of re-computing the mesh as part of the radiosity calculation, which tackles areas with strong discontinuities by putting more patches (elements) into that area, or moving the mesh boundary to coincide with the greatest change