Week 4 - Models and Illumination

Question 1

How are continuous objects modelled?

Answer 1

Using polygonal approximations - a number of connected polygons (faces): a mesh

Question 2

When rendering opaque objects, why is only the outer boundary/surface important?

Answer 2

Since only the outer boundary/surface interacts with light

Question 3

What is a mesh?

Answer 3

A number of connected polygons/faces

Question 4

What are the two main types of mesh?

Answer 4

Triangle mesh and quadrangle mesh

Question 5

What does a polygon mesh consist of?

Answer 5

A set of vertices (3D points described by (x, y, z) coordinates)

A set of edges (line segments connecting two vertices)

A set of faces (polygons)

Question 6

When are two edges chained?

Answer 6

If they share one vertex

Question 7

What are surface normals?

Answer 7

Vectors perpendicular to the surface

Question 8

What 3 conditions must a polygon mesh satisfy?

Answer 8

The intersection of two faces is either empty or a vertex or an edge

Manifold meshes - every edge must be shared by exactly two faces (except for the border edges)

The faces must have consistent normals (defining the same outward direction)

Question 9

What is a Mobius strip?

Answer 9

A shape with inconsistent normals

Question 10

How are smooth surfaces approximated?

Answer 10

By a discrete set of points, connected by edges

Typically meshes have more polygons in high curvature areas

Question 11

What methods can you use to define a triangle mesh using OpenGL?

Answer 11

glBegin/glEnd and glVertex, using one of GL_TRIANGLES (plots each triangle separately), GL_TRIANGLE_STRIP (plots in a line, avoids repeating vertices), or GL_TRIANGLE_FAN (plots in a circle, avoids repeating vertices)

Question 12

What does explicit representation of a triangle mesh mean?

Answer 12

Each triangle is encoded by its three vertices

Question 13

What does redundant representation of a triangle mesh mean?

Answer 13

Each vertex is shared by three triangles

Question 14

How many float variables are required for redundant triangle mesh representation?

Answer 14

3 x 3 x n = 9n floats for n faces

Question 15

What are VBOs?

Answer 15

Vertex Buffer Objects - arrays for object models that can remain in GPU memory

Question 16

What is shared vertex/indexed face set representation?

Answer 16

Array of coordinates of all vertices (3m floats for m vertices) and an array of triangles with indices into the vertex list (3n integers for n faces)

It gives less redundancy than explicit representation

Question 17

Name 3 properties of polygon models per-vertex.

Answer 17

Coordinate (x, y, z)

Colour (r, g, b)

Normal (nx, ny, nz)

Question 18

Name 4 light and material properties.

Answer 18

Light source

Colour

Reflectivity

Transparency

Question 19

How can meshes be made to look solid?

Answer 19

By filling the triangles with colour

Question 20

How can you get rid of the polygonal edges when filling a mesh with colour?

Answer 20

Interpolate the colour across every face

Question 21

Name 4 types of light sources.

Answer 21

Point lights (very small candle flame)

Directed lights (sun, assumed at infinite distance)

Spot lights (torch etc)

Distributed lights

Question 22

What attenuation does directed light have?

Answer 22

No attenuation

Question 23

What is light attenuation?

Answer 23

The reduction in its intensity as it travels through a medium due to absorption or scattering of photons

Question 24

What attenuation does a point light source have and why?

Answer 24

Quadratic attenuation -> 1/d^2 because it is a sphere (A = 4pid^2)

Question 25

When a ray of light hits a surface, what 4 things can happen?

Answer 25

Absorption, reflection, refraction and fluorescence

Question 26

What does global illumination consider?

Answer 26

All interactions of light within the scene, such as lighting directly from light sources, and indirect lighting

Question 27

What does Lambert's cosine law describe?

Answer 27

Light reflected with equal intensity in all directions

Question 28

What is perfect specular reflection?

Answer 28

Reflection from a perfect mirror. Incident and reflected light make equal and opposite angles with surface normal

Question 29

What is imperfect specular reflection?

Answer 29

Some light is scattered away from principal reflected direction

Question 30

Give 2 examples of a specular reflection model.

Answer 30

The Phong Model and the Blinn-Phong Model

Question 31

Name the light source options for the Phong reflection model

Answer 31

Point light at a given position Spot light at a given position with a limited set of light directions

Question 32

How do you do flat shading of polygon surfaces?

Answer 32

A single intensity is calculated for each polygon Shade is determined by polygon normal and colour at "centre" A triangular face's normal is given by the cross product

Question 33

How does Gouraud shading work?

Answer 33

Calculate vertex normals by (possibly weighted) averaging of adjacent polygons (Bi)linerarly interpolate intensity between vertices

Question 34

How does Phong shading work?

Answer 34

Calculate vertex normals by averaging (possibly weighted) adjacent polygons (Bi)linearly interpolate normals between vertices

Question 35

How does scanline interpolation work?

Answer 35

Uses Z-buffer algorithm to determine screen locations of vertices Intensities or normals interpolated from vertices along vertices Interpolate along scanline from edges

Question 36

Name 3 methods and give a brief explanation of how to shade polygons in screen space.

Answer 36

Flat shading - shade determined by polygon normal and colour at "centre" Gouraud shading - calculate shades at vertices and interpolate to interior pixels Phong shading - interpolate normal direction to interior pixels and then calculate shade

Question 37

Name a positive and a negative of using flat shading.

Answer 37

Fast and avoids interpolation Doesn't look great

Question 38

Name a positive and a negative of using Gouraud shading.

Answer 38

Basic shading, good for diffuse reflections Poor rendering of highlights and specular reflections

Question 39

Name a positive and a negative of Phong shading.

Answer 39

Highest quality without volumetric rendering 4-5 times slower than Gouraud

Question 40

How can you use Phong shading and Gouraud shading together?

Answer 40

Use Phong for surfaces with specular reflection Use Gouraud for diffuse surfaces